What mnemonic is used to rank bases?

The mnemonic 'REO' is used to rank bases: Atom, Resonance, Induction, and Orbitals.

How does the charge affect acidity and basicity?

The charge affects acidity and basicity as a more negative charge generally means a stronger base, while a more positive charge is linked to a stronger acid.

What is the relationship between acids and their conjugate bases?

The stronger an acid, the weaker its conjugate base and vice versa.

Why is resonance important in determining acid and base strength?

Resonance stabilizes a base, making it weaker because it lowers the energy of the electrons.

How does atom size affect basicity?

Smaller atoms make stronger bases due to shorter, stronger bonds to H+.

What does induction mean in the context of acids and bases?

Induction refers to the effect of nearby electronegative atoms pulling electron density away, stabilizing a base.

How does hybridization affect base strength?

Lone pairs on sp hybridized atoms are more stable (weaker base) than on sp3 hybridized ones.

Can electronegativity influence acid strength?

Yes, less electronegative atoms have weaker bases, hence stronger conjugate acids.

What role do memorized pKa values play?

Memorized pKa values help confirm acid rankings when structural rule assessments might lead to confusion.

What is the significance of atomic positioning in relation to electronegative atoms?

The closer an electronegative atom is to the base, the more it stabilizes due to induction.

3.2 Ranking Acids and Bases | Organic Chemistry

00:35:47

https://www.youtube.com/watch?v=ucC9IhQX6JU

Ringkasan

TLDRIn this lesson, the concept of ranking acids and bases based on their structural features is explored. The discussion focuses on how to compare the relative acidities and basicities of acids and bases by applying specific rules. A key highlight is the mnemonic 'REO' used for ranking. This stands for Atom, Resonance, Induction, and Orbitals, though the rule concerning charge is also discussed as a subsidiary component. The video clarifies that a stronger acid has a weaker conjugate base and vice versa and illustrates examples where the stability of bases, the size of atoms, or resonance can impact their corresponding acid or base strengths. Key points include: 1. A more stable base is often a weaker base due to lower energy electrons, and charge can affect energy levels significantly. 2. The sequence of considering Atom, Resonance, Induction, and Orbitals helps rank the bases effectively. 3. The mnemonic is primarily for bases having similar charges. 4. Memorizing pKa values can resolve instances where structural analysis is not straightforward. 5. Inductive effects can stabilize a base, as demonstrated by electronegative atoms withdrawing electron density. 6. Changes in hybridization states (e.g., sp, sp2, sp3) impact the stability and thus basicity of atoms in compounds. The lesson emphasizes understanding these principles can aid in predicting relative reactivity in chemical reactions, imperative for organic chemistry.

Takeaways

🔍 Understanding rules is key: Understanding and applying the mnemonic 'REO' helps in ranking bases and analyzing acidity.
🔓 Charge matters: More negative charge correlates with a stronger base, while more positive with stronger acid.
🔬 Size affects basicity: Smaller atoms form stronger bases.
🌐 Resonance stabilizes bases: Resonance decreases electron energy, leading to weaker bases.
⚛️ Atom matters: The atom's position in the periodic table influences base strength.
🔧 Induction helps stabilize: Electronegative atoms nearby reduce a base's negativity, stabilizing it.
🔄 Hybridization affects basicity: sp hybridized atoms have more stable, lower energy electrons than sp2 or sp3.
📏 Memorized pKa values are crucial: They help when structural clues are ambiguous.
📉 Strong acids have weak bases: Inversely, weak acids have strong conjugate bases.
📖 Examples solidify understanding: Practical examples illustrate each rule's application.

Garis waktu

00:00:00 - 00:05:00
The lesson focuses on ranking acids and bases, highlighting various structural features that affect acidity and basicity. It emphasizes the inverse relationship between acid strength and the stability of its conjugate base and introduces the concept of electron energy relevance to base stability.
00:05:00 - 00:10:00
The mnemonic 'REO' is introduced for comparing bases, especially when they have the same charge. The rank of bases considers factors like atom type, resonance, induction, and hybridization, emphasizing charge importance in these comparisons.
00:10:00 - 00:15:00
Charge impacts the energy of electrons, influencing base strength. Bases with negative charges have higher energy electrons, making them more reactive, whereas positive charges result in lower energy, less reactive bases. Charge is crucial but specific to conjugate acid-base comparisons.
00:15:00 - 00:20:00
The atom rule's significance is discussed, focusing on size and electronegativity on the periodic table to determine base strength. It's explained that base strength can be influenced by the element's position on the periodic table regarding size and electronegativity.
00:20:00 - 00:25:00
Resonance stabilizes bases and is critical in ranking them. More resonance structures generally mean a weaker base. The lesson also cautions against relying solely on the number of resonance structures without considering the quality of those structures relative to different atoms.
00:25:00 - 00:30:00
Inductive effects of electronegative atoms help stabilize bases. Proximity and type of electronegative atom affect stabilization strength; closer or more electronegative atoms result in a more stabilized, weaker base. Multiple electronegative atoms enhance stabilization further.
00:30:00 - 00:35:47
The role of orbitals in base strength through hybridization differences is highlighted. Bases with lone pairs in sp hybridized orbitals are more stable compared to sp2 or sp3 hybridization. This rule, though less frequent, is integral to understanding base comparisons in specific scenarios.

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Pertanyaan yang Sering Diajukan

What mnemonic is used to rank bases?
The mnemonic 'REO' is used to rank bases: Atom, Resonance, Induction, and Orbitals.
How does the charge affect acidity and basicity?
The charge affects acidity and basicity as a more negative charge generally means a stronger base, while a more positive charge is linked to a stronger acid.
What is the relationship between acids and their conjugate bases?
The stronger an acid, the weaker its conjugate base and vice versa.
Why is resonance important in determining acid and base strength?
Resonance stabilizes a base, making it weaker because it lowers the energy of the electrons.
How does atom size affect basicity?
Smaller atoms make stronger bases due to shorter, stronger bonds to H+.
What does induction mean in the context of acids and bases?
Induction refers to the effect of nearby electronegative atoms pulling electron density away, stabilizing a base.
How does hybridization affect base strength?
Lone pairs on sp hybridized atoms are more stable (weaker base) than on sp3 hybridized ones.
Can electronegativity influence acid strength?
Yes, less electronegative atoms have weaker bases, hence stronger conjugate acids.
What role do memorized pKa values play?
Memorized pKa values help confirm acid rankings when structural rule assessments might lead to confusion.
What is the significance of atomic positioning in relation to electronegative atoms?
The closer an electronegative atom is to the base, the more it stabilizes due to induction.

