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The Stereoisomers of Organic Molecules with More Than One Chiral Carbon Atom

posted Nov 16, 2016, 6:12 AM by Grace Ong   [ updated Nov 16, 2016, 6:50 AM ]
The following post was first posted on Blogger on Wednesday, 1 January 2014.



Happy New Year, everyone!

This is a little embarrassing... I'm starting 2014 with a blog post which I had wanted to publish some time back, but promptly forgotten as I got too busy at school.

I came across the following question (and its proposed answers) a few months ago.

structure elucidation with incorrectly drawn enantiomers

This is a relatively straightforward question, but the solution isn’t quite as straightforward, particularly in relation to the three-dimensional structures of compounds B and C

Both (geometric) isomers of A, as shown in the given answers above, undergo mild oxidation with cold, dilute potassium manganate(VII) solution to produce butane-2,3-diol:

structural formula of butane-2,3-diol

This diol has two chiral carbon atoms, and a plane of symmetry between them.

A consequence of this structure is that while it is possible for butane-2,3-diol to have a maximum of 22 = 4 stereoisomers, in reality, it only has three – a pair of optically active enantiomers and an optically inactive meso compound.

Since the question states that the products of this oxidation, B and C, rotate light in opposite directions, they must be the two enantiomers of butane-2,3-diol. At this point, this conclusion is congruent to the answers proposed.

What I do not quite agree with, however, is the simplistic three-dimensional representation of B and C in the given answers; the teacher who drew these structures had used the ‘standard’ convention for drawing enantiomers given under Section 10.1 of the H2 Chemistry Syllabus 9647, using only one of the asterisked carbons as a chiral centre reference point. While this works fine for molecules with only one chiral carbon, this is not so for molecules with more than one chiral carbon, such as the butane-2,3-diol answer required by the question above.

The following diagrams illustrate what I mean:

meso compound of butane-2,3-diol

enantiomers of butane-2,3-diol

And M is a diastereoisomer of B and C.

As you can see, the overall chirality of butane-2,3-diol is dependent on the spatial orientation of the H atoms, −OH and −CH3 groups around both chiral carbon atoms. In other words, the only way to tell if the molecule has a plane of symmetry (and therefore achiral) is to draw wedge-and-dash bonds around not one, but both chiral carbon atoms.



Addendum (02 January 2014):

The reaction between compound A and cold, alkaline potassium manganate(VII) is an example of a syn stereoselective hydroxylation of an alkene to form a diol.

Syn hydroxylation of cis-but-2-ene gives meso butane-2,3-diol (M),

syn hydroxylation of cis-but-2-ene

while that of trans-but-2-ene results in racemic butane-2,3-diol (B and C).

syn hydroxylation of trans-but-2-ene

Thus, if the reaction starts off with a mixture of both cis- and trans- isomers of A, the diol formed will be a racemic mixture of both enantiomers, i.e. B and C, as well as the meso compound, M.
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