Take the 2-minute tour ×
Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. It's 100% free, no registration required.

The most stable conformation of optically inactive butane-2,3-diol is:

(A)enter image description here

(B)enter image description here

(C)enter image description here

(D)enter image description here

I choose C as the correct answer but, in the answer key it given to be B. C is the staggered form and the most stable one. It can also have hydrogen bonding between -OH and -H groups. B can also have hydrogen bonding but there would be repulsion between the two Me groups. So, which one is correct?

share|improve this question
Nice question and wow I just looked at Wikipedia for 2,3-butanediol and it is crazy wrong about how many stereoisomers there are. It apparently doesn't know tha (R,S)-2,3-butanediol is the same as (S,R)-2,3-butanediol because of symmetry (i.e. it is a meso compound). At least, that's what I think but I'm not a stereochemical expert. Can someone update? –  Curt F. Apr 2 at 23:07

1 Answer 1

Conformation B is probably correct because of the hydrogen bonding, if we assume this substance is either pure or in a polar solvent. The Me-Me gauche interaction is on the order of +3.8 kJ/mol (see most textbooks, or it is one half of methyl's A value, which involves two gauche interactions), while the hydrogen bond can be on the order of −5 to −25 kJ/mol. According to the Wikipedia article on hydrogen boning, the typical $\ce{O-H-OH}$ hydrogen bond has an energy stabilization of −21 kJ/mol. Since these groups are not quite properly aligned, a safe assumption is that the hydrogen bond interaction would be about half of that ideal value.

However, in a nonpolar solvent, there might be a benefit to conformation C, which has a net-zero molecular dipole moment. The entropic repulsion between the nonpolar solvent and B would have to be more serious than the two Me-OH gauche interactions (probably about +1.8 kJ/mol each). However, without that information, conformation B should be correct.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.