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I am going to start now by
telling you a little bit more

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about Lewis theory.
Last time we went through

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aspects of the cube theory of
Lewis and showed how that could

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account for single bonds and
double bonds,

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but not triple bonds.
And that when that was

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superseded by the electron pair
theory, the idea being that

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these electron pairs would,
in fact, be oriented at the

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vertices of a tetrahedron.
And I showed you that

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tetrahedron, as described in a
cube last time,

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that you could then account for
triple bonds.

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And, if you looked at the
notes, you also saw that certain

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aspects of the dynamic behavior
of certain bond types,

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rotation around single bonds,
hindered restricted rotation

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around double bonds,
and so forth.

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Those were accounted for by
this electron pair theory in

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which the four pairs of
electrons were oriented at these

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vertices of a tetrahedron.
So, that was another quite

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important triumph of that part
of Lewis's theory.

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And the problem was,
although that electron pair

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theory initially put forward by
Lewis was so successful at

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accounting for the properties of
many different kinds of

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molecules and was a good
description of their electronic

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structure, there was one class
of molecules,

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00:02:29 --> 00:02:33
a very important class of
molecules, namely those known as

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aromatic.

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And the most important member
of the class of aromatic

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compounds is,
in fact, the benzene molecule.

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Benzene has the formula C six H
six.

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And we can easily calculate the
number of valence electrons in

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benzene if we just say 4 for
carbon times 6,

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00:03:02 --> 00:03:07
plus 1 times 6 for the number
of valance electrons from each

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00:03:07 --> 00:03:13
of the six hydrogens.
And that is 30 electrons.

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00:03:13 --> 00:03:18
So, in trying to understand how
these 30 electrons in a valence

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shell of benzene hold this
molecule together,

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it is known to be a planar
molecule, how do those electrons

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not only hold it together,
--

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00:03:30 --> 00:03:36
-- but how do they account for
the structure of benzene,

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00:03:36 --> 00:03:42
and how do they account for its
amazing stability?

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00:03:42 --> 00:03:48
And this picture that I have
drawn here is a representation

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00:03:48 --> 00:03:54
of benzene, I will explain to
you in a moment,

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00:03:54 --> 00:03:59
but it comes from a person by
the name of Ernest C.

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Crocker.

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Ernest C.
Crocker was an MIT Bachelor of

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Science degree holder,
who earned that degree in 1917.

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He was an MIT undergraduate
like you.

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And he published a paper.
Let me write down the reference

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for that paper.

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00:04:43 --> 00:04:55
This is the Journal of the
American Chemical Society,

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1922, Volume 44,
Page 1618.

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That paper was the first paper
in which the Lewis electron pair

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theory was applied to an
understanding of the electronic

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structure of aromatic molecules,
in particular,

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benzene.
And the amazing thing about

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that paper is --
Well, there are many amazing

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things, but one of the amazing
things about that paper is that

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there is only one author on that
paper, Ernest C.

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Crocker.
And Ernest, who was a very

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bright individual,
nonetheless did not go on in

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graduate school to earn a Ph.D.
in chemistry,

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had many various
chemistry-related interests.

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He explained the chemistry of
many different kinds of

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fragrances and odors.
I think he was referred to as

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"the man with the million dollar
nose."

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So, this person had quite a
variety of interests.

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He worked in what was then the
Applied Chemistry Laboratory at

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MIT after his graduation with
his Bachelor of Science degree.

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And one of the things he was
thinking about was how to use

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modern electronic structure
descriptions,

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such as Lewis theory,
to explain molecules like

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benzene.
And so, if you go and read this

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paper, you are going to find a
very lucid discussion of how the

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00:06:28 --> 00:06:33
Lewis electron pair theory could
represent benzene according to

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00:06:33 --> 00:06:37
this formula that I have drawn
here.

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00:06:37 --> 00:06:41
And in this formula,
there is considered to be an

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electron pair between each
carbon and each of the hydrogen

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nuclei.
There is an electron pair along

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each carbon-carbon axis,
as shown here,

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and here, and so on,
all the way around the ring.

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And then, finally,
you have six more electrons to

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come up to the number of 30,
which is the number of

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electrons in benzene.
And so, the question really is

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what to do with this remaining
six electrons.

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00:07:15 --> 00:07:20
And I have shown them around
the outside of the ring,

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here, which is where Ernest
arranged them in his work.

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00:07:24 --> 00:07:29
And I just want to point out
that he put forward the idea

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that these six electrons were
circulating around the plane of

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the ring and involved a net
one-half bond between each pair

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00:07:39 --> 00:07:45
of adjacent carbon atoms in the
benzene ring.

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00:07:45 --> 00:07:48
So, in effect,
when we draw benzene this way,

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00:07:48 --> 00:07:52
with a circle in the middle,
we know that that circle

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00:07:52 --> 00:07:57
represents the circulating six
electrons in what we are going

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00:07:57 --> 00:08:01
to call the pi system.
But Ernest C.

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00:08:01 --> 00:08:05
Crocker was the man who put the
circle into the middle of

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00:08:05 --> 00:08:08
benzene.
And he was an MIT undergrad.

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00:08:08 --> 00:08:11
And this was a sole-authored
paper.

