22.4: Alpha Halogenation of Aldehydes and Ketones

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A particularly common α-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their α positions by reaction with Cl₂, Br₂, or I₂ in acidic solution. Bromine in acetic acid solvent is often used.

Acetophenone reacts with bromine in acetic acid to form alpha-bromoacetophenone in 72 percent yield.

Remarkably, ketone halogenation also occurs in biological systems, particularly in marine alga, where dibromoacetaldehyde, bromoacetone, 1,1,1-tribromoacetone, and other related compounds have been found.

The figure shows three structures, dibromo acetaldehyde, bromoacetone, and 1,1,1 tribromoacetone. These are labeled as from the Hawaiian alga Asparagopsis taxiformis.

This form of halogenation is a typical α-substitution reaction that proceeds by acid-catalyzed formation of an enol intermediate, as shown in Figure 22.4.

Figure 22.4 MECHANISM Mechanism of the acid-catalyzed bromination of acetone.

Four-step reaction mechanism for formation of bromoacetone from acetone and bromine with acid catalyst: protonation of carbonyl, abstraction of alpha proton to enol, addition of bromine on alpha carbon, deprotonation.

Evidence for the mechanism shown in Figure 22.4 includes the observation that acid-catalyzed halogenations show second-order kinetics and follow the rate law

$Reaction rate = k [Ketone] [H^{+}]$

In other words, the rate of halogenation depends only on the concentrations of ketone and acid and is independent of halogen concentration. Since halogen is not involved in the rate-limiting step, chlorination, bromination, and iodination of a given substrate all occur at the same rate.

Furthermore, if an aldehyde or ketone is treated with D₃O⁺, the acidic α hydrogens are replaced by deuterium. For a given ketone, the rate of deuterium exchange is identical to the rate of halogenation, implying that a common intermediate—presumably the enol—is involved in both processes.

Ketone or aldehyde reacts slowly with hydronium ions to form enol intermediate, which reacts fast with trideuteriooxidanium ion and X 2 to form the product with D and X, respectively.

α-Bromo ketones are useful in the laboratory because they can be dehydrobrominated by base treatment to yield α,β-unsaturated ketones. For example, 2-methylcyclohexanone gives 2-bromo-2-methylcyclohexanone on halogenation, and the α-bromo ketone gives 2-methyl-2-cyclohexenone when heated in pyridine. The reaction takes place by an E2 elimination pathway (Section 11.8) and is a good method for introducing a $C═C Section 7.6).$

2-methyl cyclohexanone reacts with bromine in acetic acid to form 2-bromo-2-methyl-cyclohexanone, which when heated reacts with pyridine to form 2-methyl-2-cyclohexenone in 63 percent yield.

Write the complete mechanism for the deuteration of acetone on treatment with D₃O⁺.

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