will'o'wisp
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Optical isomerism

I never understood what "Optical isomers rotate plane polarised light in opposite directions" means.

I still don't understand can someone please explain in super simple laymans terms.
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Kallisto
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I give myself a try.

Optical isomerism is quasi the ability of enantiomers to turn the light in a certrain direction. Enantiomers themselves are chemical substances which have the same atoms and functional groups, but different structures to each other, you know?

First you have a light source. This light source is producing light vibrations in all directions. Also you have a polarising filter and an analyzing filter and a plane between them. What happens now? the light vibrations are emitting to the polarising filter first. If the light vibrations touch the polarising filter, the filter allows just to pass one light vibration in one direction through. That is to say the other vibrations are 'out' of the filter. After that the light vibration is in the plane inside. The plane itself is filled with the chemical substance, one of the enantiomers. According to what kind of enantiomer exists in plane, the light is either turn in left or right direction. This turning direction can be observed by the analyzing filter. As the turning direction has something with the structures of enantiomers to do, this property can be used to get those structures out.
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GetOverHere
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(Original post by will'o'wisp)
Optical isomerism

I never understood what "Optical isomers rotate plane polarised light in opposite directions" means.

I still don't understand can someone please explain in super simple laymans terms.
Light is a wave, but naturally it will go out in many different directions; you don't notice every single individual light wave - your eye receives all that is directed into it, giving you a sense of distance, perspective, etc.

However, we can get it to go in one direction (plane) by polarising it. This will give you a singular wave of light going in one direction, and if we were to aim that singular wave of light at a chiral centre, it would rotate the direction of that wave to the left (anticlockwise) or to the right (clockwise).
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will'o'wisp
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(Original post by Kallisto)
I give myself a try.

Optical isomerism is quasi the ability of enantiomers to turn the light in a certrain direction. Enantiomers themselves are chemical substances which have the same atoms and functional groups, but different structures to each other, you know?

First you have a light source. This light source is producing light vibrations in all directions. Also you have a polarising filter and an analyzing filter and a plane between them. What happens now? the light vibrations are emitting to the polarising filter first. If the light vibrations touch the polarising filter, the filter allows just to pass one light vibration in one direction through. That is to say the other vibrations are 'out' of the filter. After that the light vibration is in the plane inside. The plane itself is filled with the chemical substance, one of the enantiomers. According to what kind of enantiomer exists in plane, the light is either turn in left or right direction. This turning direction can be observed by the analyzing filter. As the turning direction has something with the structures of enantiomers to do, this property can be used to get those structures out.
i lost you at quasi
(Original post by GetOverHere)
Light is a wave, but naturally it will go out in many different directions; you don't notice every single individual light wave - your eye receives all that is directed into it, giving you a sense of distance, perspective, etc.

However, we can get it to go in one direction (plane) by polarising it. This will give you a singular wave of light going in one direction, and if we were to aim that singular wave of light at a chiral centre, it would rotate the direction of that wave to the left (anticlockwise) or to the right (clockwise).
so if u polarise it the the light will go left or right?
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GetOverHere
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(Original post by will'o'wisp)
i lost you at quasi

so if u polarise it the the light will go left or right?
If you polarise it, the wave will go in one direction. When it hits the chiral centre, it'll be rotated clockwise or anticlockwise, depending on which optical isomer is present. If there are equal amounts of both (what we call a racemic mixture), the wave will pass straight through, as the clockwise and anticlockwise rotations cancel each other out.
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will'o'wisp
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(Original post by GetOverHere)
If you polarise it, the wave will go in one direction. When it hits the chiral centre, it'll be rotated clockwise or anticlockwise, depending on which optical isomer is present. If there are equal amounts of both (what we call a racemic mixture), the wave will pass straight through, as the clockwise and anticlockwise rotations cancel each other out.
maybe i'm being a bit ******ed but really please take me seriously

so the light does a 360° anticlockwise or clockwise continuously when the light hits the centre?
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Kallisto
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(Original post by will'o'wisp)
i lost you at quasi

so if u polarise it the the light will go left or right?
Where you have lost me? don't be shy to ask questions.

And to answer your question: not going, turning (rotation). The light vibrations is still going straight ahead to the analyzing filter, but it is turned (rotated) left or right during going.
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GetOverHere
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(Original post by will'o'wisp)
maybe i'm being a bit ******ed but really please take me seriously

so the light does a 360° anticlockwise or clockwise continuously when the light hits the centre?
It's not '******ed' at all, nothing wrong with asking a question! (:

No, it's just 90 degrees. So the light will either go completely left or completely right relative to the direction that it approached the chiral centre.