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00:08:11 --> 00:08:14
Aromaticity has a vast history
in chemistry,

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00:08:14 --> 00:08:19
and it is still a very active
and unfolding history because of

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00:08:19 --> 00:08:22
the problems,
to our understanding,

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00:08:22 --> 00:08:27
posed by electrons that seem to
be circulating around a whole

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molecule rather than localized
between pairs of nuclei.

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00:08:33 --> 00:08:36
So, Ernest Crocker,
Bachelor of Science,

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1917, MIT, had a huge hand in
that.

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00:08:39 --> 00:08:42
I thought you might find that
interesting.

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00:08:42 --> 00:08:46
And, having looked at benzene
rings like that,

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I will now draw them perhaps
another way that is also useful,

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which comes from Kekulé.

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00:08:54 --> 00:09:08


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00:09:08 --> 00:09:10
Because I want to continue our
discussion of Lewis acid-base

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theory.
And I am going to draw two

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molecules here that are going to
be pretty similar.

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00:09:35 --> 00:09:39
When I write a molecule,
as I have done here on the

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left, I haven't explicitly
indicated each of the hydrogens

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that are present on the
periphery of this substituted

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00:09:48 --> 00:09:52
benzene ring.
But you should understand from

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a formula like this that this is
the molecule boron C 18 H 15.

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And over here I am drawing

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explicitly, at each of these
peripheral positions on the

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00:10:08 --> 00:10:14
substituted benzene rings,
fluorine atoms in place of the

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hydrogens.
So, this is a different

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00:10:17.609 --> 15.
molecule with formula B C 18 F

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15. --> 00:10:22


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00:10:22 --> 00:10:26
These are both Lewis acids.

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00:10:26 --> 00:10:33


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00:10:33 --> 00:10:38
And based on our discussion,
last time, if you were to add

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00:10:38 --> 00:10:43
ammonia to one of these
molecules, where would the lone

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00:10:43 --> 00:10:46
pair of electrons on the ammonia
bind?

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00:10:46 --> 00:10:49
The boron.
So, yes, you have here a

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00:10:49 --> 00:10:54
trigonal planer boron center.
And, if you were to add an

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ammonia molecule,
the lone pair of electrons

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00:10:58 --> 00:11:03
would come in and stick to the
boron --

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00:11:03 --> 00:11:07
-- because the boron is
electron deficient.

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00:11:07 --> 00:11:11
Just like the aluminum we
discussed last time,

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00:11:11 --> 00:11:17
it has only six electrons
around it, and it wants eight.

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00:11:17 --> 00:11:23
But what if we were to add one
ammonia molecule to a flask

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00:11:23 --> 00:11:28
containing both of those Lewis
acids that would then be

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competing for the ammonia
molecule?

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00:11:33 --> 00:11:38


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00:11:38 --> 00:11:42
What I am asking you to do is
something I will ask you to do

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00:11:42 --> 00:11:46
throughout this semester,
and that is to analyze a

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00:11:46 --> 00:11:50
molecule's properties based on
its structure and its

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composition.

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00:11:52 --> 00:12:10


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Exactly.
She said that it would

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preferentially stick to the one
with the fluorines because these

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very electronegative fluorines
are drawing electron density

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00:12:21 --> 00:12:25
away from the boron.
This is one of our most

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electronegative elements.

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00:12:28 --> 00:12:43


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00:12:43 --> 00:12:46
So you have a whole bunch of
fluorines in that molecule.

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00:12:46 --> 00:12:49
The whole thing,
we call it perfluorinated.

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00:12:49 --> 00:12:52
It is a perfluorinated triaryl
boron reagent.

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00:12:52 --> 00:12:55
These, in fact,
are great Lewis acids,

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00:12:55 --> 00:13:00
really powerful Lewis acids.
And they are modern Lewis acids

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00:13:00 --> 00:13:05
whose implementation in chemical
processes has come about really

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in the last 10,
15 years.

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00:13:07 --> 00:13:11
And, in fact we were talking a
little bit about Professor

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Schrock last time.
In some of his research,

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00:13:14 --> 00:13:19
he has used that perfluorinated
Lewis acid as an activator in

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catalysis to get catalytic
polymerization reactions to

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work.
And that is a very popular

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approach these days in Lewis
acid chemistry.

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00:13:30 --> 00:13:33
The design of new kinds of
Lewis acids with interesting

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molecular architectures is
something that is very much a

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current topic of interest in
research in chemistry,

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00:13:41 --> 00:13:45
because you can make lots of
chemical processes happen when

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you have something that can tug
on electron pairs.

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And this one tugs a lot harder
than that one because this one

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has very electronegative
fluorines to pull electron

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00:13:56 --> 00:14:00
density away from that boron and
to adjust the distribution of

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00:14:00 --> 00:14:04
the electron density in the
molecule.

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00:14:04 --> 00:14:07
And we will be talking more
about electron density and

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00:14:07 --> 00:14:11
distribution in a few minutes in
connection with what I am going

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to show you now.
And that is --

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00:14:14 --> 00:14:22


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That has to do with this
molecule, which is the SO two

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molecule.
Anyone know where SO two

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comes from in nature?
Or in the environment,

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00:14:37 --> 00:14:39
I should say?