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Teks

Gulir Otomatis:

00:00:01
ranking acids and bases that'll be the
00:00:02
topic in this lesson now
00:00:04
we just had an introduction to acids and
00:00:05
bases in the last lesson and now we're
00:00:07
really going to learn
00:00:08
uh all the different structural features
00:00:10
that allow us to actually compare the
00:00:11
relative acidities between two acids
00:00:13
or the relative basicities between a
00:00:15
couple of bases as well
00:00:16
now this is my brand new organic
00:00:18
chemistry playlist i'll be releasing
00:00:20
these lessons weekly throughout the
00:00:21
2020-21 school year
00:00:22
so if you don't want to miss one of
00:00:24
these subscribe to the channel click the
00:00:25
bell notifications you'll be notified
00:00:27
every time i post a new video
00:00:34
so ranking acid bases i just want to
00:00:36
start off with a little reminder from
00:00:37
our introduction in the last lesson that
00:00:38
the stronger an acid the weaker its
00:00:41
conjugate base and vice versa so if you
00:00:43
have a stronger conjugate base you're
00:00:44
going to have a weaker
00:00:46
acid associated with that and so next
00:00:47
part of that is going to be that a more
00:00:49
stable base is a weaker base so that's
00:00:52
the next part and then
00:00:53
more stable base is usually going to
00:00:56
mean lower energy electrons on the base
00:00:58
now recall that
00:00:59
lewis definition of base was an electron
00:01:00
pair donor and usually lower energy
00:01:03
electrons mean more
00:01:04
stable electrons less reactive electrons
00:01:07
we'll see one major exception to that
00:01:08
but most of the time
00:01:09
that's kind of how we're going to rank
00:01:11
bases we're going to look at how low the
00:01:13
energy of the electrons are
00:01:14
and their relative stabilities
00:01:16
associated with that so lower energy
00:01:17
electrons
00:01:18
more stable base more stable base weaker
00:01:21
base
00:01:21
all right so we're going to use a little
00:01:24
mnemonic here and in this case i'm going
00:01:26
to use
00:01:26
reo so but the truth is we only use reo
00:01:29
if we're comparing
00:01:30
bases first off these are mnemonic for
00:01:32
comparing bases
00:01:34
but we only use it if we're comparing a
00:01:35
bunch of bases that all have the same
00:01:37
charge now and most of the time we'll be
00:01:40
using bases that have a negative charge
00:01:41
but this could apply to comparing bases
00:01:43
that are all neutral in charge and
00:01:44
things of a sort as well so
00:01:46
so i want to want to put charge at the
00:01:47
top of the list although it's not a
00:01:49
part of the the proper mnemonic now
00:01:50
other textbooks
00:01:52
uh and professors will often use a
00:01:54
version of this mnemonic that includes
00:01:55
charge
00:01:56
and they would call it cardio and
00:01:58
induction here instead of calling it
00:01:59
induction they might call it
00:02:00
dipole induction so they can put the d
00:02:02
and the i on that because cardio is just
00:02:04
a really cool one so
00:02:06
and it starts with charge so charge is
00:02:07
really important we'll get into that in
00:02:08
a little bit and
00:02:09
and then the atom actually acting as the
00:02:11
base and where it's located on the
00:02:12
periodic table that'll be important in
00:02:13
determining relative basicity
00:02:15
uh and then resonance we you know we've
00:02:17
learned about resonance stabilization
00:02:18
and how
00:02:19
that could stabilize the base and make
00:02:20
it a weaker base as well and induction
00:02:22
is one we haven't specifically talked
00:02:24
about yet
00:02:24
maybe you saw it in gen chem maybe it
00:02:26
didn't depends on your class so
00:02:28
but induction means do you have any
00:02:29
electronegative atoms nearby
00:02:31
so that might help stabilize a base it
00:02:33
turns out having electronegative atoms
00:02:34
are going to pull electrons towards them
00:02:36
and lower the electron density on that
00:02:38
base making it a more stable base less
00:02:40
negative sort of thing
00:02:41
uh we'll get more into that a little bit
00:02:42
and then finally orbitals and this last
00:02:44
one really
00:02:45
should you know be called hybridization
00:02:46
because it's really a difference in
00:02:47
hybridization of the orbitals of where
00:02:50
you find a lone pair of electrons but
00:02:51
that wouldn't make a good
00:02:53
mnemonic you know are or something like
00:02:55
that you know so reo they go with
00:02:57
orbitals but it's really a matter of
00:02:58
hybridization
00:02:59
but this little mnemonic is the one
00:03:01
we're going to use and you typically
00:03:02
follow it
00:03:03
in order and most the time again 99 of
00:03:06
the time
00:03:07
you're just going to be comparing a
00:03:08
bunch of bases that all have the same
00:03:10
charge and so this little charge rule
00:03:12
becomes irrelevant and that's why it
00:03:14
really just reduces down to reo but for
00:03:15
that
00:03:16
one time out of a hundred i put the
00:03:18
charge at the top of the list here
00:03:19
so let's take a look we're gonna take an
00:03:20
in-depth look at every single one of
00:03:22
these rules
00:03:23
so that you can rank acids and bases in
00:03:25
their relative acidity or relative
00:03:26
basicity
00:03:27
just based on looking at their
00:03:28
structures with no pkas provided
00:03:32
all right so charge here so it turns out
00:03:34
when you get a
00:03:35
negative charge that raises the energy
00:03:37
of the electrons and you have more
00:03:38
electrons around that oxygen nucleus
00:03:40
they can't get as close to the nucleus
00:03:41
because they're repelling each other and
00:03:42
so they spread out
00:03:43
and the further they get out from that
00:03:45
nucleus the higher their energy and the
00:03:47
greater the reactivity
00:03:48
as a base and so having a negative
00:03:51
charge causes your electron cloud to be
00:03:53
bigger the electrons are on average
00:03:54
further from the nucleus
00:03:55
and it leads to a stronger base now a
00:03:58
positive charge is going to do the exact
00:04:00
opposite if i was actually talking about
00:04:01
this guy is acting like a base here
00:04:03
that lone pair because now we have a
00:04:05
positive formal charge in the auction
00:04:06
with
00:04:07
fewer electrons around the nucleus which
00:04:08
means the remaining electrons can
00:04:10
actually get even closer to the nucleus
00:04:12
which lowers their energy and makes them
00:04:13
less reactive and so in this case if we
00:04:15
were going to look at a trend in
00:04:17
basicity
00:04:23
so it would be going to the left and if
00:04:25
we want to talk about the stability of
00:04:26
the electrons
00:04:27
the electrons would actually be getting
00:04:28
more stable as they went to the right
00:04:31
so that would relate to basicity here so
00:04:33
it turns out for charge though you could
00:04:35
also look at this in terms of
00:04:36
acidity in this one case and and notice
00:04:38
we've got hydronium over here hydroxide
00:04:41
over here we know hydronium