So when you drive up a T junction on the long part of it, you can either turn left or right (90 degrees relative to the direction you approached). It's the same with the manner in which plane-polarised light behaves when there is a chiral centre present.
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will'o'wisp
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(Original post by Kallisto)
Where you have lost me? don't be shy to ask questions.

And to answer your question: not going, turning (rotation). The light vibrations is still going straight ahead to the analyzing filter, but it is turned (rotated) left or right during going.
don't get the analysing filter stuff
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username1539513
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(Original post by will'o'wisp)
Optical isomerism

I never understood what "Optical isomers rotate plane polarised light in opposite directions" means.

I still don't understand can someone please explain in super simple laymans terms.
This might be a bit confusing if you didn't do AS physics tbf; plane polarised light is like normal light being shone through a filter of sorts, so that it can only transverse/ travel in one direction instead of all directions like normal light does. Have you ever worn polarised sunglasses? It's the same principle

So if you have an optically active compound of two isomers, if you shone a ray of polarised single coloured light through them the emerging light beam would be rotated
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will'o'wisp
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(Original post by GetOverHere)
It's not '******ed' at all, nothing wrong with asking a question! (:

No, it's just 90 degrees. So the light will either go completely left or completely right relative to the direction that it approached the chiral centre.

So when you drive up a T junction on the long part of it, you can either turn left or right (90 degrees relative to the direction you approached). It's the same with the manner in which plane-polarised light behaves when there is a chiral centre present.
Oh i kinda get it now.


so it'll hit the sample and instead of the light carrying on straight it'll come out at a right angle? and the light will all go right or left?
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username1539513
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(Original post by will'o'wisp)
Oh i kinda get it now.


so it'll hit the sample and instead of the light carrying on straight it'll come out at a right angle? and the light will all go right or left?
It will turn 90 degrees.
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will'o'wisp
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ok in any direction?
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username1539513
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(Original post by will'o'wisp)
ok in any direction?
It will turn 90 degrees relative to whatever angle it entered into the solution at. Not that any of this is really important as they'll be unlikely to test you on this
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will'o'wisp
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ok, is jus for piece for mind
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alow
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(Original post by GetOverHere)
It's not '******ed' at all, nothing wrong with asking a question! (:

No, it's just 90 degrees. So the light will either go completely left or completely right relative to the direction that it approached the chiral centre.

So when you drive up a T junction on the long part of it, you can either turn left or right (90 degrees relative to the direction you approached). It's the same with the manner in which plane-polarised light behaves when there is a chiral centre present.


Not sure where you are getting the "90 degrees" thing from. The polarisation of the light from interaction with a chiral molecule (or any molecule for that matter) depends on the electronic structure of the molecule, and its orientation relative to the incident photon's wave vector. This is not necessarily 90 degrees.

This stack exchange question has a couple of really good answers, one involving more maths and the other a bit more wordy: http://physics.stackexchange.com/que...olarized-light
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username1539513
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(Original post by alow)
Not sure where you are getting the "90 degrees" thing from. The polarisation of the light from interaction with a chiral molecule (or any molecule for that matter) depends on the electronic structure of the molecule, and its orientation relative to the incident photon's wave vector. This is not necessarily 90 degrees.

This stack exchange question has a couple of really good answers, one involving more maths and the other a bit more wordy: http://physics.stackexchange.com/que...olarized-light
Thanks, I think I got the 90 degrees thing from an a-level textbook
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alow
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That makes me sad

This is a fun blog post which discredits that a bit: http://comporgchem.com/blog/?p=349
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username1539513
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(Original post by alow)
That makes me sad

This is a fun blog post which discredits that a bit: http://comporgchem.com/blog/?p=349
I think it was a physics one, of all things too

Thanks again, my computer doesn't like that page for some reason though
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GetOverHere
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(Original post by alow)
Not sure where you are getting the "90 degrees" thing from. The polarisation of the light from interaction with a chiral molecule (or any molecule for that matter) depends on the electronic structure of the molecule, and its orientation relative to the incident photon's wave vector. This is not necessarily 90 degrees.

This stack exchange question has a couple of really good answers, one involving more maths and the other a bit more wordy: http://physics.stackexchange.com/que...olarized-light
I got it in a Chemistry textbook and also from my teacher when I was doing my A-Levels, my bad haha.
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