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00:14:39 --> 00:14:43


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00:14:43 --> 00:14:46
Volcanoes, absolutely.
And also coal-burning power

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plants.
Coal is a very dirty fuel,

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00:14:48 --> 00:14:51
and it contains a lot of
sulfur.

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00:14:51 --> 00:14:55
And, when you burn that coal
without controlling the way you

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burn it, you emit SO two
into the atmosphere.

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00:15:00 --> 00:15:05
So, that can be a big problem.
And we will try to understand

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00:15:05 --> 00:15:07
why.
One of the things that SO two

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00:15:07 --> 00:15:13
can do when it gets into
the atmosphere is it can react

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00:15:13 --> 00:15:17
with dioxygen.
And that can give you --

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00:15:17 --> 00:15:23


199
00:15:23 --> 00:15:25
-- SO three.

200
00:15:25 --> 00:15:30


201
00:15:30 --> 00:15:35
And if SO two and SO
three are present in the

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atmosphere and if there is also
water present in the atmosphere,

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00:15:41 --> 00:15:46
acid rain, that is exactly the
type of process that we will be

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talking about here.
SO two and SO three

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00:15:50 --> 00:15:56
are examples of what we
call anhydrides.

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00:15:56 --> 00:16:00


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00:16:00 --> 00:16:03
Anhydride is a word that means
without water.

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00:16:03 --> 00:16:08
And so you should not be
surprised that SO two

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00:16:08 --> 00:16:12
can react with water.
And when it reacts with water,

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00:16:12 --> 00:16:15
it takes up water.
And the product of that

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00:16:15 --> 00:16:19
reaction will look like this.

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00:16:19 --> 00:16:37


213
00:16:37 --> 00:16:40
I draw it like that.
Okay.

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00:16:40 --> 00:16:48
So, H two O plus SO two going
to H two SO three.

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00:16:48 --> 00:16:56
The name of this
molecule is

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00:16:56 --> 00:17:01
sulfurous acid.
And, alternatively,

217
00:17:01 --> 00:17:05
when SO three reacts
with water --

218
00:17:05 --> 00:17:17


219
00:17:17 --> 00:17:22
I have one extra electron pair
that should not have been there.

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00:17:22 --> 00:17:25
So, both SO two and SO three,

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00:17:25 --> 00:17:30
as they react with water,
are going from a situation in

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00:17:30 --> 00:17:34
which they are electron
deficient to a situation in

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00:17:34 --> 00:17:38
which the sulfur attains an
octet.

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00:17:38 --> 00:17:44


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00:17:44 --> 00:17:46
Okay?
And you should verify that the

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00:17:46 --> 00:17:49
number of electrons that I have
drawn up here actually is

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00:17:49 --> 00:17:53
consistent with the elements
that I am using with the stated

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00:17:53 --> 00:17:56
charge that I am using.
But when SO three

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reacts with H two O to
give H two SO four,

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00:17:59 --> 00:18:02
we have now got sulfuric acid.

231
00:18:02 --> 00:18:08


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00:18:08 --> 00:18:11
Okay.
And, as I did over there,

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00:18:11 --> 00:18:17
I have two acids that I want
here now to compare in terms of

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00:18:17 --> 00:18:22
their relative strengths.
Here I have a Lewis acid SO

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00:18:22 --> 00:18:28
three and a Lewis acid
SO two engaging in a

236
00:18:28 --> 00:18:34
hydration reaction,
which produces sulfurous acid

237
00:18:34 --> 00:18:39
and sulfuric acid.
And the type of acids that

238
00:18:39 --> 00:18:42
these are on the bottom is
Bronsted acids,

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00:18:42 --> 00:18:43
--

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00:18:43 --> 00:18:48


241
00:18:48 --> 00:18:53
-- distinguished from Lewis
acids in that the way that they

242
00:18:53 --> 00:18:58
behave as acids is through
ionization that produces a

243
00:18:58 --> 00:19:02
proton.
They are also Lewis acids

244
00:19:02 --> 00:19:06
because the Lewis definition of
acidity is far more general,

245
00:19:06 --> 00:19:11
in saying that acids are simply
entities that can accept a pair

246
00:19:11 --> 00:19:15
of electrons.
Protons can accept a pair of

247
00:19:15 --> 00:19:17
electrons, so they are Lewis
acids.

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00:19:17 --> 00:19:21
But if you are talking about
Bronsted acids,

249
00:19:21 --> 00:19:26
you are talking exclusively
about protons that are produced

250
00:19:26 --> 00:19:31
by ionization of some kind of a
Bronsted acid.

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00:19:31 --> 00:19:38
Which one of these is the
stronger acid?

252
00:19:38 --> 00:19:44
Sulfuric acid.
And why?

253
00:19:44 --> 00:19:50


254
00:19:50 --> 00:19:53
Yes, down here.
You got the other one right.

255
00:19:53 --> 00:20:00


256
00:20:00 --> 00:20:02
Exactly.
When this ionizes,

257
00:20:02 --> 00:20:05
you get SO four minus.

258
00:20:05 --> 00:20:10
I will draw is a slightly
different way that is quicker.

