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Why does the Grignard reagent not react as a base in case of ketones even though the α-hydrogen atom in case of ketones is acidic? Grignard reagents do react with ethyne to give hydrocarbons, so why not here?
The only thing I can put my finger on is electrophilicity of the carbon.
Grignard reagent is a hard base. The HSAB theory states that soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases, all other factors being equal (Wikipedia). Thus, Grignard reagent (a hard base) would preferably attack the harder position when it is available and in offer, like in the situation with its reaction with ketone carbonyl carbon even with the presence of $\alpha$-$\ce{H}$s. Keep in mind that the Grignard reagent probably react with $\alpha$-$\ce{H}$s to certain extent, but when the carbonyl group is readily accessible, that reaction proceeds to much faster rate. Yes, Grignard reagents do react with ethyne to give hydrocarbons, but it is a different story if the molecule contains a carbonyl group.
