Detailed and comprehensive notes on unit 3.3.11 (amines) for AQA A level chemistry. Covers: amine nomenclature, base properties of amines, nucleophilic substitution with ammonia, reduction of nitriles, formation of aromatic amines, acylation reactions and quaternary ammonium salts. Includes mechani...
Specification Points
Preparation
Content:
• Primary aliphatic amines can be prepared by the reaction of ammonia with halogenoalkanes and by
the reduction of nitriles.
• Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of
dyes.
Base Properties
Content:
• Amines are weak bases.
• The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.
Skills:
• Explain the difference in base strength in terms of the availability of the lone pair of electrons on the
N atom.
Nucleophilic Properties
Content:
• Amines are nucleophiles.
• The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form
primary, secondary, tertiary amines and quaternary ammonium salts.
• The use of quaternary ammonium salts as cationic surfactants.
• The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides
and acid anhydrides.
Skills:
• Outline the mechanisms of
o These nucleophilic substitution reactions
o The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl
chlorides.
,Naming Amines
Amines
• Amines are organic compounds containing an –NH2 group.
• They derive from ammonia where an alkyl or aryl group substitutes for one or more hydrogen atoms
of NH3.
Note: A C=N bond will effectively count as 2 carbons bonded to the nitrogen.
Note: Quaternary ammonium salts are derived from ammonia but not classed as amines.
Naming Rules
• If the amines is the highest priority functional group, use the suffix –amine, e.g. propan-1-amine.
• If there is a higher priority functional group, use the prefix amino-, e.g. 2-aminopropanoic acid.
Secondary Amines:
• The longest carbon chain attached to the nitrogen is named as the parent.
• Shorter chains are named as N-substitutions, e.g. N-methylethanamine.
Tertiary Amines:
• The longest carbon chain attached to the nitrogen is named as the parent.
• Shorter chains are named as N-substitutions.
• 2 N-prefixes are used to show both N-substitutions.
• The prefixes are named alphabetically, e.g. N-ethyl-N-methylpropan-1-amine.
• N,N-di- is used for identical R-groups, e.g. N,N-diethylphenylamine.
, Amine Base Properties
Amines as Bases
• Primary aliphatic amines act as Bronsted-Lowry Bases because the lone pair of electrons on the
nitrogen is readily available for forming a dative covalent bond with a H+ ion, accepting a proton.
• Ammonia in solution exists in equilibrium.
• NH3 + H2O ⇌ NH4+ + OH-
• The position of equilibrium lies far to the left as NH3 is a weak base.
Reactions with Acids
• Primary, secondary and tertiary amines will react with acids to form ammonium salts:
o Primary: CH3NH2 + H+ ! CH3NH3+
o Secondary: (CH3CH2)2NH + H+ ! (CH3CH2)2NH2+
o Tertiary: (CH3CH2)3N + H+ ! (CH3CH2)3NH+
• These reactions will typically form ionic salts, e.g. CH3NH2 + HCl ! CH3NH3+Cl-
• The ionic salts formed means that the compounds are soluble in acid and can have high
melting/boiling points.
Factors Affecting Base Strength
1. The ease at which the lone pair picks up a proton.
2. The stability of the ions formed.
Aliphatic Amines
• Alkyl groups are referred to as electron donating, or
they have a ‘positive-inductive effect’.
• They tend to cause a small additional build up of
negative charge on the nitrogen atom
• This makes the lone pair even more attracted to
protons, or in other words, more readily donated.
• Once formed, the methylammonium ion will
be more stable than an ammonium ion due
to the inductive effect.
• These factors make primary aliphatic amines
better bases than ammonia.
• Furthermore, secondary aliphatic amines will be better bases than primary aliphatic amines because
they have more alkyl groups that are substituted onto the N atom in place of H atoms.
• Therefore more electron density is pushed onto the N atom
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