259
00:20:10 --> 00:20:13
You get HSO four minus.

260
00:20:13 --> 00:20:19
There is your ionization.
And the idea now is that this O

261
00:20:19 --> 00:20:23
minus that you have,
the negative charge can

262
00:20:23 --> 00:20:27
actually be shared among a
greater number of

263
00:20:27 --> 00:20:31
electronegative oxygens here,
namely four,

264
00:20:31 --> 00:20:36
as compared to here,
where we have only three

265
00:20:36 --> 00:20:42
electronegative oxygens.
It is a consideration of the

266
00:20:42 --> 00:20:47
very electronegative elements in
your molecule that will help you

267
00:20:47 --> 00:20:51
understand the properties that
these molecules will have in

268
00:20:51 --> 00:20:55
terms of acid-base chemistry.
Now, how many electrons do we

269
00:20:55 --> 00:21:00
have in the valance shell of SO
three?

270
00:21:00 --> 24.


271
24. --> 00:21:03
And, that being the case,

272
00:21:03 --> 00:21:10
what molecule from last time
does that remind you of?

273
00:21:10 --> 00:21:20


274
00:21:20 --> 00:21:27
Maybe seeing a picture of it
will help refresh your memory.

275
00:21:27 --> 00:21:30
AlCl three.
Exactly.

276
00:21:30 --> 00:21:34
I am going to show it to you
anyway.

277
00:21:34 --> 00:21:40
And this is going to be faster
than the last time,

278
00:21:40 --> 00:21:45
if I set this up right.
Just to remind you.

279
00:21:45 --> 00:21:52
And if we could have the lights
down just a little bit for a

280
00:21:52 --> 00:21:54
moment.

281
00:21:54 --> 00:22:03


282
00:22:03 --> 00:22:06
I want to refresh your memory
of the electron density

283
00:22:06 --> 00:22:09
distribution here in AlCl
three.

284
00:22:09 --> 00:22:12
This is an electron density
isosurface of AlCl three.

285
00:22:12 --> 00:22:14
And what you are noticing is

286
00:22:14 --> 00:22:18
that the electron density drops
to a low value in between the

287
00:22:18 --> 00:22:21
central aluminum and the
radially disposed chlorides,

288
00:22:21 --> 00:22:25
the three chlorides that
surround that central aluminum

289
00:22:25 --> 00:22:27
ion.
And the coloring in this is

290
00:22:27 --> 00:22:31
such that the blue regions
represent regions in space where

291
00:22:31 --> 00:22:36
there is a high probability of
finding paired electrons.

292
00:22:36 --> 00:22:40
So, basically you have three Cl
minus's that are packed tightly

293
00:22:40 --> 00:22:42
around an Al three plus.

294
00:22:42 --> 00:22:46
This is a very ionic compound.
See the empty region in space

295
00:22:46 --> 00:22:50
between aluminum and chloride,
and the polarization of that

296
00:22:50 --> 00:22:54
otherwise spherical cloud of
electrons around the chloride in

297
00:22:54 --> 00:22:57
the direction of that positively
charged aluminum?

298
00:22:57 --> 00:23:02
There is your electron density
distribution for that.

299
00:23:02 --> 00:23:06
And now I want you to keep that
in mind, and we will compare the

300
00:23:06 --> 00:23:10
isoelectronic SO three
molecule to it.

301
00:23:10 --> 00:23:21


302
00:23:21 --> 00:23:24
SO three.
Here is another case where we

303
00:23:24 --> 00:23:27
have 3 times 8,
24 valance electrons in a

304
00:23:27 --> 00:23:29
system.
But the character of this

305
00:23:29 --> 00:23:33
molecule in terms of electron
density distribution is very

306
00:23:33 --> 00:23:37
different.
While on the blackboard,

307
00:23:37 --> 00:23:41
I am not able to really tell
you very much about the

308
00:23:41 --> 00:23:45
difference between SO three
and AlCl three,

309
00:23:45 --> 00:23:50
here I think you can
see that indeed they are quite

310
00:23:50 --> 00:23:53
different.
This is an electron density

311
00:23:53 --> 00:23:58
isosurface at the same contour
level as what we were looking at

312
00:23:58 --> 00:24:03
for AlCl three.
Now, to explain this and to

313
00:24:03 --> 00:24:08
understand just what is going on
here, you need to remember that

314
00:24:08 --> 00:24:12
the electronegativity difference
between the central sulfur and

315
00:24:12 --> 00:24:16
the peripheral oxygens is not
very great compared to the

316
00:24:16 --> 00:24:19
electronegativity difference
between aluminum,

317
00:24:19 --> 00:24:23
which is a very electropositive
and metallic element,

318
00:24:23 --> 00:24:26
and chlorine,
which is a very electronegative

319
00:24:26 --> 00:24:30
halogen.
And so, what that results in,

320
00:24:30 --> 00:24:33
as shown here,
is a much more equal sharing of

321
00:24:33 --> 00:24:36
the electrons between that
central sulfur and these

322
00:24:36 --> 00:24:40
peripheral oxygens.
So, even though these things

323
00:24:40 --> 00:24:44
are both Lewis acids and they
both have 24 valance electrons,

324
00:24:44 --> 00:24:46
the electron density
distribution in

325
00:24:46 --> 00:24:51
three-dimensional space for
these molecules and the covalent

326
00:24:51 --> 00:24:55
verses ionic character of these
molecules, is really quite

327
00:24:55 --> 00:24:57
different.

