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Chemistry help

image.jpegHello. We've been studying esters recently and I wondered why they always have the same conformation with the OR group pointing away so to speak from the carbonyl rather than down and parallel to it?
Original post by SonOfNineMothers
image.jpegHello. We've been studying esters recently and I wondered why they always have the same conformation with the OR group pointing away so to speak from the carbonyl rather than down and parallel to it?


What level are you at? If you're not at uni yet then this explanation will probably mean nothing to you but I'll provide it anyway in case you're interested and want to research it or whatever.

The reason is two-fold. The first obvious consideration is sterics. In s-cis the OR group is out "away" from the carbonyl and so there is less steric hindrance and electron repulsion between the groups.

But the deciding and most important factor for why esters adopt this configuration is that the oxygen lone pair of the OR group can overlap with the antibonding orbital of the C-O sigma bond of the carbonyl group. Filling antibonding orbitals normally leads to bond breaking (as the name antibonding suggests), but perhaps counterintuitively it's a favourable interaction as the electrons are thus more delocalised and "spread out", even if it is in an antibonding orbital. This is also why sugars often adopt alpha configurations at the 1 position, due to the anomeric effect.

In the s-trans conformation the oxygen lone pair is pointing away from the C-O sigma antibonding orbital and thus no electron delocalisation is possible.

Again, if you're not at Uni this may sound like a different language but it's the actual explanation for why esters preferentially adopt one form over the other.
(edited 7 years ago)
omg thank you so much !! :biggrin:

I'm only at AS and have no clue what an antibonding orbital is? Is it like the opposite of an orbital like matter and antimatter? Omg this is so interesting i think I'm going to ask my chemistry teacher about this !!
Original post by SonOfNineMothers
omg thank you so much !! :biggrin:

I'm only at AS and have no clue what an antibonding orbital is? Is it like the opposite of an orbital like matter and antimatter? Omg this is so interesting i think I'm going to ask my chemistry teacher about this !!


This picture from Google may help you visualise it a bit better

image.png

It's not as simple as that, antibonding orbital theory requires quite in-depth knowledge and I'm not sure it would be very useful to get into it if you're only at AS :tongue:
If you wanna read more about it, then google molecular orbital theory.

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