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Chapter 22: Q15P (page 1163)

Propose a mechanism for the acid-catalyzed bromination of pentan-3-one.

Short Answer

Expert verified

The halogenation of ketones is catalyzed by acid. The ketone is dissolved in acetic acid, which serves as both the solvent and the acid catalyst.

Acidic halogenation can selectively replace just one hydrogen or more than one, depending on the amount of the halogen added.

Step by step solution

01

Acid-Catalyzed Alpha Halogenation

The halogenation of ketones is catalyzed by acid. The ketone is dissolved in acetic acid, which serves as both the solvent and the acid catalyst.

Acidic halogenation can selectively replace just one hydrogen or more than one, depending on the amount of the halogen added.

02

Mechanism of Acid-catalyzed bromination of pentan-3-one

Under acid catalysis, the ketone( pentan-3-one) is in equilibrium with its enol form. The enol acts as a weak nucleophile, attacking bromine to give a resonance-stabilized intermediate. Loss of a proton (deprotonation) results in the formation of the product 2-bromopentan-3-one.

Formation of 2-bromopentan-3-one

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Most popular questions from this chapter

The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each case, work backward from the target molecule to an aldol product, and show what compounds are needed for the condensation.)

The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound reacts with an aldehyde or ketone, in the presence of a secondary amine as a basic catalyst, to produce a new C=C. Show the starting materials that made each of these by a Knoevenagel condensation.

Question:Show how you would use the Robinson annulation to synthesize the following compounds.

C.

Show how you would use the malonic ester synthesis to make the following compounds

(a) Although the following compound is a substituted acetone derivative, it cannot be made by the acetoacetic ester synthesis. Explain why (two reasons).

(b) The use of LDA to make enolate ions (Sections 22-2B and 22-3) has provided alternatives to the acetoacetic ester synthesis. Show how you might make the compound shown in part (a), beginning with 1,3-diphenylacetone.

(c) Enamine reactions (Section 22-4) occur under relatively mild conditions, and they often give excellent yields of compounds like the one shown in part (a). Show how you might use an enamine reaction for this synthesis, beginning with 1,3-diphenylacetone.
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