Notices
Results 1 to 6 of 6
Like Tree1Likes
  • 1 Post By Warron

Thread: enantiomers, rotation of polarised light

  1. #1 enantiomers, rotation of polarised light 
    SHF
    SHF is offline
    Forum Sophomore
    Join Date
    Sep 2012
    Location
    London
    Posts
    107
    If you have a molecule which has 2 sterioisomers for example Alanine. Each form rotates polarised light in opposite directions (levorotatory vs. dextrorotatory). If you have a solution of just L-Alanine surely the molecules in the solution will be distributed in terms of their orientation such that there will be an equal number of the molecules facing any given direction. So my question is what is the property of the molecule that rotates the light and if, for the sake of argument, 50% of the molecules are ‘facing toward’ the light and 50% ‘facing away’ from the light surely on balance the light should be rotated in both directions in equal measure and so no net rotation?? This sounds logical (to me at least) but of course must be wrong as L-Alanine will rotate polarised light, and so do in the opposite direction to D-Alanine. So what is the property of a molecule that rotates light? And how is it that the light doesn’t get rotated both ways seen as how the molecules will be oriented every which way?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Sophomore
    Join Date
    Jan 2014
    Posts
    106
    It is a difficult concept to understand. Molecules will interact with light differently depending on their orientation. But what is happening in solution is the influence of lots of molecules, so you get the combined effect of them all. In other words the average influence of all orientations.

    Perhaps it is easiest to understand if you recognise that the only way that one molecule can completely cancel out the action of another is if it is a mirror image. And there are no mirror images in a stereoisomer or more particularly in an optical isomer.


    exchemist likes this.
    Reply With Quote  
     

  4. #3  
    KJW
    KJW is offline
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,174
    I think the property you are looking for, and the property being measured by optical rotation, is helicity. Irrespective of the orientation of a chiral molecule, a left-handed corkscrew action will differ from a right-handed corkscrew action. Indeed, take a helix and view it from either end, and you'll see that it looks the same, and not the same as its mirror-image.
    There are no paradoxes in relativity, just people's misunderstandings of it.
    Reply With Quote  
     

  5. #4  
    exchemist
    Join Date
    May 2013
    Location
    London
    Posts
    2,656
    Quote Originally Posted by Warron View Post

    Perhaps it is easiest to understand if you recognise that the only way that one molecule can completely cancel out the action of another is if it is a mirror image. And there are no mirror images in a stereoisomer or more particularly in an optical isomer.
    I think that is an extremely elegant way of putting it. Bravo.
    Reply With Quote  
     

  6. #5  
    exchemist
    Join Date
    May 2013
    Location
    London
    Posts
    2,656
    Quote Originally Posted by SHF View Post
    If you have a molecule which has 2 sterioisomers for example Alanine. Each form rotates polarised light in opposite directions (levorotatory vs. dextrorotatory). If you have a solution of just L-Alanine surely the molecules in the solution will be distributed in terms of their orientation such that there will be an equal number of the molecules facing any given direction. So my question is what is the property of the molecule that rotates the light and if, for the sake of argument, 50% of the molecules are ‘facing toward’ the light and 50% ‘facing away’ from the light surely on balance the light should be rotated in both directions in equal measure and so no net rotation?? This sounds logical (to me at least) but of course must be wrong as L-Alanine will rotate polarised light, and so do in the opposite direction to D-Alanine. So what is the property of a molecule that rotates light? And how is it that the light doesn’t get rotated both ways seen as how the molecules will be oriented every which way?
    Warron gives a beautifully simple explanation of why optically active molecules with random orientations are not expected to cancel each others' effects.

    As to what causes the phenomenon, it is to do with refractive index. All transparent substances interact with the electric field of light passing through. This interaction is responsible for the slowing of the apparent speed ("phase velocity)" of light that causes the different refractive indices of various materials.

    Plane-polarised light is equivalent to a combination of left and right circularly polarised light. In circularly polarised light, the electric and magnetic vectors don't oscillate in the same plane as the photon progresses through space, but progressively twist, so that the pattern they make looks like a twisted paper Christmas decoration. Chiral molecules interact more strongly with one than the other. This makes the phase velocity of one component less than for the other one, so the "plane" wave, being made up of the superposition of the two, will itself progressively twist.
    Reply With Quote  
     

  7. #6  
    exchemist
    Join Date
    May 2013
    Location
    London
    Posts
    2,656
    Quote Originally Posted by SHF View Post
    If you have a molecule which has 2 sterioisomers for example Alanine. Each form rotates polarised light in opposite directions (levorotatory vs. dextrorotatory). If you have a solution of just L-Alanine surely the molecules in the solution will be distributed in terms of their orientation such that there will be an equal number of the molecules facing any given direction. So my question is what is the property of the molecule that rotates the light and if, for the sake of argument, 50% of the molecules are ‘facing toward’ the light and 50% ‘facing away’ from the light surely on balance the light should be rotated in both directions in equal measure and so no net rotation?? This sounds logical (to me at least) but of course must be wrong as L-Alanine will rotate polarised light, and so do in the opposite direction to D-Alanine. So what is the property of a molecule that rotates light? And how is it that the light doesn’t get rotated both ways seen as how the molecules will be oriented every which way?
    Supplementary, if you are interested. The difference between "phase velocity" (the speed with which waves propagate) and the "group velocity" (the speed of the wave packet that makes up a quantum of light, or photon), is nicely shown in the attached moving picture: File:Frontgroupphase.gif - Wikipedia, the free encyclopedia

    In this you can see that the waves move more slowly than the envelope. So the phase velocity can become less than c, whilst the group velocity can still be c, in accordance with the invariance of the speed of light.

    At least, I hope I've got this right - if not I'm sure a physicist will correct me - I learnt this stuff 40 years ago.
    Reply With Quote  
     

Similar Threads

  1. about earth's rotation
    By ksdivyaprasad1969 in forum General Discussion
    Replies: 0
    Last Post: September 6th, 2012, 08:18 AM
  2. why sound cant be polarised?
    By precious in forum Physics
    Replies: 5
    Last Post: November 26th, 2011, 01:45 PM
  3. Crop rotation
    By Twittery in forum Biology
    Replies: 6
    Last Post: June 3rd, 2009, 11:05 AM
  4. Rotation
    By Vector Equilibrium in forum Earth Sciences
    Replies: 19
    Last Post: December 13th, 2008, 06:47 AM
  5. polarised sockets/plugs
    By goodgod3rd in forum Electrical and Electronics
    Replies: 4
    Last Post: May 28th, 2006, 05:34 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •