Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub
Which of the following two compounds will exhibit easy keto-enol tautomerism? Give explanation.
(a) 
(b) 
Options:
Answer:
Solution:
Compound (a), benzene-1,3,5-triol (phloroglucinol), exhibits easy keto-enol tautomerism. Its triketo tautomer, cyclohexane-1,3,5-trione, is a genuine, isolable compound - phloroglucinol even reacts with hydroxylamine to give a trioxime - because the loss of aromatic resonance energy of the benzene nucleus is counterbalanced by the bond energy of three C=O groups (about 18 kcal/mol each). Compound (b), benzene-1,2,4-triol (hydroxyquinol), can in principle tautomerise, but its keto form places two C=O groups on adjacent carbon atoms and is very unstable, while the enol is further stabilised by intramolecular hydrogen bonding - so its tautomerism is not observed easily. Hence (a).
