Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: madscientist on March 28, 2007, 07:47:58 PM
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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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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).
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Thanks for the quick reply Yggdrasil, sort of makes sense now.
cheers
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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.
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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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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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Don't quote me on this, but I think axial chirality can lead to optical activity as well as point chirality.
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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
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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.
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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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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.
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Is citrate another way to say citric acid? are they the same?
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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.
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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.
"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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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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You can also throw in Planar Chirality to make it more confusing.
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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,
"Optical activity" is a physical quality like a melting point or boiling point. A pure single enantiomer of a chiral compound will display optical activity but so might a mixture.
A chiral molecule has two enantiomers which are non-superimposable mirror images. Each pure enantiomer will display optical activity, that is, they will rotate plane polarised light - by the same magnitude and in opposite directions. The only reliable test of whether a molecule is chiral is to see if its mirror images are superimposable. A clue that it may be chiral is the presence of a stereogenic or asymmetric centre (e.g. an sp3 carbon with four different substituents). This is not proof however, because the stereogenic centre may be duplicated elsewhere leading to a meso compound. The meso compound below has two asymmetric centres, but is not chiral.
Chirality can also arise from asymmetry in the molecule that is not associated with a stereogenic centre. Two classic examples - allenes, and biphenyls with restricted rotation - are shown below. These compounds are not superimposable on their mirror images but they have no stereogenic centres. They have axial chirality.
Helical compounds can also be chiral (planar chirality) depending on whether they have a left-hand or right hand threads.
By the way, you might be interested to read here (http://www.rsc.org/chemistryworld/News/2007/March/28030701.asp) that new limits of detectability of chirality have been set by the measurement of the Raman optical activity of (R)-[2H1, 2H2, 2H3]-neopentane -- that is, a carbon atom with 4 different substituents being CH3, CHD2, CH2D and CD3. Pretty amazing subtly in enatiomeric difference.
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Thankyou Custos and everyone else that has responded, its still pretty confusing but i am starting to understand it.
Cheers,
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hi
this will help you to find the optical activity
Finding Stereocenters in Chains: Pointers
Pointer #1:
Remember that line drawings commonly leave hydrogens out. If a carbon atom has only 2 bonds shown, the other two bonds are to hydrogen and it is not a stereocenter.
Pointer #2:
Remember that you must compare the whole substituent rather than simply the single atoms connected to the atom you are considering. (A carbon can be chiral with four carbons attached to it if one is a methyl, one an ethyl, one a propyl, and one a butyl group)
you can find help in this site http://www.chem.arizona.edu/courses/chem242/stereochem1/sub.html
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Thanks for that mona! ;D