Question

Which of the following can exhibit keto-enol tautomerism?

• A. $ H-\underset{\begin{smallmatrix} |\,\,| \\ O \end{smallmatrix}}{\mathop{C}}\,-H $
• B. $ {{(C{{H}_{3}})}_{3}}C-\underset{\begin{smallmatrix} |\,\,| \\ O \end{smallmatrix}}{\mathop{C}}\,-H $
• C. $ C{{H}_{3}}-\overset{\begin{smallmatrix} O \\ |\,\,| \end{smallmatrix}}{\mathop{C}}\,-H $
• D. $ CC{{l}_{3}}-\underset{\begin{smallmatrix} |\,\,| \\ O \end{smallmatrix}}{\mathop{C}}\,-H $

Answer 1

Answer: (C)

Tautomerism is a special kind of functional isomerism in which the isomers exist in dynamic equilibrium with each other. It arises due to migration of hydrogen atom from one polyvalent atom to the other within the same molecule. The isomers thus obtained are called tautomers. Thus, $ HCHO,\text{ }CC{{l}_{3}}CHO $ and $ {{(C{{H}_{3}})}_{3}}CCHO $ do not show tautomerism due to the absence of $ \alpha - $ hydrogen atom w.r.t carbonyl $ (>C=O) $ group while $ C{{H}_{3}}CHO $ shows tautomerism as $ \underset{\begin{smallmatrix} (acetaldehyde) \\ \,\,\,\,\,keto\text{ }form \end{smallmatrix}}{\mathop{C{{H}_{3}}-\overset{\begin{smallmatrix} O \\ |\,\,| \end{smallmatrix}}{\mathop{C}}\,-H}}\,\underset{\begin{smallmatrix} (vinyl\text{ }alcohol) \\ \,\,\,enolic\text{ }form \end{smallmatrix}}{\mathop{C{{H}_{2}}=\overset{\begin{smallmatrix} O-H \\ |\,\,| \end{smallmatrix}}{\mathop{C}}\,-H}}\, $