00:04:42
is a strong acid we know hydroxide's a
00:04:43
strong base and we got water right in
00:04:45
the middle which is neutral
00:04:46
and so you can look at this
00:04:53
like so cool so charge in this one
00:04:57
example here this set of examples is
00:04:59
you got to be careful on when it's
00:05:00
relevant because it's not going to
00:05:01
always be relevant now
00:05:03
as long as you're comparing a conjugate
00:05:04
base to a conjugate acid conjugate
00:05:06
conjugate base
00:05:07
that's when charge will work every time
00:05:09
as long as you're just comparing a
00:05:10
conjugate acid a conjugate base
00:05:12
yes whichever one of these is more
00:05:13
negative is the better base
00:05:15
whichever one of these is more positive
00:05:17
or less negative is the better acid
00:05:19
that's when it works now if you try to
00:05:21
compare just two totally different
00:05:23
molecules that have a different charge
00:05:25
maybe it'll work and maybe it won't and
00:05:27
it's not a comparison that comes up a
00:05:28
lot as a result because it's hard to ask
00:05:30
you about it
00:05:31
now if it involves one of the acids that
00:05:34
you're supposed to memorize the pkas for
00:05:35
one of those typical functional groups
00:05:36
that we went through
00:05:37
in the last lesson well the model means
00:05:39
that's fair game and you wouldn't even
00:05:40
need to use rules and you shouldn't use
00:05:42
rules in that case and i'll show you a
00:05:43
couple examples where that happens
00:05:45
so but i just want to point out that
00:05:48
charge is important and it's probably
00:05:50
the most important thing however
00:05:53
most of the time you're not going to be
00:05:54
comparing acids or bases that have any
00:05:57
difference in charge and so most of the
00:05:58
time
00:05:59
this will just be a moot point but i did
00:06:01
want to cover my bases and i definitely
00:06:02
want to relate it to the energies of
00:06:04
electrons as well and again negative
00:06:06
charge higher energy electrons positive
00:06:08
charge
00:06:08
lower energy electrons okay so now let's
00:06:11
get into that mnemonic proper so
00:06:13
once you verify that what you're
00:06:14
comparing has the same charge you next
00:06:16
kind of follow these
00:06:17
this mnemonic in order so the first rule
00:06:19
you'll consider is adam
00:06:21
and if you don't find a difference there
00:06:22
then you'd move on to resonance and if
00:06:23
you still don't find
00:06:24
the distinguishing difference there
00:06:25
you'd move on to induction and if you
00:06:27
still don't
00:06:28
find the you know a distinguishing
00:06:29
difference there you'd move on to
00:06:30
orbitals
00:06:31
so these are ranked kind of in like
00:06:33
order of priority so the atom rule is
00:06:35
the most important again after charge
00:06:36
so but then resonance then induction
00:06:38
then orbitals and again this is not a
00:06:40
perfect set of rules and there is no
00:06:41
perfect set of rules so but this will
00:06:43
work usually you know at least 90
00:06:45
percent of the time this will lead you
00:06:47
to the right conclusions as far as
00:06:49
relative acidity and basicity
00:06:50
and when these won't lead to the right
00:06:52
path that's why we memorized some pkas
00:06:54
earlier because that
00:06:56
would kind of take care of those
00:06:57
exceptions so before i ever use these
00:06:59
rules if i'm ranking acids i look at
00:07:00
pkas first
00:07:01
the ones i've memorized so but if i
00:07:03
don't know the appropriate pkas
00:07:04
then i go to my rules and rank my
00:07:07
relative acidity and basically so
00:07:09
this atom rule the way this works if we
00:07:11
look at relative basicity on the
00:07:12
periodic table and
00:07:13
i know you can't see the whole periodic
00:07:14
table but it just needed you to be able
00:07:15
to see the non-metals
00:07:17
that'll show up in our organic compounds
00:07:19
and it turns out basicity increases as
00:07:21
you get
00:07:22
smaller and as you get less
00:07:25
electronegative
00:07:26
so and there's two parts to this so the
00:07:28
vertical part of that trend
00:07:29
is all about size and it turns out a
00:07:31
smaller base makes a shorter bond
00:07:34
to an h plus ion and a shorter bond is a
00:07:36
stronger bond and so as a result the
00:07:38
smaller
00:07:39
base is the stronger base so and then
00:07:41
finally as you go from right to left
00:07:43
within a period now
00:07:44
if you're comparing elements within a
00:07:45
period it's not about size anymore
00:07:47
because they're all really close in size
00:07:49
and it turns out to be more about
00:07:50
electronegativity instead
00:07:53
and it turns out the less
00:07:54
electronegative base is the less
00:07:56
stable base and the stronger base so as
00:07:58
long as you're comparing in the same
00:08:00
group
00:08:00
your comparison is about
00:08:01
electronegativity as long as you're
00:08:03
comparing
00:08:03
and i said that backwards as long as
00:08:05
you're comparing in the same period it's
00:08:07
about electronegativity but if you're
00:08:08
comparing within the same group
00:08:09
it's about size so that's kind of the
00:08:12
deal here so you gotta remember which is
00:08:13
which
00:08:14
but they're both important but notice
00:08:15
size must be more important
00:08:17
because again if you're comparing them
00:08:19
in the same period
00:08:20
the more electronegative it is it's a
00:08:22
weaker base the less electronegative the
00:08:23
stronger base
00:08:24
but in the same group the more
00:08:26
electronegative is actually the stronger
00:08:29
base
00:08:29
that's how we know that size must be
00:08:31
more important consideration
00:08:32
so but again if you're comparing them in
00:08:34
the same group it's about size if it's
00:08:35
in the same
00:08:36
period it's about electronegativity all
00:08:39
right
00:08:40
incidentally so with electronegativity
00:08:42
you can totally correlate the energy of
00:08:44
the electrons
00:08:44
when you have like a negative charge
00:08:46
let's say they're just lone pairs on a
00:08:47
more electronegative atom
00:08:49
on the more electronegative atom it
00:08:50
pulls them closer to the nucleus which
00:08:51
lowers their energy and makes them more
00:08:53
stable in a weaker base
00:08:55
now in size this is the one place where
00:08:58
the energy of the electrons does not
00:08:59
correlate with
00:09:01
basicity so it turns out the larger atom
00:09:03
has higher energy electrons
00:09:04
but it's still the weaker base so
00:09:06
because it turns out the energy of the
00:09:08
electrons
00:09:08
is not the whole story but most of the
00:09:11
time it's an adequate story but this is
00:09:12
the one example where it's not
00:09:14
when you're comparing atoms here in the
00:09:16
same uh
00:09:17
group size is more important and instead
00:09:19
of just worrying about the energy of the
00:09:21
electrons it's also about the process of
00:09:22
acting as a base
00:09:23
of making a new bond to h plus and again
00:09:26
those big atoms they just make really
00:09:27
weak bonds to h
