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Q.
Keto-enol tautomerism is not shown by
NTA AbhyasNTA Abhyas 2020Organic Chemistry – Some Basic Principles and Techniques
Solution:
The compounds A, B and C show keto-enol tautomerism.
A. $CH_{3}-\underset{O}{\underset{\left|\right. \left|\right.}{C}}-CH_{2}CH_{3}\rightleftharpoons\underset{\left(i\right) \, }{C H_{3} - \underset{O H \, \, \, }{\underset{\left|\right. \, \, }{C =}} C H - C H_{3}}\rightleftharpoons\underset{\left(i i\right) \, \, }{H_{2} C = \underset{O H \, \, \, }{\underset{\left|\right. \, }{C -}} C H_{2} C H_{3}}$
The first enol is more stable, because it has the more substituted double bond.
B. $PhCH_{2}-\underset{O}{\underset{\left|\right. \left|\right.}{C}}-CH_{2}-CH_{3}\rightleftharpoons\underset{\left(i\right) \, }{P h C H = \underset{O H \, \, \, }{\underset{\left|\right. \, \, }{C -}} C H_{2} - C H_{3}}\rightleftharpoons\underset{\left(i i\right) \, \, \, }{P h C H_{2} - \underset{O H \, \, \, }{\underset{\left|\right. \, }{C =}} C H - C H_{3}}$
The first enol is more stable, because $C=C$ and Ph are conjugated.
C. $p-O_{2}NC_{6}H_{4}CH_{2}COCH_{3}\rightleftharpoons\underset{ \, \left(\right. i \left.\right)}{p - O_{2} N C_{6} H_{4} C H = \underset{O H \, \, }{\underset{\left|\right.}{C} -} C H_{3}}\rightleftharpoons\underset{ \, \, \, \left(\right. i i \left.\right)}{p - O_{2} N C_{6} H_{4} C H_{2} \underset{O H \, }{\underset{\left|\right.}{C} =} C H P h}$
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 $\alpha $ -H-atom.