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Q46.

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Organic Chemistry
Found in: Page 452

Short Answer

Question: Tautomerization in base resembles tautomerization in acid, but deprotonation precedes protonation in the two-step mechanism. (a) Draw a stepwise mechanism for the following tautomerization. (b) Then, draw a stepwise mechanism for the reverse reaction, the conversion of the keto form to the enol.

Answer

a.

Base-catalyzed conversion of enol to the keto form

b.

Mechanism for the reverse reaction

See the step by step solution

Step by Step Solution

Step-by-Step SolutionStep 1: Definition of tautomerization

Tautomerization is a kind of constitutional isomerism in which the isomers differ from each other in the position of a double bond and a hydrogen atom. The two tautomers exist in equilibrium with each other.

One tautomer exists in the enol form, and the other exists in the keto form. The equilibrium usually favors the formation of the keto form because the -C=O bond is much stronger than the carbon-carbon double form.

Step 2: Mechanism for the tautomerization in base

In the tautomerization of the base-catalyzed keto form, the hydrogen next to the carbonyl group is removed by the hydroxide ion to produce the enolate ion. In the next step, the water reacts with the enolate ion to result in the formation of the enol.

a. The stepwise mechanism for the given reaction is as follows.

Base-catalyzed conversion of enol to the keto form

b. The stepwise mechanism for the reverse reaction, i.e., the conversion of the keto form to the enol, involves the following two steps:

Mechanism for the reverse reaction

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