Vinylogy the α-carbon immediately adjacent to the carbonyl, as would a simple enolate. Allylic electrophiles often react by vinylogous attack of a nucleophile rather than direct addition. A further example of vinylogous reactivity: ascorbic acid (Vitamin C) behaves as a vinylogous carboxylic acid by involvement of its carbonyl moiety, a vinyl group within the ring, and the lone pair on the hydroxyl group acting as the conjugated system. Acidity of the hydroxyl proton at the terminus of the vinyl group in ascorbic acid is more comparable to a typical carboxylic acid than an alcohol because two major resonance structures stabilize the
Chemistry of ascorbic acid titration with an oxidizing agent, and several procedures have been developed. The popular iodometry approach uses iodine in the presence of a starch indicator. Iodine is reduced by ascorbic acid, and, when all the ascorbic acid has reacted, the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end-point of the titration. As an alternative, ascorbic acid can be treated with iodine in excess, followed by back titration with sodium thiosulfate using starch as an indicator. This iodometric method has been revised to exploit reaction of ascorbic acid with iodate and iodide in