328
00:24:57 --> 00:25:05


329
00:25:05 --> 00:25:09
Our section that is going to be
devoted to bonding has not

330
00:25:09 --> 00:25:13
really kicked into gear yet,
but the nice thing is that

331
00:25:13 --> 00:25:16
Lewis theory applies both to
acid-base chemistry and to

332
00:25:16 --> 00:25:20
bonding, so we are able to talk
a little bit about that.

333
00:25:20 --> 00:25:24
In a few moments,
I will tell you a little more

334
00:25:24 --> 00:25:27
about an issue that is very
important in chemistry as

335
00:25:27 --> 00:25:30
regards bonding.

336
00:25:30 --> 00:25:36


337
00:25:36 --> 00:25:42
And it has to do with what
happens when acids,

338
00:25:42 --> 00:25:46
like sulfuric acid,
ionize in water.

339
00:25:46 --> 00:25:55
When Bronsted acids ionize in
water, we get this ion produced.

340
00:25:55 --> 00:26:03
H three O plus,
which is the hydronium ion.

341
00:26:03 --> 00:26:10


342
00:26:10 --> 00:26:13
That is to say that if you
ionize in water some Bronsted

343
00:26:13 --> 00:26:18
acid, the protons that are
produced through that ionization

344
00:26:18 --> 00:26:22
are not floating around freely,
naturally, because they are

345
00:26:22 --> 00:26:26
positively charged and they are
attracted to negatively charged

346
00:26:26 --> 00:26:29
electrons.
So, they look around in

347
00:26:29 --> 00:26:33
solution and find the next
source of an electron that they

348
00:26:33 --> 00:26:35
can.
And you know that if we draw

349
00:26:35 --> 00:26:39
out a molecule like water,
according to the Lewis dot

350
00:26:39 --> 00:26:42
structure, it has two extra
pairs of electrons,

351
00:26:42 --> 00:26:46
in addition to those two pairs
of electrons it is using in

352
00:26:46 --> 00:26:50
making bonds to the two
hydrogens that are on the oxygen

353
00:26:50 --> 00:26:54
of the water molecule.
So, H plus is not just

354
00:26:54 --> 00:26:58
isolated around by itself in
solution.

355
00:26:58 --> 00:27:01
It perches on an oxygen lone
pair.

356
00:27:01 --> 00:27:07
So, H three O plus is
what you get when Bronsted acids

357
00:27:07 --> 00:27:10
ionize in water.
And furthermore,

358
00:27:10 --> 00:27:16
when you put these things in
solution you find that you

359
00:27:16 --> 00:27:22
organize the water molecules
that are close to the hydronium

360
00:27:22 --> 00:27:25
ion.
Let's draw here a neighboring

361
00:27:25 --> 00:27:28
water molecule.

362
00:27:28 --> 00:27:33


363
00:27:33 --> 00:27:34
And another one.

364
00:27:34 --> 00:27:38


365
00:27:38 --> 00:27:43
And I think you can imagine
that throughout a solution,

366
00:27:43 --> 00:27:48
we might have many of the kinds
of interactions that I have

367
00:27:48 --> 00:27:52
drawn here as dotted
peach-colored lines.

368
00:27:52 --> 00:27:56
Those lines represent what we
call hydrogen bonds.

369
00:27:56 --> 00:28:01
And hydrogen bonds are
enormously important in

370
00:28:01 --> 00:28:05
chemistry.
Later, when we talk about the

371
00:28:05 --> 00:28:08
structure of proteins in DNA,
in particular,

372
00:28:08 --> 00:28:12
you may be aware that the DNA
double helix is held together by

373
00:28:12 --> 00:28:16
a network of hydrogen bonds
between complimentary base

374
00:28:16 --> 00:28:19
pairs.
So hydrogen bonds are not only

375
00:28:19 --> 00:28:23
restricted to the hydronium ion
in aqueous solution.

376
00:28:23 --> 00:28:27
There are many other types of
molecules that can form what we

377
00:28:27 --> 00:28:32
call hydrogen bonds.
Another really interesting

378
00:28:32 --> 00:28:37
thing is that in water,
the hydronium ion can move

379
00:28:37 --> 00:28:41
around really rapidly,
much more rapidly than

380
00:28:41 --> 00:28:46
molecules normally diffuse
through aqueous solution.

381
00:28:46 --> 00:28:51
And the reason for that is if
you look at the arrangement of

382
00:28:51 --> 00:28:56
electrons and nuclei here,
all I have to do is,

383
00:28:56 --> 00:29:01
without even moving the nuclei
much at all, reorganize the

384
00:29:01 --> 00:29:06
hydrogen bonding network as
such.

