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Video transcript

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