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Chapter 16: Problem 45

The following bicyclic ketone has two \(\alpha\)-carbons and three \(\alpha\)-hydrogens. When this molecule is treated with \(\mathrm{D}_{2} \mathrm{O}\) in the presence of an acid catalyst, only two of the three \(\alpha\)-hydrogens exchange with deuterium. The \(\alpha\)-hydrogen at the bridgehead does not exchange.

Short Answer

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Short Answer: When a bicyclic ketone with two α-carbons and three α-hydrogens is treated with D2O in the presence of an acid catalyst, only two of the three α-hydrogens exchange with deuterium because the base has a hard time accessing the bridgehead hydrogen due to steric hindrance. This restriction prevents enolization at the bridgehead position, leading to selective deuteration where only two α-hydrogens are replaced with deuterium.

Step by step solution

01

Understanding the given bicyclic ketone

Look at the structure of the bicyclic ketone - it has a carbonyl group, two α-carbons (adjacent to the carbonyl group), and three α-hydrogens. We also know that the bridgehead hydrogen does not participate in the reaction.
02

Analyzing the deuterium exchange reaction

When the bicyclic ketone is treated with D2O in the presence of an acid catalyst, an enolization reaction occurs where the α-hydrogens (except for the bridgehead hydrogen) exchange with deuterium from D2O. The acid catalyst protonates the carbonyl oxygen, making the carbonyl carbon more electropositive and more susceptible to attack by a nucleophile (a qualified base). The base reacts with these α-hydrogens and forms an enolate intermediate, which then reacts with D2O, substituting the α-hydrogen with deuterium.
03

Understanding why the bridgehead hydrogen does not exchange

The bridgehead hydrogen does not participate in the reaction because it is in a more sterically hindered environment compared to the other two α-hydrogens. The base would have difficulty accessing the bridgehead hydrogen due to the crowded nature of this position, preventing enolization of the molecule at that site. This steric hindrance restricts the reaction from happening at the bridgehead position, and therefore, only two of the three α-hydrogens are able to exchange with deuterium.
04

Writing the overall reaction

The overall deuterium exchange reaction can be written as follows: Ketone + D2O + Acid Catalyst -> Transient Enolate Intermediate -> Deuterated Ketone (2 of the 3 α-hydrogens replaced with deuterium) This reaction demonstrates the selective deuteration of the bicyclic ketone, where only two of the three α-hydrogens are replaced with deuterium, and the bridgehead hydrogen remains unchanged due to steric hindrance.

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Most popular questions from this chapter

Both \((S)\)-citronellal and isopulegol are naturally occurring terpenes (Section 5.4). When \((S)\)-citronellal is treated with tin(IV) chloride (a Lewis acid) followed by neutralization with aqueous ammonium chloride, isopulegol is obtained in \(85 \%\) yield. (a) Show that both compounds are terpenes. (b) Propose a mechanism for the conversion of \((S)\)-citronellal to isopulegol. (c) How many stereocenters are present in isopulegol? How many stereoisomers are possible for a molecule with this number of stereocenters? (d) Isopulegol is formed as a single stereoisomer. Account for the fact that only a single stereoisomer is formed.

Both 1,2 -diols and 1,3 -diols can be protected by treatment with 2-methoxypropene according to the following reaction. (a) Propose a mechanism for the formation of this protected diol. (b) Suggest an experimental procedure by which this protecting group can be removed to regenerate the unprotected diol.

A primary or secondary alcohol can be protected by conversion to its tetrahydropyranyl ether. Why is formation of THP ethers by this reaction limited to primary and secondary alcohols?

Write structural formulas for all aldehydes with the molecular formula \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}\), and give each its IUPAC name. Which of these aldehydes are chiral?

The base-promoted rearrangement of an \(\alpha\)-haloketone to a carboxylic acid, known as the Favorskii rearrangement, is illustrated by the conversion of 2 -chlorocyclohexanone to cyclopentanecarboxylic acid. (a) Propose a mechanism for base-promoted conversion of 2 -chlorocyclohexanone to the proposed intermediate. (b) Propose a mechanism for base-promoted conversion of the proposed intermediate to sodium cyclopentanecarboxylate.
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