385
00:29:06 --> 00:29:12


386
00:29:12 --> 00:29:15
And now you can see that
through just a slight set of

387
00:29:15 --> 00:29:20
motions, our hydronium ion has
moved from the left-hand side of

388
00:29:20 --> 00:29:24
this hydrogen bonded network,
where you can see that it is

389
00:29:24 --> 00:29:28
indicated with the positive
charge and the three solid lines

390
00:29:28 --> 00:29:33
drawn to the oxygen,
over to the right-hand side.

391
00:29:33 --> 00:29:36
But it did that not by coming
off and moving over,

392
00:29:36 --> 00:29:40
but rather through just
redistribution of the electron

393
00:29:40 --> 00:29:45
density, so that the positively
charged part ends up down on the

394
00:29:45 --> 00:29:48
other side.
And so this way of propagating

395
00:29:48 --> 00:29:52
hydronium ions in aqueous
solution is one of the really

396
00:29:52 --> 00:29:56
special aspects of Bronsted acid
chemistry that takes place in

397
00:29:56 --> 00:30:00
water.
And I think I will also show

398
00:30:00 --> 00:30:04
you what a hydrogen bond looks
like from the standpoint of

399
00:30:04 --> 00:30:06
electron density.

400
00:30:06 --> 00:30:18


401
00:30:18 --> 00:30:21
First, I am just going to show
you the position of the nuclei

402
00:30:21 --> 00:30:24
in a very simple hydrogen bonded
system.

403
00:30:24 --> 00:30:37


404
00:30:37 --> 00:30:40
Here what you can see,
the oxygens are drawn in red as

405
00:30:40 --> 00:30:44
spheres and the hydrogens are
drawn in white as spheres.

406
00:30:44 --> 00:30:48
You can see that the geometry
around the oxygen atoms is

407
00:30:48 --> 00:30:51
slightly pyramidal.
And that is due,

408
00:30:51 --> 00:30:55
of course, to the presence of
that extra lone pair here,

409
00:30:55 --> 00:31:00
up above one oxygen and here,
up above the other oxygen.

410
00:31:00 --> 00:31:04
And what we have now is a
hydrogen serving in a bridging

411
00:31:04 --> 00:31:07
fashion.
And the number of electrons in

412
00:31:07 --> 00:31:11
this system is exactly 2 times
8, because we have two water

413
00:31:11 --> 00:31:14
molecules and we have an
H plus.

414
00:31:14 --> 00:31:18
So this is a positively charged
ion in which a hydronium ion,

415
00:31:18 --> 00:31:23
and you can pick either side,
actually, is interacting with

416
00:31:23 --> 00:31:28
one of the lone pairs of the
other water molecule.

417
00:31:28 --> 00:31:31
And you could imagine lots of
different types of water

418
00:31:31 --> 00:31:34
clusters like this that are
singly positively charged.

419
00:31:34 --> 00:31:38
And people have done a lot of
work to study such clusters in

420
00:31:38 --> 00:31:40
solution.
What you should remember,

421
00:31:40 --> 00:31:44
though, is that the size of the
spheres that I have drawn there

422
00:31:44 --> 00:31:48
to represent those oxygens and
hydrogens is somewhat arbitrary.

423
00:31:48 --> 00:31:51
But what is not arbitrary is
the way that the electron

424
00:31:51 --> 00:31:53
density represents a molecule
like this.

425
00:31:53 --> 00:31:57
So, we will show that to you
next.

426
00:31:57 --> 00:32:13


427
00:32:13 --> 00:32:15
Here it is.
And if we could have the lights

428
00:32:15 --> 00:32:19
down just a little bit,
please, since this one is a

429
00:32:19 --> 00:32:22
little harder to see.
What you should see here is

430
00:32:22 --> 00:32:26
that we have the same structure
now surrounding that

431
00:32:26 --> 00:32:30
representation of the water
molecule hydrogen bonded to the

432
00:32:30 --> 00:32:34
hydronium ion that I drew a
moment ago.

433
00:32:34 --> 00:32:38
We now have this sort of mesh,
which is exactly what we have

434
00:32:38 --> 00:32:41
been looking at with these other
molecules, namely,

435
00:32:41 --> 00:32:46
an electron density isosurface.
And what you can see is that

436
00:32:46 --> 00:32:50
the electron density is falling
to a pretty small value in the

437
00:32:50 --> 00:32:53
middle, here,
where we have the proton that

438
00:32:53 --> 00:32:57
is the connecting glue binding
together these two water

439
00:32:57 --> 00:33:02
molecules in this 16 valance
electron system.

440
00:33:02 --> 00:33:06
And after one more
representation of that,

441
00:33:06 --> 00:33:10
we will be onto our next topic.

442
00:33:10 --> 00:33:20


443
00:33:20 --> 00:33:24
And this one is a solid display
of the electron density

444
00:33:24 --> 00:33:30
isosurface associated with this
hydrogen bonded cluster.

445
00:33:30 --> 00:33:34
And it is, once again,
color mapped with this function

446
00:33:34 --> 00:33:39
that tells us about the
probability of finding electrons

447
00:33:39 --> 00:33:43
paired up together in space.
There is H three O plus

448
00:33:43 --> 00:33:47
hydrogen bonded to H
two O.

449
00:33:47 --> 00:33:52
The blue color represents those
regions in space where you are

450
00:33:52 --> 00:33:55
most likely to find pairs of
electrons.

