Keto-enol tautomerism is not shown by

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The compounds A, B and C show keto-enol tautomerism.

A. $C H_{3} - \underset{O}{\underset{| |}{C}} - C H_{2} C H_{3} ⇌ \underset{(i)}{C H_{3} - \underset{O H}{\underset{|}{C =}} C H - C H_{3}} ⇌ \underset{(i i)}{H_{2} C = \underset{O H}{\underset{|}{C -}} C H_{2} C H_{3}}$

The first enol is more stable, because it has the more substituted double bond.

B. $P h C H_{2} - \underset{O}{\underset{| |}{C}} - C H_{2} - C H_{3} ⇌ \underset{(i)}{P h C H = \underset{O H}{\underset{|}{C -}} C H_{2} - C H_{3}} ⇌ \underset{(i i)}{P h C H_{2} - \underset{O H}{\underset{|}{C =}} C H - C H_{3}}$

The first enol is more stable, because $C = C$ and Ph are conjugated.

C. $p - O_{2} N C_{6} H_{4} C H_{2} C O C H_{3} ⇌ \underset{(i)}{p - O_{2} N C_{6} H_{4} C H = \underset{O H}{\underset{|}{C} -} C H_{3}} ⇌ \underset{(i i)}{p - O_{2} N C_{6} H_{4} C H_{2} \underset{O H}{\underset{|}{C} =} C H P h}$

The first enol is more stable, because the $p - O_{2} N$ extends the delocalization of the conjugated system.

PhCOPh does not show tautomerism as do not have any $α$ -H-atom.

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