00:09:28
plus and so they're not good bases it
00:09:30
turns out and so as a result the energy
00:09:32
the electrons not a good reflection in
00:09:34
that one
00:09:35
comparison again comparing different
00:09:37
atoms
00:09:38
in the same group but outside of that
00:09:40
the energy the electrons and
00:09:42
and will correspond greatly with
00:09:44
basicity lower energy electrons
00:09:46
weaker base one exception there
00:09:50
all right so again bases get stronger as
00:09:52
we go up the periodic table
00:09:53
and as we move to the left so if we
00:09:56
compare some make some comparisons here
00:09:57
so
00:09:58
in this case we're going to first
00:10:00
compare these two and i just want you to
00:10:01
circle on your
00:10:02
study guide there which one is the
00:10:04
stronger base and in this case i can see
00:10:06
auction is smaller than sulfur they're
00:10:08
in the same group so it's all about size
00:10:10
and the smaller base is the stronger
00:10:12
base
00:10:13
so we're good to go there and in the
00:10:15
next pair again circle in the stronger
00:10:16
base now it's between oxygen and
00:10:18
nitrogen
00:10:19
so and now these are in the same period
00:10:21
so now it's about electronegativity
00:10:23
oxygen's more electronegative that makes
00:10:24
him the weaker base nitrogen is less
00:10:26
electronegative that makes him
00:10:27
the stronger base in this pair
00:10:32
cool now at the same breath i also want
00:10:33
to compare and rank some acids here
00:10:35
and the problem is is from reo here is
00:10:38
doesn't this
00:10:39
doesn't help you rank acids directly the
00:10:42
rules ario here are going to be for
00:10:43
ranking
00:10:44
bases but this is where the idea that
00:10:46
the stronger acid
00:10:47
has the weaker conjugate base and vice
00:10:49
versa comes in handy
00:10:50
because if you're asked to compare acids
00:10:52
and that's what i'm asking you over here
00:10:53
now in these pairs
00:10:55
what you're actually going to do is look
00:10:56
back and draw out their conjugate bases
00:10:58
if you haven't already notice
00:11:00
this is the conjugate base of this one
00:11:02
and this is the conjugate base of this
00:11:04
one
00:11:04
and so if you're comparing bases that's
00:11:06
what the rules for you don't want to
00:11:07
draw any conjugates out or anything like
00:11:08
that
00:11:09
but if you're comparing acids first
00:11:12
thing you'd ever do
00:11:13
is draw out the conjugate bases and you
00:11:14
look at them and be like okay who's
00:11:15
stronger over here
00:11:17
well he's stronger so if this one is the
00:11:19
stronger base well then he comes from
00:11:20
the weaker conjugate acid
00:11:22
and if this is the weaker base the
00:11:24
weaker more stable base
00:11:25
then he comes from the stronger
00:11:28
conjugate acid
00:11:29
and so we're not actually ranking the
00:11:30
acids directly we're only ranking them
00:11:33
in the context of having
00:11:34
already ranked their conjugate bases so
00:11:37
when you're asked to rank bases
00:11:38
good to go that's what the rules are for
00:11:40
but when you're asked to rank acids
00:11:41
you're going to do it indirectly every
00:11:42
time
00:11:43
so well almost every time so but you're
00:11:45
going to do it indirectly most of the
00:11:46
time
00:11:47
by looking at the conjugate bases
00:11:48
instead same thing here so here we
00:11:50
circle the stronger base
00:11:52
and if this is the stronger base it has
00:11:53
the weaker conjugate acid
00:11:56
so if this was the weaker base he has
00:11:57
the stronger
00:11:59
conjugate acid and so a lot of times
00:12:01
when we're ranking acids we just say
00:12:02
okay which one of these has the weaker
00:12:04
more stable conjugate base
00:12:06
in this case it was sulfur and in these
00:12:08
two if i said which one again has the
00:12:09
weaker
00:12:10
more stable conjugate base in this case
00:12:11
it was oxygen now up here it was the
00:12:13
weaker conjugate base because sulfur is
00:12:15
bigger than oxygen
00:12:16
and down here so the weaker more stable
00:12:19
conjugate base was oxygen because oxygen
00:12:21
is more electronegative
00:12:22
than nitrogen cool that is your
00:12:26
atom rule all right so after atom is the
00:12:29
resonance rule and again these rules for
00:12:32
ranking bases and resonance simply
00:12:34
stabilizes bases that's the way it works
00:12:36
so if you have something with resonance
00:12:37
and something without
00:12:38
well the thing with resin is generally
00:12:39
going to be more stable assuming you've
00:12:42
already done your atom rule so again
00:12:43
we would never start out with the
00:12:44
resonance rule we first verify that the
00:12:46
bases we're comparing all have the same
00:12:47
charge
00:12:48
then we verify that they're all the same
00:12:50
atom acting as the base and then we'd
00:12:52
move on to resonance so
00:12:53
if we look at the example here we're
00:12:54
going to compare these three bases and
00:12:56
i've drawn out all the resonance
00:12:58
structures here and so
00:12:59
in this case the first question would be
00:13:00
do they have the same charge well
00:13:01
negative charge negative charge negative
00:13:03
charge
00:13:03
done cool then we move on to the atom
00:13:05
rule and say which atom actually acts as
00:13:07
the base well it's oxygen here
00:13:09
it's oxygen here and it's oxygen here
00:13:12
and so we have a tie and so then we'd
00:13:15
have to move on and go to the resonance
00:13:17
rule so that's kind of how this works
00:13:19
so you wouldn't start with the resonance
00:13:20
rule unless you've already done the
00:13:21
charge in the atom rule first
00:13:23
all right so doing the resonance rule
00:13:24
now we see that here the negative charge
00:13:26
is
00:13:27
no resonance it's on a single oxygen
00:13:29
completely here
00:13:30
the negative charge due to resonance is
00:13:32
shared between two oxygen atoms that
00:13:33
makes it more
00:13:34
stable and a more stable base is a
00:13:36
weaker base this is a weaker base than
00:13:38
the first one that has no resonance
00:13:40
and then finally this last one the
00:13:41
negative charge is actually shared on
00:13:43
three oxygen atoms and is even more
00:13:46
stable and even more stable means even
00:13:49
weaker base
00:13:50
and so if we were to rank these as bases
00:13:52
with one being the strongest and
00:13:54
three being the weakest well our most
00:13:55
stable base is our
00:13:57
weakest base and let's do this in blue
00:14:01
so and then our strongest base that had
00:14:02
no resonance number one and then
00:14:04
obviously
00:14:04
this one that had two resonant
00:14:06
structures sharing on two auctions was
00:14:07
number two
00:14:09
now something you should know about
00:14:10
comparing resonance it's not just about
00:14:12
the total number of resonance structures
00:14:14
it's also about the quality of resonance
00:14:16
structures because if you notice like
00:14:17
here
00:14:18
i'm sharing the negative charge on two
00:14:20
oxygens and here it's on three options
00:14:22
well that's an easy comparison
