Question

Bring out the following conversion-
Acetic acid to Malonic acid

Answer 1

Hint : There are two important steps in this conversion. The first step is the Hell-Volhard-Zelinsky reaction. This is a very important named reaction and helps in the addition of a halogen atom to the alpha carbon. Then we are doing a nucleophilic substitution followed by hydrolysis to obtain the final product as Malonic Acid.

Complete Step By Step Answer:
Carboxylic acids having alpha hydrogen can be halogenated at the alpha position on treatment with chlorine in the presence of a small amount of red phosphorus to give alpha-halo carboxylic acids. In this case, we are treating acetic acid with chlorine and we obtain alpha chloro-acetic acid. This reaction is known as the Hell-Volhard-Zelinsky reaction.
The presence of red phosphorus is very important to this reaction because it helps in the formation of necessary intermediates which are crucial for proceeding the reaction.
This is represented by the following equation:
$\Rightarrow C{H_3}COOH + C{l_2}\xrightarrow{{\operatorname{Re} d{\text{ P}}}}C{H_2}(Cl)COOH + POC{l_3} + HCl$
In the second part of the conversion, the alpha chloroacetic acid is treated with potassium Cyanide in ethanol. This will substitute the chloride group with a CN group. Now this will add a carbon atom more to the molecule. By hydrolyzing this in the presence of an acid we get Malonic acid.
The reaction is given by:
$\Rightarrow C{H_2}(Cl)COOH + KCN \to C{H_2}(CN)COOH + KCl$
$\Rightarrow C{H_2}(CN)COOH\xrightarrow{{{H_2}O/{H^ + }}}COOH - C{H_2} - COOH$ .

Note :
Whenever we are asked to convert a molecule with n carbon atoms to a molecule with n+1 carbon atoms we can do this by introducing a cyano group. If the opposite result is desired then we do reduction to eliminate the carbon atom in the reactant and obtain a product having less number of carbon atoms.