Optical Activity Video Lecture | Chemistry for GRE Paper II

here we have a pair of enantiomers on
the Left we have our car bone and on the
right we have s car bone both of these
compounds have the same melting point
the same boiling points and the same
density however there are a few
important differences our car bone is
the major component of spearmint oil so
our carvone smells like spearmint s car
bone is the major component of caraway
oil and this smells like caraway so it's
pretty amazing that our noses can tell
the difference between these two
enantiomers and the science of smell is
a really really fascinating topic
another important difference between
these two enantiomers is their optical
activity so enantiomers exhibit
different behavior when exposed to plane
polarized light so let's examine what I
mean by that so here we have our
unpolarized light which is usually from
a sodium lamp and normally we're talking
about the d-line of sodium with a
wavelength of 589 nanometers so this
unpolarized light tries to pass through
our filter and this filter here if you
look at it notice we have these slits
these vertical slits and so not all of
the unpolarized light can pass through
only this vertical plane of light can
pass through one of our vertical filters
and so we now have a plane of polarized
light and this plane the polarized light
comes to a tube so this is the tube of
the polarimeter so that's what this
device is called so let me go ahead and
write this down so this is our tube and
nr2 we have a solution of optically
active of an optically active compound
so let me go ahead and draw in some of
our compound here so imagine a solution
so this compound is dissolved in
something our plane of polarized light
rotates when it hits our compound so
imagine this plane here which starts off
up-and-down it starts to rotate and the
more molecules it hits here the more it
rotates and so by the time it leaves our
tube it's at a different angle from how
it entered next we have the analyzer
portion so this is our analyzer
and imagine that you are right here so
your eye is here looking at the analyzer
and the analyzer let's say started off
with the slits up and down so just like
we had on the filter here but that
wouldn't allow this plane of light to
pass through so we would have to rotate
the analyzer to allow our plane of light
to pass through and you can see I've
already shown that with this drawing so
the slits are now going in this
direction to allow our plane to pass
through so we had to rotate our analyzer
to the right to allow that plane to get
through and that's called this angle
here alpha is called the observed
rotation so this is the observed
rotation and in this case we had to
rotate the analyzer to the right so
let's start up here let's say we started
vertically and the plane of light was
rotated to the right when it ran into
our compound here so that means that we
rotated the analyzer to the right and
that's said to be an observed rotation
that is positive that's a positive
rotation that's a clockwise rotation
this is also called dextrorotatory so
let me write that down here so this is
dextro rotatory what if our plane was
rotated to the left so what if the light
was rotated to the left let's say we
started off vertical and our light was
rotated in this direction this time so
we had to rotate the analyzer to the
left the observed rotation is said to be
negatives this is a negative rotation a
counterclockwise rotation and this is
called levo rotatory so let me write
that in here so this is levo rotatory
the observed rotation the observed
rotation alpha depends on the number of
molecules that are hit by our polarized
light so let's say we increase the
concentration so I'm going to
and draw some more red dots in here to
indicate increasing the concentration of
our compound well that means that our
light is going to rotate even more all
right so our light starts off vertical
it runs into more molecules it rotates
even more it exits our tube at a
different at a different angle so that
changes our observed rotation turns out
if you double the concentration you
double the observed rotation you can
also change the observed rotation by
changing the path length by changing the
length of this tube here so I'll call
that length L so if you hold the
concentration constant and you double
the path length you double the observed
rotation because that means that your
light is running into more molecules
because your tube is longer so let's
take these ideas of observed rotation
and concentration and path length and
let's turn them into an equation here so
if we take the observed rotation which
is alpha and this is measured in degrees
in degrees so something could rotate you
know you could have an angle in here
right so think about an angle and
degrees for your observed rotation if
you divide your observed rotation by the
concentration right of what's in your
tube and the concentration is in grams
per ml and then the concentration is
multiplied by the path length L which is
in decimetres so this is in decimetres
you'll get something called the specific
rotation so that would be alpha in
brackets so this is the specific
rotation and the nice thing about the
specific rotation is this is a constant
this is a constant your observed
rotation might change right depending on
what concentration you're using
depending on what your path length is
but if you take the observed rotation
you divide it by the concentration times
the path length you get the specific
rotation and having this as a physical
constant is very useful because you can
look up these specific rotations for
specific compounds so for example you
could look up the specific rotation for
we talked about s carvone earlier so
that would be the specific rotation and
this changes this can also change
depending on temperature and wavelength
so you need to specify the temperature
here and the wavelength here so for s
carvone at 20 degrees let me write this
up here so the specific rotation of s
carvone at 20 degrees celsius and using
the d-line of sodium this is equal to
positive 61 so that's the specific
rotation of s carvone and the for
specific rotation is normally a unitless
so normally you don't see anything with
this number however a lot of times you
do i've seen a a breeze sign here for a
lot of things i'm going to take it off
right here because usually the the
degree sign is left for the observed
rotation so that's how you would see a
specific rotation we just saw that s
carbone has a specific rotation of
positive 61 so this enantiomer is
dextrorotatory we have a positive
rotation so we put a positive sign up
here our car bone has a specific
rotation of negative 61
so this enantiomer is levo rotatory so
we have a negative rotation so we put a
negative sign up here notice the
difference in the specific rotations for
our pair of enantiomers enantiomers have
specific rotations that are equal in
magnitude so this one's 61 and this
one's 61 but opposite in sign this one's
negative and this one is positive louis
pasteur was the first one to realize
this relationship so pretty amazing that
he was able to figure this out
I also want to point out that R and s
have nothing to do with negative and
positive so the fact that this is s
carbone has nothing to do with the fact
that this is positive R and s are used
to assign a configuration to a chiral
center and the negative and positive
specific rotations have to be determined
experimentally so we've seen that chiral
compounds
our optically active but a chiral
compounds are not