00:14:23
but what if the negative charge was
00:14:24
shared like instead of on two auctions
00:14:26
what if it was shared on one auction and
00:14:28
three carbons well carbon's not as
00:14:30
electronegative as oxygen
00:14:31
it's not as good of a place to put a
00:14:33
negative charge
00:14:34
so you gotta ask yourself is it two
00:14:36
resonant structures sharing it on two
00:14:38
oxygens better
00:14:38
or is four resonant structures but it's
00:14:40
only one oxygen and three carbons better
00:14:43
well i don't expect you to know that
00:14:44
well it turns out sharing it on two
00:14:45
auctions
00:14:46
is better so but again that's one of
00:14:49
those
00:14:49
examples where you're just kind of until
00:14:51
you've seen it worked out for you but
00:14:52
you just don't know however that's
00:14:54
exactly the situation we have when it
00:14:56
compare
00:14:56
carboxylic acid to a phenol but if
00:14:59
you've memorized your pkas you'd know
00:15:01
that a carboxylic acid has a pka of 4-5
00:15:04
a phenol has a pka of 10 so the
00:15:06
carboxylic acid is stronger
00:15:08
and you wouldn't have to rely on the
00:15:09
rules for an example that's a little bit
00:15:11
challenging
00:15:12
because you've memorized some pkas so
00:15:14
just want to point
00:15:15
out where that might come in handy okay
00:15:18
so here's
00:15:19
we've ranked some bases now what if i
00:15:20
asked you to rank these
00:15:22
acids and so for these acids notice
00:15:24
these are just the conjugate
00:15:26
acids here and if you were asked to rank
00:15:28
these acids
00:15:29
first thing you'd want to do is draw the
00:15:31
conjugate bases if you didn't have them
00:15:33
already on the board like i do
00:15:34
so we've got them on the board now and
00:15:36
in this case you just know that the
00:15:38
acids have the exact
00:15:39
opposite ranking so the strongest acid
00:15:42
has the weakest conjugate base
00:15:44
well the weakest conjugate base was this
00:15:45
one it was the one that was most stable
00:15:47
due to resonance the weakest base so
00:15:49
then this must be the strongest
00:15:51
acid we'll make him number one
00:15:55
and it's just the exact opposite ranking
00:15:58
our strongest and least stable base
00:16:02
comes from the weakest conjugate acid
00:16:04
cool
00:16:06
so once again you can rank bases
00:16:08
directly that's what the
00:16:09
reo mnemonic and the rules are for but
00:16:12
acids we rank
00:16:13
indirectly by looking at the conjugate
00:16:15
bases and just knowing again the
00:16:16
stronger the acid the weaker the
00:16:18
conjugate base
00:16:18
and vice versa all right the next rule
00:16:21
in our reo mnemonic is
00:16:23
induction so an induction deals with
00:16:26
polarity
00:16:27
now for now i'm going to look at things
00:16:29
that are simply electronegative
00:16:30
we like to call them electron
00:16:32
withdrawing when they're electronegative
00:16:34
because they're pulling electrons away
00:16:35
and if you recall
00:16:36
we talked about charge at the very
00:16:38
beginning we said that something with a
00:16:39
more negative charge is a stronger base
00:16:41
whereas with a less negative charge i.e
00:16:43
more positive charge would be a
00:16:45
weaker base and so what an
00:16:46
electronegative atom does is it pulls
00:16:47
electron density away from a base
00:16:50
making the base less negative and
00:16:52
therefore more stable
00:16:54
lower energy electrons and a weaker base
00:16:57
cool we'll find out later on in the
00:16:58
course not in this chapter that
00:17:00
in addition to electron withdrawing
00:17:01
groups there are also things called
00:17:03
electron
00:17:03
donating groups and they're not going to
00:17:05
really have much relevance here
00:17:07
but they will have some relevance down
00:17:09
usually in the beginning a second
00:17:11
semester when you study alcohols and
00:17:12
phenols
00:17:15
all right so let's go through our list
00:17:17
and again we wouldn't ever start with
00:17:19
induction as a rule we just work our way
00:17:21
down
00:17:21
so first thing i want to look at in
00:17:23
comparing the first pair here i want you
00:17:24
to circle the stronger pair
00:17:26
or the stronger base in the pair is
00:17:28
charge well they're both negative one
00:17:29
then go to atom well oxygen is the base
00:17:33
in either case the atom with the
00:17:35
negative charge so no difference there
00:17:37
so then we move on to resonance well
00:17:38
these are both the conjugate bases of
00:17:40
carboxylic acids
00:17:41
they're both going to have two resonant
00:17:42
structures and share the negative charge
00:17:45
between two oxygens we still have no
00:17:47
distinguishing difference
00:17:49
and so now we move on to induction and
00:17:52
so
00:17:52
we've already identified that oxygen is
00:17:54
the base that's the atom rule don't get
00:17:56
that confused a lot of students confuse
00:17:57
the atom rule in the induction rule
00:17:59
my question now is now that we know
00:18:00
oxygen is the base in both cases
00:18:03
are there any electronegative atoms
00:18:05
nearby that can act as a helpful
00:18:07
neighbor to help stabilize the base
00:18:10
and over here we've got chlorine and
00:18:11
over here there's nobody electronegative
00:18:13
around those auctions at all so again
00:18:15
those options are sharing the negative
00:18:17
charge by resonance
00:18:18
they are the base and if you are the
00:18:20
base that's what the atom rule's about
00:18:22
and the atom rules about size and then
00:18:24
electronegativity
00:18:26
but induction here means you're not the
00:18:28
base you're just a helpful neighbor
00:18:30
and you're in the area to help that base
00:18:33
out
00:18:33
and it has nothing to do with size it
00:18:35
has everything to do with just pure
00:18:37
electronegativity
00:18:38
pure polarity if you will and so in this
00:18:41
case
00:18:41
the chlorine here is going to pull
00:18:43
electron density
00:18:44
so away from this carbon technically
00:18:48
which is then going to pull it away from
00:18:49
this carbon which then pulls away from
00:18:50
this carbon
00:18:51
which is then going to pull some away
00:18:52
from those oxygens eventually making
00:18:54
those
00:18:54
oxygens less negatively charged which
00:18:57
makes them more stable lower energy and
00:18:59
a weaker base so the one stabilized by
00:19:02
induction
00:19:03
is the weaker base okay so that's the
00:19:06
first part
00:19:07
so and again induction is all about
00:19:09
electronegativity so far as we're
00:19:11
concerned with in this lesson
00:19:13
all right in the next example here so
00:19:16
we're going to go through the rules
00:19:17
again charge no difference atom
00:19:19
oxygen in both cases no difference
00:19:21
resonance stabilized by resonance
00:19:23
between two auctions in both cases no
00:19:25
difference so we make it to induction
00:19:27
and we ask do either of these have a
00:19:29
helpful neighbor nearby
00:19:30
well yeah they both do and they have the
00:19:33
same neighbor
00:19:34
chlorine which is equal in
00:19:36
electronegativity so however