451
00:33:55 --> 00:34:00
And you can see that the two OH
bonds over here are nicely

452
00:34:00 --> 00:34:05
colored blue.
The lone pair of electrons up

453
00:34:05 --> 00:34:09
here is nicely colored blue.
And then we have an interesting

454
00:34:09 --> 00:34:14
situation where there is some
blue in between that bridging H

455
00:34:14 --> 00:34:18
plus and the two lone
pairs that are pointed at it

456
00:34:18 --> 00:34:21
that produces,
in fact, our hydrogen bond.

457
00:34:21 --> 00:34:25
So, there is a picture of
hydrogen bonding in terms of

458
00:34:25 --> 00:34:30
electron density.
And it is a type of bonding

459
00:34:30 --> 00:34:35
that compliments the straight,
covalent, and ionic bonding

460
00:34:35 --> 00:34:41
that I was talking about in
terms of SO three and

461
00:34:41 --> 00:34:45
AlCl three.
So, we have added this third

462
00:34:45 --> 00:34:50
type of hydrogen bonding to our
list of bonding interests.

463
00:34:50 --> 00:34:55
And now, we will talk more
about what we can do when we

464
00:34:55 --> 00:35:00
consider Bronsted acids
ionizing.

465
00:35:00 --> 00:35:15


466
00:35:15 --> 00:35:20
Here is a generic
representation of the formula of

467
00:35:20 --> 00:35:22
a Bronsted acid,
HA.

468
00:35:22 --> 00:35:28
A might be, for example,
the HSO four minus

469
00:35:28 --> 00:35:34
ion that we showed over there.
When we put a Bronsted acid in

470
00:35:34 --> 00:35:36
aqueous solution,
as I said before,

471
00:35:36 --> 00:35:39
we can get ionization into H
plus and A minus.

472
00:35:39 --> 00:35:41
But we know that it is not just

473
00:35:41 --> 00:35:44
H plus.
It is actually H three O plus

474
00:35:44 --> 00:35:46
**H3O^+**.
And H three O plus is

475
00:35:46 --> 00:35:50
further hydrogen bonded in
networks in the water system.

476
00:35:50 --> 00:35:53
But, for simplicity,
I will just write it as H plus

477
00:35:53 --> 00:35:57
right here.
Recently, in your crash review

478
00:35:57 --> 00:36:00
of thermodynamics,
you were talking about

479
00:36:00 --> 00:36:03
equilibria and equilibrium
constants.

480
00:36:03 --> 00:36:09
And we are going to make use of
some of that right here because

481
00:36:09 --> 00:36:12
we are going to talk about the
acidity constant,

482
00:36:12 --> 00:36:15
Ka.
And that is going to be defined

483
00:36:15 --> 00:36:20
as equal to the hydrogen or
hydronium ion concentration

484
00:36:20 --> 00:36:24
times the concentration of the
conjugate base,

485
00:36:24 --> 00:36:29
A minus --
When a Bronsted acid ionizes it

486
00:36:29 --> 00:36:34
produces what we call the
conjugate base of the acid.

487
00:36:34 --> 00:36:37
Here is conjugate base --

488
00:36:37 --> 00:36:45


489
00:36:45 --> 00:36:49
-- divided by the concentration
of the acid.

490
00:36:49 --> 00:36:53
And this is at equilibrium.

491
00:36:53 --> 00:36:59


492
00:36:59 --> 00:37:02
And let me just emphasize
something so that you don't

493
00:37:02 --> 00:37:05
forget.
This is an important piece of

494
00:37:05 --> 00:37:08
nomenclature.
These square brackets here

495
00:37:08 --> 00:37:12
refer to concentration,
usually in molarity.

496
00:37:12 --> 00:37:19


497
00:37:19 --> 00:37:21
Okay?
So that is what we are talking

498
00:37:21 --> 00:37:24
about.
And concentration is something

499
00:37:24 --> 00:37:28
that can be measured.
You may be familiar,

500
00:37:28 --> 00:37:31
for example,
with the pH meter invented by

501
00:37:31 --> 00:37:34
Arnold O.
Beckman and its utility in

502
00:37:34 --> 00:37:40
measuring the concentration of
hydrogen ions in solution.

503
00:37:40 --> 00:37:43
Well, we can make use of
information like that to talk

504
00:37:43 --> 00:37:47
about the properties of our
Bronsted acids.

505
00:37:47 --> 00:37:54


506
00:37:54 --> 00:38:00
How can we do that?
Well, let's say we are going to

507
00:38:00 --> 00:38:08
take a particular acid such as
this one, which is acetic acid.

508
00:38:08 --> 00:38:14


509
00:38:14 --> 00:38:19
You know the smell of acetic
acid if you have ever been in an

510
00:38:19 --> 00:38:24
establishment where they were
making barbecued chicken wings.

511
00:38:24 --> 00:38:27
That is the smell of acetic
acid.

512
00:38:27 --> 00:38:32
A beautiful smell.
Anyway, what you do here is you

513
00:38:32 --> 00:38:35
are trying to figure out what is
going on.

