Question

Convert ethyl chloride to propanoic acid.

Answer 1

To answer this question, you should recall the concept of nucleophilic substitution reaction and hydrolysis reaction. Nucleophilic substitution reaction refers to the reaction where one nucleophile replaces another. The number of carbon atoms in a chain can be increased by reaction with potassium nitrile.

Complete step by step solution:
In the case of nucleophilic substitution reactions the group which takes the electron pair and displaces it from the carbon is known as leaving the group and the molecule on which substitution takes place is known as substrate. The alkyl chloride is reacted with potassium cyanide. The cyano group replaces the chloride ion.
When nitriles are hydrolysed you can think of them reacting with water in two stages - first to produce an amide, and then the ammonium salt of a carboxylic acid.
The nitrile is instead heated with either a dilute acid such as dilute hydrochloric acid or with an alkali such as sodium hydroxide solution. In case of acidic hydrolysis of nitriles, the nitrile is heated under reflux with dilute hydrochloric acid. Instead of getting an ammonium salt as you would do if the reaction only involves water, you produce the free carboxylic acid.
The ethanoate ions in the ammonium ethanoate react with hydrogen ions from the hydrochloric acid to produce ethanoic acid. Ethanoic acid is only a weak acid and so once it has got the hydrogen ion, it tends to hang on to it. The final mechanism is:
Step 1:
$C{H_3} - C{H_2} - Cl + KCN \to C{H_3} - C{H_2} - CN + KCl$
Step 2:
$C{H_3} - C{H_2} - CN\xrightarrow{{{H_2}O/{H^ + }}}C{H_3} - C{H_2} - COOH$

Note:
You should know about the alkaline hydrolysis of nitriles. The nitrile is heated under reflux with a sodium hydroxide solution. This time, instead of getting an ammonium salt as you would do if the reaction only involves water, you get the sodium salt. Ammonia gas is given off as well. If you wanted the free carboxylic acid in this case, you would have to acidify the final solution with a strong acid such as dilute hydrochloric acid or dilute sulphuric acid. The ethanoate ion in the sodium ethanoate will react with hydrogen ions as mentioned above.