We have $\ce{(CH3)2CHCHO}$. Is the alpha-hydrogen acidic enough to react with a base? There is +I effect of two methyl groups, however there the carbanion is getting stabilised through resonance. And, as we know that inductive is a weak effect, so will it proceed through aldol reaction or will Cannizzaro reaction occur? Moreover, if we are considering the inductive effect to be significant, won't it reduce the electrophilicity of carbon of the carbonyl group, hence reducing the chances of Cannizzaro reaction? What could be the products if the reaction is done separately by using conc. base in one time and dil. base for other?


1 Answer 1


Is the alpha hydrogen acidic enough to react with base?

The answer is yes. As you said +I effect of the methyl group stabilizes the carbocation. It is weakly acidic but not weak enough to not be taken by any base.

Will it proceed through aldol reaction or will Cannizzaro reaction occur?

It will proceed through Cannizzaro mechanism even though it has an alpha hydrogen. That is because if you take it through aldol, the attacking nucleophile in the second step is $\ce{-C(CH3)2(CHO)}$ which is a very bulky nucleophile and so it isn't able to attack on $\ce{CH(CH3)2(CHO)}$. Think, if this reaction undergoes than the product will have $\ce{CH3}$, $\ce{CH3}$, $\ce{CHO}$ and $\ce{-C(OH)(CH(CH3)2)}$ groups all on one carbon. The steric crowding will be so immense that the reaction becomes infeasible. That is why it undergoes Cannizzaro reaction which has $\ce{H-}$ as the attacking nucleophile (in the second step).

I think your other questions won’t arise if you think this way.

  • $\begingroup$ Thanks for the answer, so what's the major thing which is preventing the aldol reaction, steric or the reason of alpha hydrogen not being acidic enough, for example if we add formaldehyde , considering only the cross aldol, will it happen, since the steric hinderance is not very much? $\endgroup$
    – green_32
    May 24, 2021 at 22:40
  • $\begingroup$ @green_32: You might want to look at this. chemistry.stackexchange.com/questions/65089/… $\endgroup$
    – user55119
    May 24, 2021 at 22:59

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