19. Nitrogen compounds
- Syllabus
- 9701–2028–2029
- Section
- 19
- Level
- AS
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Topic 19.1
19.1 Primary amines
Objectives in this topic
Amines can be prepared by heating a halogenoalkane with ammonia in ethanol
Ammonia acts as a nucleophile and replaces the halogen in a halogenoalkane. Heating under pressure in ethanol gives an amine and hydrogen halide, which can react further if ammonia is not in excess.
Use excess ammonia to favour the primary amine and note that detailed amine classification is outside AS scope. The carbon skeleton of the halogenoalkane is retained.
CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br. The second ammonia molecule accepts HBr formed in the substitution.
Ammonia is not simply added across a double bond, and the pressure/solvent conditions are part of the preparation.
Topic 19.2
19.2 Nitriles and hydroxynitriles
Objectives in this topic
KCN substitution makes a nitrile and increases the carbon chain by one
CN⁻ attacks a halogenoalkane in ethanol on heating, replacing X and forming a nitrile, R–C≡N. The cyanide carbon becomes part of the product skeleton.
This route is useful for chain extension: count the starting carbon atoms and add one for the nitrile carbon. The reaction is nucleophilic substitution, not elimination.
CH₃CH₂Br + KCN → CH₃CH₂CN + KBr. Bromoethane has two carbons; propanenitrile has three.
Do not write HCN as the carbon source or forget the extra carbon in the product.
HCN adds to aldehydes and ketones to form hydroxynitriles
Cyanide attacks the carbonyl carbon and protonation gives a molecule containing both –OH and –C≡N: a hydroxynitrile. KCN helps generate CN⁻ and heat supports the reaction.
The product has one more carbon than the carbonyl compound because the nitrile carbon is added. Aldehyde and ketone starting structures lead to different substitution patterns.
CH₃CHO + HCN → CH₃CH(OH)CN. The new carbon bears OH, CN, H and CH₃.
Hydroxynitrile formation is not nitrile hydrolysis and does not remove the carbonyl carbon.
Nitrile hydrolysis converts –C≡N into a carboxylic acid after acidification
Dilute acid or dilute alkali hydrolyses a nitrile through amide/carboxylate stages. Acid work-up gives the carboxylic acid; alkaline conditions initially give a carboxylate salt.
The nitrile carbon becomes the carboxyl carbon, so the chain is retained after the earlier KCN extension. State whether the reaction mixture is acidic or alkaline at the final step.
CH₃CH₂CN + 2H₂O → CH₃CH₂COOH + NH₃ under acid hydrolysis (simplified overall equation). In alkali, propanoate forms first and is then acidified.
Do not lose the nitrile carbon or report a carboxylate as the acid before acidification.