00:19:38
if you notice by the way the we pull
00:19:40
electrons
00:19:41
from the adjacent atom through the bonds
00:19:43
based on electronegativity
00:19:44
so if we were like a hundred carbons
00:19:47
long and then there was just a chlorine
00:19:48
way over here
00:19:49
do you think the other side of the
00:19:50
molecule would even know there's a
00:19:51
chlorine down there no
00:19:52
and so proximity is really important as
00:19:54
well typically the closer it is
00:19:56
the bigger the impact it's going to have
00:19:58
in terms of stabilization so
00:20:00
in this case the chlorine is closer to
00:20:02
these oxygens it's only one
00:20:04
two and then three bonds away whereas
00:20:06
this one is one two
00:20:07
three four bonds away from the oxygens
00:20:10
and so the closer it is the more
00:20:11
stabilizing of an impact
00:20:13
the greater its stabilization due to
00:20:15
induction
00:20:16
and so that makes this the more stable
00:20:18
base and the more is the weaker
00:20:20
base so the one that's less stabilized
00:20:22
is the stronger base
00:20:25
all right so now we've learned having an
00:20:27
electronegative atom
00:20:28
in the area is helpful and the closer it
00:20:31
is the more helpful it is
00:20:32
and so now what if we have two different
00:20:35
helpful neighbors
00:20:37
uh carrying out inductive stabilization
00:20:39
here so once again
00:20:40
charge no difference atom auction
00:20:43
auction no difference resonance shared
00:20:46
between two oxygen two resonant
00:20:47
structures on both no difference
00:20:49
so then we make it to induction and now
00:20:51
there is a difference now they're in the
00:20:52
same location they're the same distance
00:20:54
away from the oxygens from the bases
00:20:56
but one's chlorine and one's fluorine
00:20:58
and this is the most one of the most
00:20:59
common mistakes students make
00:21:00
is they say okay but chad you just told
00:21:03
me that fluorine was a better base than
00:21:04
chlorine
00:21:05
no no no i told you when fluorine is
00:21:08
the base is the atom acting as the base
00:21:11
that it's a better base
00:21:12
but again here fluorine and chlorine
00:21:14
they're not the base in either one of
00:21:15
these
00:21:16
the oxygens have the negative charge
00:21:18
they're the base
00:21:19
fluorine and chlorine are not acting as
00:21:21
the base at all but they can help
00:21:22
stabilize the oxygens who are acting as
00:21:24
the base
00:21:25
and again your ability to help stabilize
00:21:27
those auctions has
00:21:28
nothing to do with how big you are only
00:21:32
with how electronegative you are and
00:21:33
fluorine is more electronegative than
00:21:35
chlorine
00:21:36
and as fluorine is more electronegative
00:21:37
than chlorine this will be a more
00:21:39
stabilized base that negative charge
00:21:41
will be diminished more on these
00:21:42
auctions than on the auctions on the
00:21:44
molecule next door
00:21:45
and so as a result the more stable base
00:21:46
on the right here is the weaker base
00:21:49
more stabilized by induction so the one
00:21:51
on the left
00:21:52
is the stronger base cool can't
00:21:55
emphasize this enough though
00:21:57
so the difference between the atom rule
00:21:58
and induction the atom rule
00:22:00
is you're examining the atom that's
00:22:02
acting as the base
00:22:03
which in our case was oxygens in both
00:22:05
cases no difference
00:22:07
induction though is you're looking for a
00:22:09
helpful neighbor who's near to the base
00:22:12
and it's all about electronegativity so
00:22:14
the atom rule is about size first and
00:22:16
then electronegativity are you the base
00:22:18
then we're going to talk about your size
00:22:20
and then your electronegativity but if
00:22:21
we're talking about induction and you're
00:22:23
not the base you're just nearby you're
00:22:24
in the neighborhood
00:22:25
it's purely about your electronegativity
00:22:27
your size is irrelevant
00:22:31
all right finally last example here and
00:22:33
in this case same comparisons same
00:22:35
charge same atom same resonance
00:22:36
comes down to induction again and two
00:22:39
fluorines
00:22:40
is better than one so to speak so two
00:22:42
fluorines two electronegative atoms will
00:22:44
have even more of a stabilizing
00:22:45
impact than one flooring so this will be
00:22:48
the even more stable base and therefore
00:22:49
the weaker base
00:22:50
therefore this one is the stronger base
00:22:54
cool so now i've given you an example of
00:22:55
every kind of comparison you might see
00:22:57
and again we only compared two at a time
00:22:59
you might get three or four of these and
00:23:00
be asked to rank them in order well
00:23:02
everything that's important just came up
00:23:04
do you have
00:23:04
an electron you know withdrawing group
00:23:06
and one doesn't do you have
00:23:08
both the same electronic group but one's
00:23:09
closer and one's further or do you have
00:23:11
two different ones and one's more
00:23:12
electronegative one's less less or
00:23:13
negative
00:23:13
or do you have more electron withdrawing
00:23:15
groups versus another
00:23:17
those are the typical comparisons you're
00:23:18
likely to see okay
00:23:20
so i didn't want to clutter up the board
00:23:21
initially so but in addition to
00:23:22
comparing these lovely bases
00:23:24
now we're going to go compare their
00:23:26
conjugate acids
00:23:27
and again the key is again the rules are
00:23:29
for bases and we just jumped right into
00:23:31
comparing the bases
00:23:32
but if you had been asked to compare any
00:23:34
of these pairs of conjugate acids
00:23:35
first instead well the first thing to
00:23:37
realize is like oh they're asking me to
00:23:38
compare acids i don't compare acids
00:23:40
i would draw out their conjugate bases
00:23:42
and then compare those
00:23:43
and fortunately we can already take
00:23:44
advantage of the fact that we've done
00:23:46
that
00:23:47
and so we're already ready to compare
00:23:48
these because we've already compared the
00:23:49
conjugate bases
00:23:50
and so in this case in the first example
00:23:53
in these two conjugate acids i would
00:23:54
again draw out their conjugate bases
00:23:56
i'd realize that this one is the more
00:23:58
stable conjugate base the more stable
00:24:00
conjugate base is the weaker conjugate
00:24:01
base
00:24:02
and the more stable weaker conjugate
00:24:03
base comes from the
00:24:05
stronger conjugate acid so now we're
00:24:08
circling the stronger conjugate acid
00:24:09
same thing in the next example
00:24:11
we already figured out again in
00:24:12
comparing these two acids we already
00:24:14
figured out the relative
00:24:15
basicities of the conjugate bases so
00:24:18
this had the chlorine closer
00:24:19
which had a more stabilizing influence
00:24:21
by induction and the more stable base is
00:24:23
the weaker base and the weaker base
00:24:25
comes from the
00:24:26
stronger conjugate acid cool next
00:24:30
comparison again we're comparing these
00:24:31
two acids first thing you do is draw
00:24:33
these two conjugate bases and here
