Question

In the presence of a small amount of phosphorus, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which $\alpha -$hydrogen has been replaced by halogen. This reaction is known as:
A) Hell-Volhard-Zelinsky reaction
B) Wolf-Kishner reaction
C) Etard reaction
D) Rosenmund reaction

Answer 1

The answer here is based on the basic named reaction which makes use of phosphorus and chlorine which is known generally as halogenation reaction of carboxylic acids which is named after three chemists. The answer lies in this fact.

Complete step by step answer:
We have studied in our previous classes of organic chemistry about the named reactions and also various types of reactions that are oxidation, reduction etc.
We shall now see one by one about the named reactions given in the options in detail that can lead us to the required answer.
- In the first option, the named reaction is given a name after the three chemists which were invented by them for the halogenation of carboxylic acids at the$\alpha -$carbon. The reaction can be represented as shown below,
$RC{{H}_{2}}COOH\xrightarrow[-HBr(Cl)]{B{{r}_{2}}(C{{l}_{2}})/P}RCHBr(Cl)COOH$
- Now, Wolf-Kishner reaction is a reduction reaction used to convert carbonyl compounds to alkanes which makes use of hydrazine in the presence of a strong base.
- Etard reaction is the reaction in which there is direct oxidation of aromatic or heterocyclic bound methyl groups into aldehydes that makes use of chromyl chloride as a reagent.
- Rosenmund reaction is also a reduction reaction in which the acyl chlorides are reduced to aldehydes in the presence of Lindler’s catalyst or sometimes also called as Rosenmund catalyst.
Based on all these facts, we can say that the Hell-Volhard-Zelinsky reaction yields a compound in which $\alpha -$hydrogen has been replaced by halogen.

Therefore, the correct answer is option A) Hell-Volhard-Zelinsky.

Note: Note that for the named reactions, be thorough with the reagents used and also about what type of reaction it goes through that is whether oxidation, reduction, addition or substitution. Some of these basic named reactions are to be remembered to solve these types of questions.