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Topic: optical activity  (Read 16784 times)

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Offline enahs

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Re: optical activity
« Reply #15 on: March 29, 2007, 10:23:41 AM »
You can also throw in Planar Chirality to make it more confusing.


Offline Custos

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Re: optical activity
« Reply #16 on: March 29, 2007, 07:55:21 PM »
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 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.

Offline madscientist

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Re: optical activity
« Reply #17 on: March 30, 2007, 12:10:35 AM »
Thankyou Custos and everyone else that has responded, its still pretty confusing but i am starting to understand it.

Cheers,

The only stupid question is a question not asked.

Offline mona

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Re: optical activity
« Reply #18 on: March 30, 2007, 01:44:01 AM »

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

Offline madscientist

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Re: optical activity
« Reply #19 on: March 30, 2007, 03:32:35 AM »
Thanks for that mona! ;D
The only stupid question is a question not asked.

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