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The following is an extract from my book-

For an aldehyde or a ketone to exhibit keto-enol tautomerism, it is essential that it must have atleast one $\alpha$-hydrogen atom. Thus, acetophenone, butan-2-one and propionaldehyde all contain $\alpha$- hydrogen atoms and hence show keto-enol tautomerism. In contrast, benzaldehyde, benzophenone etc. do not contain $\alpha$-hydrogen atoms.

What is meant by $\alpha$-hydrogen atom? How does it affect keto-enol tautomerism?

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In general, the "alpha" hydrogen is a hydrogen attached to a carbon that is "alpha" (adjacent to) the substituent.

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So in this case, an "alpha" hydrogen is a hydrogen attached to the carbon alpha to the carbonyl.

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The following drawing represents a typical keto-enol equilibrium. If we removed the last alpha hydrogen and replaced it with a substituent (alkyl group, phenyl group, bromine, etc.) then we could not form a double bond with that alpha carbon, it would have 4 non-removable substituents. If we can't form a double bond that means we can't form an enol.

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  • $\begingroup$ Why is the secondary alpha carbon more acidic than the primary alpha carbon? I thought primary carbanions were MORE stable than secondary/tertiary carbanions... $\endgroup$ – Nova Oct 3 '16 at 23:57
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An $\alpha$ hydrogen atom is bound to a carbon next to some functional group, e.g. a ketone or an aldehyde. Without this hydrogen there is no hydrogen that can be removed by enolization. Other hydrogen atoms on the carbon chain are much less acidic due to the lack of an electron stabilizing group such as a keto-group. Benzaldehyde contain no $\alpha$ hydrogen atoms because of the $\ce{sp^2}$ hybridized carbon atom. On the other hand, acetone for example has 6 $\alpha$ hydrogen atoms.

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