If methanol rather than water is added at the end of a...

Name: Problem 5, Chapter 17: Organic chemistry with biological applications
Uploaded: 2020-06-12T19:30:55-08:00
Duration: 1 min 19 s
Channel: Grigoriy Sereda
Description: If methanol rather than water is added at the end of a Hell-Volhard-Zelinskii reaction, an ester rather than an acid is produced. Show how you could carry out the following transformation, and propose a mechanism for the ester-forming step.

If methanol rather than water is added at the end of a Hell-Volhard-Zelinskii reaction, an ester rather than an acid is produced. Show how you could carry out the following transformation, and propose a mechanism for the ester-forming step.

Key Concepts

Hell?Volhard?Zelinskii Reaction

The Hell?Volhard?Zelinskii reaction is a well?established method for selectively halogenating the alpha position of carboxylic acids. By using a phosphorus reagent (often PBr3 or similar) in combination with a halogen, the carboxylic acid is temporarily converted into a more reactive intermediate that facilitates substitution of an alpha hydrogen by a halogen. This reaction is an important example of how functionalization at the alpha?carbon can be achieved under controlled conditions.

Alpha Halogenation of Carboxylic Acids

Alpha halogenation refers to the process of replacing an alpha hydrogen (a hydrogen on the carbon adjacent to the carboxylic group) with a halogen atom. In the context of the Hell?Volhard?Zelinskii mechanism, the reaction proceeds via the formation of a reactive intermediate that allows the halogen to substitute at the alpha position. This transformation is crucial as it opens pathways for further synthetic manipulation of the molecule.

Esterification via Nucleophilic Acyl Substitution

Esterification by nucleophilic acyl substitution involves an alcohol, such as methanol, attacking an electrophilic acyl intermediate (often generated in situ from a carboxylic acid). The nucleophilic attack on the carbonyl group leads to a tetrahedral intermediate, which subsequently collapses with the expulsion of a suitable leaving group (like a halide) to form an ester. This mechanism highlights the impact that changing the nucleophile (from water to an alcohol, for example) can have on the final product of the reaction.

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