[orgo814]

I am looking at the gauche conformations for propylene glycol. Why is the conformer where the OH is next to the other OH (rather than anti) more stable than when the OH is anti to the other OH?

[swintarka]

Maybe an intramolecular hydrogen bond?

[Babcock_Hall]

Can you think of any forces that might stabilize the gauche conformation?

[orgo814]

Yeah, the only thing I can think of is intermolecular hydrogen bonding. So I'm assuming if a hydrogen bond can be formed between two groups then those groups being adjacent to each other would be more stable than the anti

[Babcock_Hall]

Do you mean intramolecular or intermolecular?

[orgo814]

Intra. Sorry

[spirochete]

This type of thing is observed also in some molecules that cannot make an intramolecular hydrogen bond. There is an explanation here: https://en.wikipedia.org/wiki/Gauche_effect . The other important factor is the different electronegativity of the atoms, which changes the relative energy of the different bonding and anti bonding orbitals.

This reasoning is similar to the explanation for the anomeric effect, which is arbitrarily used to specifically describe chair conformations.

[clarkstill]

Isn't this what you'd expect sterically anyway? The A-value of OH is smaller than that of CH3, so you'd expect the CH3 to be anti- to the OH on the adjacent carbon, right?

But I suspect hydrogen bonding is probably pretty important here, although without knowing the solvent it's pretty hard to talk about hydrogen bonding.

I agree with spirochete too, there's probably some stereoelectronic (lone pair--->CO sigma*) action going on here too.

Taken together, I'm struggling to find anything that would actually favour the anti conformation in this case!