00:24:35
fluorine was more electronegative than
00:24:36
chlorine
00:24:37
and so fluorine is going to pull more
00:24:38
electrons away from the oxygens
00:24:40
making them less negative and more
00:24:42
stable and more stable means weaker base
00:24:43
and the weaker base again comes from the
00:24:46
stronger conjugate acid and finally in
00:24:50
the last example we found out that
00:24:51
two fluorines was more stabilizing than
00:24:53
one fluorine so this was the more stable
00:24:54
base
00:24:55
the more stable base comes once again
00:24:58
from the
00:24:58
stronger conjugate acid
00:25:02
cool so obviously very fortunate that we
00:25:05
already had the conjugate bases on the
00:25:07
board and we already had them ranked
00:25:09
but if we hadn't that would have been
00:25:10
the first thing we'd have to do
00:25:12
so the last rule in our mnemonic is
00:25:14
orbital so the o here and
00:25:16
once again you wouldn't just jump into
00:25:18
using this rule you'd first have to
00:25:19
verify that
00:25:20
there's no distinction made between the
00:25:21
rest of the rules and the comparison
00:25:22
that you're making
00:25:24
so and again it's orbitals here if the o
00:25:25
for orbitals but again it really refers
00:25:27
to a difference in the hybridization
00:25:29
of your basic atom so if we take a look
00:25:32
here at comparing these three
00:25:34
lovely bases here first thing we do is
00:25:36
say do they all have the same charge yep
00:25:38
negative one negative one negative one
00:25:40
okay no difference there then we move on
00:25:41
to talk about the basic atom and it's
00:25:43
carbon compared to carbon compared to
00:25:45
carbon
00:25:46
okay atom rule doesn't help us either so
00:25:48
we'd move on to resonance
00:25:49
and it turns out none of these are
00:25:51
stabilized by resonance so if you look
00:25:53
you might go chad wait a minute load
00:25:54
about like this guy
00:25:55
well again when your lone pair of
00:25:56
electrons is exactly one bond away from
00:25:58
the pi electrons ie like allilic or
00:26:01
benzylic
00:26:01
that's when you can expect resonance but
00:26:03
here it's not one bond away it's right
00:26:04
where the pi electrons are so no
00:26:07
resonance in any of these
00:26:08
so move on to induction are there any
00:26:10
electronegative atoms in the area to
00:26:11
help stabilize this
00:26:12
nope not in any of the cases so finally
00:26:15
we move on to orbitals and we say is
00:26:18
there a difference in the hybridization
00:26:19
of the basic atom in all these cases and
00:26:22
there is indeed a difference so this
00:26:23
carbon right here is only two electron
00:26:25
domains
00:26:25
it's bonded to one atom and has one lone
00:26:27
pair so
00:26:28
it's going to be sp hybridized so this
00:26:31
carbon right here
00:26:33
is bonded to two atoms a carbon and a
00:26:34
hydrogen maybe i'll draw that hydrogen
00:26:36
in to make this a little more obvious
00:26:38
so it bonded to two atoms one lone pair
00:26:40
three electron domains
00:26:42
sp2 hybridized and then finally this one
00:26:45
right here is bonded to three atoms a
00:26:46
carbon two hydrogens and it has a lone
00:26:48
pair four domains is going to be sp3
00:26:51
hybridized
00:26:53
so there is a difference in
00:26:54
hybridization and it turns out that's a
00:26:56
significant thing now
00:26:57
if you kind of look at the difference
00:26:58
between an sp and sp2 and sp3
00:27:00
so your sp is 50 s 50
00:27:04
p we'd say 50 s character and
00:27:07
probably looks like something like this
00:27:09
your sp2 is now only 33 percent s one
00:27:12
third s
00:27:13
two thirds p and resembling more like a
00:27:16
p orbital now only 33
00:27:18
s character it's a little longer and the
00:27:21
electrons are going to be a little
00:27:22
further from the nucleus at the node
00:27:23
there than they would be if they were
00:27:25
not sp hybrid orbital
00:27:26
so having a lone pair and an sp as
00:27:28
compared to having a lone pair in an sp2
00:27:30
they're going to be on average closer to
00:27:32
the nucleus in an sp hybrid than an sp2
00:27:35
and then finally in an sp3 hybrid
00:27:37
orbital
00:27:39
it's even elongated even further and so
00:27:42
the electrons on average are going to be
00:27:43
even further from the nucleus and be
00:27:45
even
00:27:45
higher energy and less stable in an sp3
00:27:48
and so the idea then is that your lone
00:27:50
pair is going to be most stable
00:27:52
on an sp hybridized carbon as compared
00:27:54
to an sp2 or an sp3
00:27:55
and if here it's most stable that makes
00:27:58
him the weakest
00:27:59
base which we'll put number three in
00:28:02
this case
00:28:03
and then here on the sp3 hybridized atom
00:28:05
that lone pair is going to be the least
00:28:07
stable which in our case is going to
00:28:08
mean the strongest space and then this
00:28:10
guy's in the middle
00:28:11
and so that's our trend for basicity
00:28:13
that's the orbital rule really a
00:28:14
difference in hybridization and so if
00:28:16
you were
00:28:17
asked to compare the conjugate acids
00:28:19
instead well again you'd realize that
00:28:21
i'm not comparing acids
00:28:22
and first thing you do is draw out all
00:28:23
the conjugate bases go through the rules
00:28:25
and come up with this relative basicity
00:28:28
which would then allow you to come up
00:28:29
with exactly the opposite trend
00:28:32
in acidity for the conjugate acids so
00:28:35
the most stable and weakest conjugate
00:28:37
base
00:28:37
has the strongest conjugate acid so on
00:28:41
and so forth this rule doesn't come up
00:28:44
very often
00:28:45
but it is something you're probably
00:28:46
going to see at least once or twice
00:28:48
between your homeworks and your tests
00:28:50
so now we've made it completely through
00:28:51
the mnemonic and we've kind of gone step
00:28:53
by step and
00:28:54
you know we kind of knew what
00:28:55
comparisons were coming because we were
00:28:56
going in order so now i want to work
00:28:58
three examples and with these three
00:29:01
examples we're going to do
00:29:01
specifically comparing acidity so and
00:29:04
we're going to keep in mind the pkas
00:29:06
we're supposed to know as well as all
00:29:07
these lovely rules
00:29:09
all right so we've got three different
00:29:10
comparisons here and the first one's
00:29:12
going to be
00:29:13
this lovely species versus this one and
00:29:15
first thing you should do is keep in
00:29:16
mind what pkas do you know well
00:29:18
this is a carboxylic acid and his pka
00:29:21
is in the four to five range usually
00:29:23
it's pretty typical
00:29:24
whereas this is an alcohol and its pka
00:29:27
is usually in the range of like 16.
00:29:29
now the truth is this one's got a
00:29:31
chlorine that's going to inductively
00:29:32
stabilize the conjugate base
00:29:34
which is going to ultimately make the
00:29:35
conjugate base weaker and this acid a
00:29:37
stronger acid
00:29:38
but 16 versus four to five it's not
00:29:40
going to make it down that low
00:29:42
not a chance and so in this case
00:29:44
technically you could just be like yeah
