Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub
Which of the following compounds are capable to undergo tautomerism in base catalysed medium?
(1) 
(2) 
(3) 
(4) 
Options:
Answer:
Solution:
(1) Capable. Cyclohexa-2,5-dien-1-one has sp3 CH₂ groups alpha to the carbonyl; base removes an alpha proton and the enolate tautomerises to phenol - the driving force is aromatisation :
(2) Capable. The oxime (4-hydroxyiminocyclohexa-2,5-dien-1-one) transfers a proton from oxygen to the ring and aromatises to 4-nitrosophenol - oxime-nitroso tautomerism, again driven by aromatisation :
(3) No tautomerism. This is cyclohexa-2,5-dien-1,4-dione (p-benzoquinone): every ring carbon is sp2, so there is no sp3 alpha C-H that base could remove, and no enol form is accessible.
(4) Capable. Bicyclo[2.2.1]heptan-2-one has alpha hydrogens (at C-3 and at the bridgehead C-1); base forms the enolate, which reprotonates to the enol shown (the C=C forms at C-2/C-3, never at the bridgehead, which would violate Bredt's rule) :
