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Which one of the following compounds will be most readily dehydrated?
A
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B
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C
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D
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Detailed Solution
All four compounds are hydroxy ketones; they differ in the position of the –OH group relative to the C=O group.
A β-hydroxy ketone (–OH on the carbon two positions away from the carbonyl carbon) loses water very easily, as in the dehydration step of the aldol condensation.
Reasons: the hydrogen on the α-carbon, lying between the C=O and the C–OH, is acidic and is easily removed; and the product is an α,β-unsaturated ketone, in which the new C=C is conjugated with C=O and is therefore resonance stabilised.
In an α-hydroxy ketone or a γ-hydroxy ketone, loss of water cannot give such a conjugated system so readily.
The compound $CH_3-CO-CH_2-CH(OH)-CH_2-CH_3$, which has the –OH group at the β-position to the keto group, is therefore the most readily dehydrated, giving $CH_3-CO-CH=CH-CH_2-CH_3$.
A β-hydroxy ketone (–OH on the carbon two positions away from the carbonyl carbon) loses water very easily, as in the dehydration step of the aldol condensation.
Reasons: the hydrogen on the α-carbon, lying between the C=O and the C–OH, is acidic and is easily removed; and the product is an α,β-unsaturated ketone, in which the new C=C is conjugated with C=O and is therefore resonance stabilised.
In an α-hydroxy ketone or a γ-hydroxy ketone, loss of water cannot give such a conjugated system so readily.
The compound $CH_3-CO-CH_2-CH(OH)-CH_2-CH_3$, which has the –OH group at the β-position to the keto group, is therefore the most readily dehydrated, giving $CH_3-CO-CH=CH-CH_2-CH_3$.