Summary — What This Chapter Covers
- Primary amines from a halogenoalkane and NH3 in ethanol, heated under pressure.
- Nitriles from a halogenoalkane and KCN in ethanol, heated.
- Hydroxynitriles from an aldehyde or ketone with HCN, KCN catalyst, heat.
- Nitrile hydrolysis with dilute acid, or dilute alkali then acidification, gives a carboxylic acid.
1. Primary Amines
19.1
From halogenoalkanes and ammonia
Heat the halogenoalkane with excess ammonia dissolved in ethanol, under pressure (in a sealed tube). The lone pair on the nitrogen of NH3 acts as a nucleophile and substitutes the halogen.
Amine formation
CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br
The first NH3 substitutes the Br; the second removes H+ from the nitrogen, forming NH4Br. The amine product also has a lone pair and can react with more halogenoalkane, so a large excess of ammonia is used to make the primary amine the main product. (Classifying amines as primary, secondary and tertiary is not tested at AS Level.)
Naming: the alkyl group plus “amine”, e.g. CH3CH2NH2 ethylamine, CH3CH2CH2NH2 propylamine.
Worked Example
10.9 g of bromoethane (Mr 108.9) reacts with excess ammonia. 2.70 g of ethylamine (Mr 45.0) is isolated. Calculate the percentage yield.
2. Nitriles and Hydroxynitriles
19.2
Making nitriles
Heat a halogenoalkane under reflux with KCN in ethanol. CN− is the nucleophile. The carbon chain gets one carbon longer, and the nitrile carbon counts in the name.
Nitrile formation
CH3CH2Br + KCN → CH3CH2CN + KBr (propanenitrile)
Making hydroxynitriles
Aldehydes and ketones react with HCN, with KCN as catalyst, and heat (nucleophilic addition). CH3CHO + HCN → CH3CH(OH)CN, 2-hydroxypropanenitrile.
Hydrolysis to carboxylic acids
Heat under reflux with dilute acid, or with dilute alkali followed by acidification. The –C≡N becomes –COOH and the nitrogen ends up as NH4+ (acid) or NH3 (alkali).
- CH3CH2CN + 2H2O + H+ → CH3CH2COOH + NH4+
- CH3CH2CN + OH− + H2O → CH3CH2COO− + NH3, then CH3CH2COO− + H+ → CH3CH2COOH