514
00:38:35 --> 00:38:40
You have some concentration of
the acid HA, which is,

515
00:38:40 --> 00:38:43
we are going to talk about,
acetic acid.

516
00:38:43 --> 00:38:48
And in solution there may also
be a hydrogen ion or a hydronium

517
00:38:48 --> 00:38:51
ion.
And then there may also be A

518
00:38:51 --> 00:38:55
minus, which in the case of
acetic acid would be acetate,

519
00:38:55 --> 00:38:57
--

520
00:38:57 --> 00:39:02


521
00:39:02 --> 00:39:06
-- where we have two
electronegative oxygens,

522
00:39:06 --> 00:39:11
among which the negative charge
can be shared and the acetate

523
00:39:11 --> 00:39:15
ion, which is the conjugate base
of acidic acid.

524
00:39:15 --> 00:39:20
So we make a table.
We need to have some initial

525
00:39:20 --> 00:39:24
concentration.
That is to say let's consider,

526
00:39:24 --> 00:39:27
for example,
tenth molar acetic acid.

527
00:39:27 --> 00:39:32
We are just choosing tenth
molar as a concentration for

528
00:39:32 --> 00:39:38
acetic acid solution.
What that means is you have

529
00:39:38 --> 00:39:42
pure acetic acid.
And then you dissolve it in

530
00:39:42 --> 00:39:46
water and bring it up to a total
volume such that the

531
00:39:46 --> 00:39:51
concentration was 0.1 molar,
assuming that none of it had

532
00:39:51 --> 00:39:54
been ionized yet.
And so that means we have an

533
00:39:54 --> 00:40:00
initial concentration of acetic
acid of 0.1 molar.

534
00:40:00 --> 00:40:03
And initially,
before the ionization,

535
00:40:03 --> 00:40:08
we have zero H plus or
hydronium and zero A minus.

536
00:40:08 --> 00:40:12
And then, the concentration

537
00:40:12 --> 00:40:15
changes.
And it changes because the HA

538
00:40:15 --> 00:40:20
ionizes to some particular
extent, depending on the KA

539
00:40:20 --> 00:40:26
value for the acetic acid.
And what is going to happen is

540
00:40:26 --> 00:40:32
that some of the HA ionizes.
And the amount of the HA that

541
00:40:32 --> 00:40:37
is undergoing ionization is x,
so we are going to lose x.

542
00:40:37 --> 00:40:42
And then, for every HA that
ionizes, we get that same amount

543
00:40:42 --> 00:40:46
of H plus produced and
that same amount of A minus.

544
00:40:46 --> 00:40:48
And so then,

545
00:40:48 --> 00:40:53
after the system reaches
equilibrium, we will finally

546
00:40:53 --> 00:40:57
have 0.1 minus x as our
concentration of HA

547
00:40:57 --> 00:41:01
and x and x will be our
concentrations,

548
00:41:01 --> 00:41:04
respectively,
of H plus and A minus.

549
00:41:04 --> 00:41:10
And so let me put this board

550
00:41:10 --> 00:41:13
all the way up.

551
00:41:13 --> 00:41:18


552
00:41:18 --> 00:41:24
Therefore, we can write the
following, that Ka is equal to x

553
00:41:24 --> 00:41:29
squared over 0.1 over x by

554
00:41:29 --> 00:41:35
substituting into the expression
for the acidity constant.

555
00:41:35 --> 00:41:39
Ka is our acidity constant.

556
00:41:39 --> 00:41:46


557
00:41:46 --> 00:41:52
And we can go to a table and
look up the acidity constant for

558
00:41:52 --> 00:41:57
acetic acid because it is a
known quantity.

559
00:41:57 --> 00:42:03
And it turns out that that is
1.8x10^-5.

560
00:42:03 --> 00:42:06
And now that we have this
equation for the acidity

561
00:42:06 --> 00:42:09
constant and we know what the
acidity constant is,

562
00:42:09 --> 00:42:13
we can solve this for x.
Of course, this is a cubic

563
00:42:13 --> 00:42:15
equation.
We are going to get two roots.

564
00:42:15 --> 00:42:19
You will see that you get a
positive root and a negative

565
00:42:19 --> 00:42:21
root.
The negative root is

566
00:42:21 --> 00:42:24
meaningless because
concentration cannot be

567
00:42:24 --> 00:42:28
negative, so you pick the
positive root.

568
00:42:28 --> 00:42:33
And when you have done that,
you can then go ahead and

569
00:42:33 --> 00:42:37
answer questions,
like, what is the pH of the

570
00:42:37 --> 00:42:40
solution?
And what is the percent

571
00:42:40 --> 00:42:42
ionization?

572
00:42:42 --> 00:42:47


573
00:42:47 --> 00:42:49
And we can talk about that.
So next time,

574
00:42:49 --> 00:42:51
at the beginning of class,
we will do that calculation.

575
00:42:51 --> 00:42:54
We will find what the pH of a
tenth molar solution of acetic

576
00:42:54 --> 00:42:57
acid would be.
We will also go on and talk

577
00:42:57 --> 00:43:00
about pH and the pKa scale,
and also a general equation for

578
00:43:00.381 --> 43:03
discussing titrations and
buffers.