Question

i) Explain the Hell-Volhard-Zelinsky reaction with example. (2)
ii) Explain the Hofmann bromamide degradation reaction with example. (2)

Hint:

HVZ: carboxylic acid with an alpha-H plus \(X_2\) and red phosphorus gives an alpha-halo acid. Hofmann bromamide: primary amide plus \(Br_2\) and NaOH gives a primary amine with one carbon less.

Answer 1

i) Hell-Volhard-Zelinsky (HVZ) reaction:
Concept: Carboxylic acids that possess an \(\alpha\)-hydrogen atom react with chlorine or bromine in the presence of a small amount of red phosphorus to give \(\alpha\)-halogenated carboxylic acids. Only the \(\alpha\)-hydrogen is replaced by the halogen.
Example: Ethanoic (acetic) acid with chlorine and red P gives chloroacetic acid.
\[CH_3COOH + Cl_2 \xrightarrow{\text{red P}} ClCH_2COOH + HCl\]
(\(\alpha\)-chloroacetic acid). Excess halogen can give di- and tri-halogenated acids.

ii) Hofmann bromamide degradation reaction:
Concept: When an aliphatic or aromatic primary amide is treated with bromine and an aqueous (or ethanolic) solution of NaOH/KOH, it is converted to a primary amine containing one carbon atom less than the starting amide. The carbon of the \(-CONH_2\) group is lost as carbonate.
Example: Acetamide gives methanamine (methylamine).
\[CH_3CONH_2 + Br_2 + 4NaOH \rightarrow CH_3NH_2 + 2NaBr + Na_2CO_3 + 2H_2O\]
\[\boxed{\text{HVZ: }\alpha\text{-halo acid; Hofmann: amine with one C less}}\]