00:29:46
there's my stronger acid i circled it
00:29:48
life is good so
00:29:50
but if you didn't you know recall your
00:29:52
pkas or didn't categorize these
00:29:53
correctly again
00:29:54
you could always just draw their
00:29:55
conjugate bases and be like
00:29:59
and use your reo mnemonic here so in
00:30:02
this case
00:30:05
you go through and say do they have the
00:30:06
same charge yep negative one negative
00:30:08
one for comparing your bases
00:30:09
then you'd say okay adam rule auction's
00:30:12
the base and
00:30:12
auction's the base it's a tie adam rule
00:30:15
didn't help me so move on to the
00:30:16
resonance rule
00:30:17
and we'd see that the one on the left
00:30:18
has resonance and the one on the right
00:30:20
does not have resonance
00:30:22
and so the one with resonance is more
00:30:24
stable and a more stable base is a
00:30:25
weaker base and a weaker base
00:30:26
comes from the stronger conjugate acid
00:30:29
the rules would have led us to the same
00:30:30
place
00:30:32
cool moving on to the next one here and
00:30:33
this is one i brought up earlier so
00:30:35
we've got a carboxylic acid here and
00:30:37
again
00:30:37
pka of a typical carboxylic acid is in
00:30:39
the four to five range and
00:30:41
this is a phenol and pka of a typical
00:30:42
phenol is in the 10 range
00:30:44
and so we can say yep we've got our
00:30:47
stronger acid right here because we
00:30:49
memorized some pkas
00:30:50
now this is the tricky one had you not
00:30:53
memorized those pkas
00:30:55
you'd had a tough time with this one
00:30:56
until you came across it somewhere along
00:30:58
the way
00:30:59
and we've seen a carboxylic acid it's
00:31:01
got two resonant structures shares the
00:31:04
negative charge
00:31:07
on two oxygen atoms here but the phenol
00:31:10
we haven't actually drawn out
00:31:11
specifically but the conjugate base
00:31:16
actually shares the negative charge
00:31:18
between the auction and then it turns
00:31:20
out
00:31:20
three of the carbon atoms in the ring
00:31:28
i'm going to draw these out but i'm not
00:31:30
going to actually show you how to get
00:31:31
there
00:31:32
i'll leave that on you for now
00:31:38
and finally one more
00:31:49
you know what i'm going to be a little
00:31:50
nicer than i said i was going to be so
00:31:52
and here lone pair comes down low pair
00:31:57
comes out onto that atom
00:31:59
here lone pair goes into form a pi bond
00:32:00
pi bond becomes a lone pair getting you
00:32:02
here
00:32:03
here lone pair becomes a pi bond pi bond
00:32:05
becomes a lone pair
00:32:06
getting you here all right so the
00:32:09
question got to ask yourself a lot of
00:32:10
students get this one wrong because here
00:32:11
they're all more resonant structures
00:32:13
more resonance more stable base more
00:32:14
stable based weaker base weaker base
00:32:15
comes from the stronger acid but we saw
00:32:17
that wasn't the case at all
00:32:18
but we're not comparing apples to apples
00:32:20
here so you can't just compare the
00:32:21
absolute number of resonance structures
00:32:23
unless the negative charge is just being
00:32:24
shared on the same atom every time well
00:32:26
that would
00:32:26
that would work so but here sharing the
00:32:28
negative charge on two options
00:32:30
as compared to sharing the negative
00:32:31
charge on one action and
00:32:33
three carbons and there's no intuitive
00:32:35
way to just be like well i have a hunch
00:32:37
so it really does come down to either
00:32:38
having seen this example before
00:32:40
or just knowing those pkas super helpful
00:32:44
in this case it turns out sharing it on
00:32:45
two auctions was more stable that was
00:32:46
the weaker base
00:32:47
which came from our stronger acid all
00:32:50
right finally this last one
00:32:52
this last one is a super tricky example
00:32:54
but you might remember pka's here and
00:32:55
pka of a terminal alkyne here
00:32:57
was right around 26. so
00:33:01
and we didn't cover this guy's pka
00:33:03
explicitly but we did cover a very
00:33:04
similar molecule
00:33:06
and that similar molecule was ammonia
00:33:08
and we said ammonius pka
00:33:10
was right around 38. and so whether you
00:33:13
deprotonate neutral nitrogen ammonia or
00:33:15
a neutral nitrogen on
00:33:17
an amine it's going to be around a pka
00:33:19
of 38
00:33:20
and i can see that my lower pka is
00:33:22
definitely this guy
00:33:23
and he's going to be our stronger acid
00:33:27
cool and i brought this one up on
00:33:28
purpose because this is a tricky example
00:33:30
if you try to follow the mnemonic here
00:33:32
you're going to get hosed
00:33:33
and let's see why so if we draw those
00:33:36
conjugate bases
00:33:46
so said at the beginning of this lesson
00:33:48
that there's no perfect set of rules
00:33:50
because there are always exceptions like
00:33:51
this
00:33:52
one so if we look here and we start with
00:33:54
charge like negative one negative one
00:33:55
life is good so we move on to atom we're
00:33:57
like carbon versus
00:33:59
nitrogen well carbon and nitrogen are in
00:34:01
the same
00:34:02
period which means it's going to be
00:34:04
about electronegativity
00:34:05
and as nitrogen is more electronegative
00:34:07
we'd think oh more electronegative
00:34:09
more stable more stable base weaker base
00:34:11
weaker base comes from the stronger acid
00:34:13
and we totally got the opposite
00:34:15
determination from our pka values that
00:34:17
we knew
00:34:18
so what's going on here so well it turns
00:34:20
out this is usually the order of
00:34:22
priority as far as it
00:34:24
you know what distinguishes two bases
00:34:26
and comparing them
00:34:27
and order importance so however so
00:34:31
there's two differences here it is a
00:34:32
difference in atom but it's also a
00:34:33
difference in hybridization of the
00:34:35
orbitals
00:34:36
so it's not just comparing carbon to
00:34:37
nitrogen and that's the only difference
00:34:39
it's an sp hybridized carbon versus an
00:34:41
sp3 hybridized nitrogen
00:34:44
and it just so turns out that the sp
00:34:47
so is more stable the sp carbon is more
00:34:50
stable than the sp3 nitrogen
00:34:53
now there's no intuitive way to have
00:34:54
known that but because we knew the pkas
00:34:57
we didn't get hosed by this exception
00:34:59
from our rules
00:35:00
so like i said there's usually two or
00:35:02
three comparisons you can make
00:35:03
where the rules are going to lead you
00:35:05
astray but if you know all your pkas
00:35:07
those can't get you cool this is asked
00:35:10
in bases and ranking them
00:35:12
and again understanding relative acidity
00:35:14
and basically basicity is really going
00:35:16
to be key
00:35:16
to understanding relative reactivity
00:35:19
when we start talking about chemical
00:35:20
reactions later i can't stress
00:35:22
how important this is for getting a good
00:35:24
working understanding of how
00:35:26
the world works from an organic
00:35:28
chemistry perspective but how chemical
00:35:29
reactions work
00:35:32
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