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Offline madscientist

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optical activity
« on: March 28, 2007, 07:47:58 PM »
Can anyone lead me in the right direction as to how to tell if a compound is optically active or not? Has it got somthing to do with whether a compound has two chiral centres, one having an R configuration and the other an S configuration? I understand that optical activity is indicated in nomenclature i.e. (-)-Lactic acid and (+)-Lactic acid.... but have read that the sign of optical rotation, (+) or (-), is not related to the R,S designation.

What im basically asking is, if given a structure, can you tell if it is optically active? and how?

is it simply the fact that if a compound is chiral that it will be optically active?

Any hints or help is appreciated.

Cheers,

madscientist
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Offline Yggdrasil

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Re: optical activity
« Reply #1 on: March 28, 2007, 07:58:06 PM »
Any chiral molecule (meaning that it cannot be superposed onto its mirror image) will be optically active.  This means that if the compound has a stereogenic carbon (aka stereocenter, chiral carbon) it will be optically active.  Compounds with two or more stereocenters will often be optically active, but not always (they could be meso compounds).

Offline madscientist

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Re: optical activity
« Reply #2 on: March 28, 2007, 08:00:50 PM »
Thanks for the quick reply Yggdrasil, sort of makes sense now.

cheers
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Offline RBF

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Re: optical activity
« Reply #3 on: March 28, 2007, 09:40:38 PM »
2-butanol is a chiral molecule, yet 2-butanol you are likely to get from Aldrich Chemical Co will not be optically acitive.  A substance is optically active if it causes a beam of polarized light to rotate either clockwise or counterclockwise.

Offline madscientist

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Re: optical activity
« Reply #4 on: March 28, 2007, 09:46:16 PM »
Quote
A substance is optically active if it causes a beam of polarized light to rotate either clockwise or counterclockwise.

I get that much, but is there a way to tell if a molecule will be optically active by looking at the structure alone?
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Offline madscientist

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Re: optical activity
« Reply #5 on: March 28, 2007, 11:26:52 PM »
If you find that a compound is achiral, and not a meso compound (no chirality centres) is that enough to say that theoretically it is optically inactive?
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Offline Ψ×Ψ

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Re: optical activity
« Reply #6 on: March 28, 2007, 11:42:48 PM »
Don't quote me on this, but I think axial chirality can lead to optical activity as well as point chirality.

Offline madscientist

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Re: optical activity
« Reply #7 on: March 29, 2007, 12:54:53 AM »
In the case of citric acid (shown below) it seems to me that this compound is achiral but i'm unsure about a few things which i have noted in the attached picture.

Any ideas?

The atoms that look like C in the left hand side of the structure are actually O, the pic didnt turn out that good. Also, ignore the question about the chirality centre, ive just realised that there are not four different groups bonded to that carbon and therefore this is not a meso compound.  :o
« Last Edit: March 29, 2007, 01:13:57 AM by madscientist »
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Offline Yggdrasil

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Re: optical activity
« Reply #8 on: March 29, 2007, 01:39:02 AM »
2-butanol is a chiral molecule, yet 2-butanol you are likely to get from Aldrich Chemical Co will not be optically acitive.  A substance is optically active if it causes a beam of polarized light to rotate either clockwise or counterclockwise.

This is a different case though.  The 2-butanol you order from Aldrich is a racemic mixture of (+) and (-) 2-butanol.  So, while the solution is composed of optically active molecules, the net rotation of light through a racemic mixture is zero because there are equal parts rotating the light clockwise and equal parts rotating the light couterclockwise.

Also, citric acid is achiral (notice that two of the substituents are exactly the same (two -CH2-COOH groups).  Amazingly, aconitase, a TCA cycle enzyme, can stereospecifically convert the achiral citrate to just one enantiomer of isocitrate, a chiral molecule.

Offline madscientist

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Re: optical activity
« Reply #9 on: March 29, 2007, 01:42:17 AM »
OK ive figured out that citric acid has no chirality centres, and is achiral (superimposable on its mirror image) but it also has an axial plane of symmetry going through the central carbon (see above pic).  

Don't quote me on this, but I think axial chirality can lead to optical activity as well as point chirality.

Can anyone back up this quote? does axial chirality lead to optical activity? My gut is telling me that citric acid is optically inactive but this quote has given me doubts  ::)

Any ideas?

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Offline Yggdrasil

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Re: optical activity
« Reply #10 on: March 29, 2007, 01:53:50 AM »
I think axial chirality may cause optical activity (but like ?*?, I'm not 100% positive of the fact), but citrate does not have axial chirality.

Offline madscientist

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Re: optical activity
« Reply #11 on: March 29, 2007, 01:56:42 AM »
Is citrate another way to say citric acid? are they the same?
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Offline Yggdrasil

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Re: optical activity
« Reply #12 on: March 29, 2007, 01:59:29 AM »
Citrate is the deprotonated form of citric acid.  I and most biochemists say citrate rather than citric acid because citric acid becomes fully deprotonated under physiological conditions, so citrate is the biologically relevant form of citric acid.  But, yes, both citrate and citric acid exhibit neither point chirality nor axial chirality.

Offline madscientist

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Re: optical activity
« Reply #13 on: March 29, 2007, 02:02:40 AM »
Could you please describe what axial chirality is? like ive indicated in the picture in the above post I would have thought that this was axial chirality,

i.e.

Quote
"Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center (the most common form of chirality in organic compounds) but an axis of chirality - an axis about which a set of substituents is held in a spatial arrangement which is not superposable on its mirror image".
Taken from wiki

Citric acid (or citrate) has no chirality centres, is achiral ( and not meso) and i would have thought that it has axial chirality? ???

Sorry, im a bit lost with the axial chirality part.
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Offline Ψ×Ψ

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Re: optical activity
« Reply #14 on: March 29, 2007, 08:36:57 AM »
By axial chirality, I mean something like a helix or a screw.  If you have a left-handed screw and a right-handed screw, they're enantiomers. 

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