Question

In the $\alpha $ halogenation of aliphatic acids (HVZ reaction), the catalyst used is:
(a) Zn
(b) P
(c) $ FeC{{l}_{3}}$
(d) $AlC{{l}_{3}}$

Answer 1

Hint: This reaction is given by only those carboxylic acids which contains the alkyl group and the catalyst used in such is that which belongs to the 15th group of the periodic table and belongs to the nitrogen family and is a non-metal with atomic number as 15. Now identify it.

Complete step by step answer:Organic compounds containing the -COOH as the functional group are called as the carboxylic acids. The -COOH group which itself is made up of a carbonyl group (C=O)and a hydroxyl group (-OH) is called as the carboxyl group carboxylic acids are said to be aliphatic if the carboxyl group is attached to the alkyl group and it is said to be the aromatic if the carboxyl group is attached to the aryl group.
Carboxylic acids can be further classified as monocarboxylic acid ( if it contains one carbonyl group), dicarboxylic (if contains two carbonyl groups),tricarboxylic (if it contains three carbonyl groups) and so on.
Now, let’s discuss the Hell-Volhard Zelensky reaction. In this reaction, the carboxylic acids react with chlorine or bromine in the presence of small quantities of red phosphorus to give alpha-choro or alpha-bromo acids( the carbon to which the carboxylic group is attached is called the alpha carbon). This reaction is given by only those carboxylic acids which contain the alkyl group i.e. formic acid (HCOOH) do not give this reaction. The reaction occurs as:
     \[\begin{matrix}
   {{C}^{\beta }}{{H}_{3}}{{C}^{\alpha }}{{H}_{2}}COOH \\
   propionic\text{ }acid \\
\end{matrix}\xrightarrow[-HBr]{B{{r}_{2}},red\text{ }P}\begin{matrix}
   {{C}^{\beta }}{{H}_{3}}{{C}^{\alpha }}HBrCOOH \\
   \alpha -Bromopropionic\text{ }acid \\
\end{matrix}\]

Thus, the catalyst used in the HVZ reaction is phosphorous.
So, the correct answer is “Option B”.

Note: Carboxylic acids have higher boiling point because of their stronger H-bonding and melting points of these carboxylic acids are higher which contains even number of carbon atoms and most of the carboxylic acids in the solid, liquid and vapour state exist as cyclic dimers.