alpha-hydrogen atom and keto-enol tautomerism 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?
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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