MolSynapsequestion bank
MolSynapse / Question bank / General Organic Chemistry II: Acids and Bases / Most acidic hydrogen in drawn molecules
JEE Advanced levelSingle correct optionDifficulty 3 of 3g2261

Which hydrogen of the compound shown (a lactone-based enediol) is the most acidic?

  1. Athe enol OH on the carbon bonded to the ring CH-O carbon
  2. Bthe primary alcohol O-H of the side chain
  3. Cthe enol OH on the carbon bonded to the lactone C=O
  4. Dthe secondary alcohol O-H of the side chain

Answer: A

The compound is ascorbic acid. The OH on the ring carbon that is attached to the CH-O carbon (C3-OH, pKa about 4.2) is a vinylogous carboxylic acid: its anion is delocalised through the C=C onto the lactone C=O. The other enol OH, on the carbon attached directly to the C=O (C2-OH), has pKa about 11.6; the side-chain alcohol OH groups have pKa about 15 to 16.
Practice General Organic Chemistry II: Acids and Bases More on Most acidic hydrogen in drawn molecules
Previous questionNext question

More questions on Most acidic hydrogen in drawn molecules