AAMC4 BS Q140. Why is a tertiary hydrogen less acidic than a secondary one?

[JSU]

The question compares the formation of a secondary and tertiary enolate anion. It comes to the conclusion that the tertiary hydrogen would be less acidic than the secondary one (and would therefore require a stronger base to remove). Im not really following why... Is it because hyperconjugation donates electrons to an already unstable negative carbon?

Thanks!

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[orgohacks]

The question compares the formation of a secondary and tertiary enolate anion. It comes to the conclusion that the tertiary hydrogen would be less acidic than the secondary one (and would therefore require a stronger base to remove). Im not really following why... Is it because hyperconjugation donates electrons to an already unstable negative carbon?

Thanks!

You're sure this isn't a question about the *rate* of formation (tertiary will be slower than secondary due to steric hindrance) versus acidity (acidity is a thermodynamic property).

here's an example for acidity:
t-butyl lithium is a stronger base than sec-butyllithium which is a stronger base than n-butyllithium. There are two ways of looking at this: 1) donation by electron-rich alkyl groups make the anion less stable (most straightforward). 2) increasing steric bulk makes it more difficult for the negative and positive charges to be stabilized through solvation, making the anion more reactive.

 

[JSU]

Yeah its acidity. Thanks for confirming that hyperconjugation form the extra alkyl groups is what destabilizes the carbanion. When I did the question I was thinking of the most stable carbocation (which would benefit from the extra electron donating properties). It was still the best answer of the bunch...

Thanks!

 

[loveoforganic]

I don't think hyperconjugation serves to destabilize carbanions. It should probably stabilize them by allowing them to "soak up" some electron density. The reason tertiary is less acidic than secondary is due to the electron donating effect of the extra alkyl group.

 

[JSU]

I don't think hyperconjugation serves to destabilize carbanions. It should probably stabilize them by allowing them to "soak up" some electron density. The reason tertiary is less acidic than secondary is due to the electron donating effect of the extra alkyl group.

Isnt the electron donating effect called hyperconjugation?

 

[loveoforganic]

No, hyperconjugation is a type of orbital interaction between nonadjacent atoms. The electron donating effect here is the inductive effect.

 

[JSU]

I can see the inductive effect (because alkyl groups are less EN than H) but isnt Hyperconjugation an effect on adjacent molecular orbitals aswell?

 

[loveoforganic]

I guess those are technically adjacent