The correct statement regarding a carbonyl compound with hydrogen atom on its alpha carbon is:
A. A carbonyl compound with hydrogen atom on its alpha carbon rapidly equilibrates with its corresponding enol and this process is known as carbonylation.
B. A carbonyl compound with hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol and this process is known as keto enol tautomerism.
C. A carbonyl compound with hydrogen atom on its alpha carbon never equilibrates with corresponding enol.
D. A carbonyl compound with hydrogen atom on its alpha carbon rapidly equilibrates with its corresponding enol and this process is known as aldehyde-ketone equilibration.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B only
A carbonyl compound with an α-hydrogen exists in equilibrium with its enol form: the α-hydrogen moves to the carbonyl oxygen and the double bond shifts between the α-carbon and the carbonyl carbon, e.g. \( \ce{CH3COCH3} \) ⇌ \( \ce{CH2=C(OH)CH3} \). Isomers that interconvert by such a shift of a hydrogen atom are tautomers, and this equilibrium is called keto-enol tautomerism. The same acidity of the α-hydrogen is behind the aldol reaction.
A. A carbonyl compound with hydrogen atom on its alpha carbon rapidly equilibrates with its corresponding enol and this process is known as carbonylation. — Incorrect. The equilibrium is right, but ‘carbonylation’ is the wrong name; carbonylation means introducing a CO group into a molecule.
B. A carbonyl compound with hydrogen atom on its alpha-carbon rapidly equilibrates with its corresponding enol and this process is known as keto enol tautomerism. — Correct. The rapid keto-enol equilibrium is keto-enol tautomerism.
C. A carbonyl compound with hydrogen atom on its alpha carbon never equilibrates with corresponding enol. — Incorrect. A carbonyl compound with an α-hydrogen does equilibrate with its enol; only compounds without an α-hydrogen (such as HCHO and \( \ce{C6H5CHO} \)) cannot.
D. A carbonyl compound with hydrogen atom on its alpha carbon rapidly equilibrates with its corresponding enol and this process is known as aldehyde-ketone equilibration. — Incorrect. The equilibrium is between the keto and enol forms of the same compound; it is not an equilibrium between an aldehyde and a ketone, and no process has that name.
Note: The question is from beyond the NCERT textbook.