29-7 Chemistry of carbon: rotation of polarized light

Uploaded by JUSTANEMONERD on 17.12.2012

PROFESSOR Cima: Our discussion of the chemistry of carbon by looking at the
symmetry of bonds around a carbon atom, this particular molecule,
2-iodobutane. So the two means go to the end of the butane
molecule which has got, of course, four carbons.
And count down-- one, two. So that's the second carbon.
And I put an iodine on it. And that's where the iodine goes.
And of course the other bond that has to be there to satisfy the four bonds
of carbon has to be a carbon hydrogen bond. And we look carefully at this carbon and said
imagine taking a molecule of 2-iodobutane and looking at it in a mirror--
so on the mirror side, what you would see is a--
now I'm trying to draw the tetrahedral arrangements here.
So this hydrogen is drawn into the board. These carbon carbon bonds are in the plane
of the board. And the carbon iodine bond is sticking out
toward us to get the 109 degrees or so separation between those tetrahedral
arrangements of bonds. And of course when I do the mirror plane,
I think you know what's going to happen here.
I create a molecule that is an isomer of 2-iodobutane. But it is not superimposable.
In other words, when I take this new molecule and rotate it back over here,
the iodine will be in the back, and the hydrogen will be in the front.
So these two molecules to share a symmetry just like your
left and right hand. They share a mirror symmetry.
I can't superimpose them on one another. And this is always possible around carbon
when you have four different groups on the carbon.
In other words, if this had been just butane-- in other words where this is hydrogen--
of course now when I take them mirror symmetry, I can now take that
molecule, bring it around here, just rotate it around 180 degrees.
And it is superimposable. So the breakdown of symmetry that created
this, what we now call a chiral center, is the fact that I have four
different groups attached to this carbon.
So it's the methyl group, the hydrogen, the iodine,
and the ethyl group-- four different ones.
If any two of those had been the same, you would not have this chiral center,
is what we call it. Now these molecules that have these chiral
centers have some interesting properties, the most important of which is
called optical rotation. What do we mean by optical rotation?
Well let's just take a non-chiral molecule to start with.
And I'm going to have some light which, as you recall, is a wave of
electromagnetic radiation. And I'm going to polarize it so that I only
have one orientation of the electric field.
Put it through a polarizer, some sunglasses. And then I'm going to take my non-chiral molecule--
so I'll just stick with butane again. Why don't I draw the bonds around the carbon
too. And I've chosen just an orientation that's
not exactly parallel with the electric field.
You know, I'm going to have some liquid butane here
in a pressure vessel. And I'll run some polarized light through
it. Now of course this molecule's long along one
axis. So in general, I would believe that the polarizability
of this molecule changes with direction in the molecule.
So if I got electrons along this structure, you might suspect that it's
easier to polarize this thing along that direction than it is let's say
perpendicular to that direction. So when this light hits this molecule, it
excites polarization along this axis more than this axis.
So in a sense, even this molecule will rotate the polarization.
When it comes out of here, it's going to be more like this.
So even-- from this picture, I've got a situation where
even a non-chiral molecule will rotate light.
Makes sense. Polarization is not uniform throughout that
molecule. But of course I've got a whole bucket or container
of butane. And so for every molecule that's oriented
this way, I got one that's oriented this way.
And so what's going to happen? It's just going to rotate this back.
And so the net effect is no rotation, no net change.
So it should be no net change. It doesn't mean that intrinsically molecules
won't rotate light. It's just that if you have a collection of
them, it goes both directions.
Now let's consider a bucket of 2-iodobutane. I got the same thing, my polarized light coming
in. But now I've got one particular chiral version.
We call these things enantimoers, I forgot to mention.
And so it'll do the same thing. It will rotate the light.
But now because there's a handedness to the molecule, there's not a chance
to get rotated back the other way. So the fact that I have a pure enantimoer
means there is a net rotation of the light.
If I had mixtures of both the mirror images then there would be a chance
and I'd get no rotation of the light, no net rotation of the light.
So we call those-- and you'll see why-- we call those when I have both versions, both
enantimoers, a racemic mixture.