AS Chemistry 9701: Preparing an Amine and a Nitrile from the Same Halogenoalkane

A student has a bottle of 1-chlorobutane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$ , and uses it to make two different nitrogen-containing products.

(a) The student heats 1-chlorobutane under pressure in a sealed tube with a large excess of ammonia, $\text{NH}_3$ , dissolved in ethanol. State the type of mechanism involved, and give the structural formula and name of the main organic product formed. [3]

(b) In a separate experiment, the student instead heats 1-chlorobutane under reflux with potassium cyanide, KCN, dissolved in ethanol. Give the structural formula and name of the organic product formed. [2]

(c) The nitrile from part (b) is heated under reflux with dilute hydrochloric acid to hydrolyse it. Give the structural formula and name of the carboxylic acid formed, and explain why this carboxylic acid has one more carbon atom than the amine formed in part (a), even though both were made from the same starting halogenoalkane. [3]

Part (a): Preparing the amine

A halogenoalkane reacts with ammonia by nucleophilic substitution: the lone pair on the nitrogen atom of $\text{NH}_3$ attacks the slightly positive carbon that bears the chlorine, displacing $\text{Cl}^-$ as the leaving group.

$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_2 + \text{NH}_4\text{Cl}$

A large excess of ammonia is used (in ethanol, heated under pressure in a sealed tube) so that the halogenoalkane is far more likely to collide with an ammonia molecule than with a molecule of the amine product; the second molecule of ammonia in the equation simply neutralises the HCl formed. The main organic product is butylamine (butan-1-amine), $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_2$ . It has the same four-carbon skeleton as the starting halogenoalkane, since $\text{NH}_3$ contributes only nitrogen and hydrogen atoms.

Part (b): Preparing the nitrile

Heating the same halogenoalkane with $\text{KCN}$ in ethanol is also a nucleophilic substitution, but here the $\text{CN}^-$ ion is the nucleophile:

$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} + \text{KCN} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN} + \text{KCl}$

The organic product is pentanenitrile, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN}$ . Note that although the reaction again simply substitutes the leaving group, the incoming nucleophile ( $\text{CN}^-$ ) itself contains a carbon atom, so the product has five carbon atoms. One more than the four-carbon halogenoalkane it was made from.

Part (c): Hydrolysing the nitrile, and comparing carbon counts

Heating pentanenitrile under reflux with dilute hydrochloric acid hydrolyses the nitrile group to a carboxylic acid:

$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COOH} + \text{NH}_4\text{Cl}$

The carboxylic acid formed is pentanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COOH}$ . It keeps the same five carbon atoms as the pentanenitrile it came from, since hydrolysis only adds the atoms of water and does not change the carbon skeleton.

Comparing the two final products from the same starting halogenoalkane:

Butylamine (from the ammonia route) has 4 carbon atoms, the same as 1-chlorobutane, because $\text{NH}_3$ contributes no carbon.
Pentanoic acid (from the cyanide/hydrolysis route) has 5 carbon atoms, because the carbon of the $\text{CN}^-$ ion used to make the nitrile becomes a new carbon atom joined onto the chain; hydrolysing the nitrile afterwards does not remove or add any further carbon atoms.

So the nitrile route is a way of extending an organic chain by exactly one carbon atom, while the ammonia route leaves the chain length unchanged.

Final answers

(a) Nucleophilic substitution; butylamine (butan-1-amine), $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_2$ .
(b) Pentanenitrile, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN}$ .
(c) Pentanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{COOH}$ ; it has one extra carbon (from the $\text{CN}^-$ ion) compared with butylamine, since $\text{NH}_3$ supplies no carbon in route (a).