name reaction h 1

Haloform Reaction

This reaction has been used in qualitative analysis to indicate the presence of a methyl ketone. The product iodoform is yellow and has a characteristic odour. The reaction has some synthetic utility in the oxidative demethylation of methyl ketones if the other substituent on the carbonyl groups bears no enolizable α-protons.

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Mechanism of the Haloform Reaction

The reaction readily proceeds to completion because of the acidifying effect of the halogen substituents.

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Hantzsch Dihydropyridine (Pyridine) Synthesis

This reaction allows the preparation of dihydropyridine derivatives by condensation of an aldehyde with two equivalents of a β-ketoester in the presence of ammonia. Subsequent oxidation (or dehydrogenation) gives pyridine-3,5-dicarboxylates, which may also be decarboxylated to yield the corresponding pyridines.

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Mechanism of the Hantzsch Dihydropyridine Synthesis

The reaction can be visualized as proceeding through a Knoevenagel Condensation product as a key intermediate:

A second key intermediate is an ester enamine, which is produced by condensation of the second equivalent of the β-ketoester with ammonia:

Further condensation between these two fragments gives the dihydropyridine derivative:

Hell-Volhard-Zelinsky Reaction

Treatment with bromine and a catalytic amount of phosphorus leads to the selective α-bromination of carboxylic acids.

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Mechanism of the Hell-Volhard-Zelinsky Reaction

Phosphorus reacts with bromine to give phosphorus tribromide, and in the first step this converts the carboxylic acid into an acyl bromide.

An acyl bromide can readily exist in the enol form, and this tautomer is rapidly brominated at the α-carbon. The monobrominated compound is much less nucleophilic, so the reaction stops at this stage. This acyl intermediate compound can undergo bromide exchange with unreacted carboxylic acid via the anhydride, which allows the catalytic cycle to continue until the conversion is complete.

Glaser Coupling
Hay Coupling

The Glaser Coupling is a synthesis of symmetric or cyclic bisacetylenes via a coupling reaction of terminal alkynes. Mechanistically, the reaction is similar to the Eglinton Reaction; the difference being the use of catalytic copper(I), which is reoxidized in the catalytic cycle by oxygen in the reaction medium.
The related Hay Coupling has several advantages as compared with the Glaser Coupling. The copper-TMEDA complex used is soluble in a wider range of solvents, so that the reaction is more versatile.

A valuable alternative is the Cadiot-Chodkiewicz Coupling which allows the preparation of asymmetric bisacetylenes.