7.5.2 Primary Aliphatic Amines

Reactions with water

• The first few members of the homologous series of primary aliphatic amines are miscible with water
  • However as the hydrocarbon part of the molecule becomes longer, the solubility decreases
  • Phenylamine is only slightly soluble in water
• They dissolve in water as they are able to form hydrogen bonds with water molecules
• Amines also react slightly with water to form alkaline solutions

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH^-

Reactions with acids

• Amines react with strong acids to form ionic ammonium salts

CH₃NH₂ (aq) + HCl (aq) → CH₃NH₃⁺Cl^- (aq)
        Methylamine           methylammonium chloride

• Addition of NaOH to an ammonium salt will convert it back to the amine
• These ionic salts will be solid crystals, if the water is evaporated, because of the strong ionic interactions
• The ionic salts formed in this reaction means that the compounds are soluble in the acid
  • e.g. Phenylamine is not very soluble in water but phenylammonium chloride is soluble

Reactions with ethanoyl chloride

• This reaction type is addition-elimination reaction meaning two molecules join together, and then a small molecule is eliminated - in these examples, hydrogen chloride
  • You do not need to know the mechanism of these reactions
• The organic product contains a new functional group - amide - in which a carbonyl group is next to an NH group
• The equation for the reaction of butylamine with ethanoyl chloride is

CH₃COCl + CH₃CH₂CH₂CH₂NH₂ → CH₃CONHCH₂CH₂CH₂CH₃ + HCl

Reaction with halogenoalkanes

• Again you do not need to know the mechanism for these reactions
• The electron-deficient carbon atom in the halogenoalkane and the electron-rich atom nitrogen atom in the amine causes these two species to react together
• The general formula for this reaction would be

R'NH₂ + R"X → R'NHR" + HX

• Where R' is the alkyl group in the amine and R" is the alkyl group in the halogenoalkane
• This reaction is an example of a substitution reaction
• The organic product is a secondary amine and the inorganic product is a hydrogen halide, often hydrogen chloride
• As an example, the equation for the reaction of butylamine and chloroethane is

CH₃CH₂CH₂CH₂NH₂ +  CH₃CH₂Cl → CH₃CH₂CH₂CH₂NHCH₂CH₃ + HCl

• The organic product contains an electron-rich nitrogen atom, so can also react with chloroethane

  CH₃CH₂CH₂CH₂NHCH₂CH₃ +  CH₃CH₂Cl →  CH₃CH₂CH₂CH₂N(CH₂CH₃)₂ + HCl

• The organic product of this reaction is a tertiary amine
• The organic product also contains an electron-rich nitrogen atom, so can also react with chloroethane

CH₃CH₂CH₂CH₂N(CH₂CH₃)₂ +  CH₃CH₂Cl → CH₃CH₂CH₂CH₂N⁺(CH₂CH₃)₃Cl^-

• In this reaction HCl is not formed because this would require the loss of H from the nitrogen from the organic reactant, which the tertiary amine doesn't have
• The product is an ionic compound related to ammonium chloride except that all the hydrogens in the ammonium ion have been replaced by alkyl groups
  • This is known as a quaternary ammonium salt

Reactions with copper(II) ions

• Ammonia can act as a lone pair donor in its reactions with transition metal ions
• For example the overall equation for the reaction of ammonia with hexaaquacopper(II) ions is

[Cu(H₂O)₆]²⁺ + 4NH₃→ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

• Amines also have a lone pair of electrons on the nitrogen, so can take part in similar reactions
• The observations are the same as with ammonia 
  • A blue precipitate forms
  • With excess butylamine the precipitate dissolves to give a blue solution
• Formation of the pale blue precipitate

[Cu(H₂O)₆]²⁺ + 2CH₃CH₂CH₂CH₂NH₂ → [Cu(H₂O)₄(OH)₂] + 2CH₃CH₂CH₂CH₂NH₃⁺

• Formation of the deep blue solution

[Cu(H₂O)₄(OH)₂] + 4CH₃CH₂CH₂CH₂NH₂ → [Cu(CH₃CH₂CH₂CH₂NH₂)₄(H₂O)₂]²⁺ + 2H₂O +2OH^-

Preparing Amines

• Primary amines can be prepared from different reactions including:
  • The reaction of halogenoalkanes with ammonia
  • The reduction of nitriles

Reaction of halogenoalkanes with ammonia

• This is a nucleophilic substitution reaction in which the nitrogen lone pair in ammonia acts as a nucleophile and replaces the halogen in the halogenoalkane
• When a halogenoalkane is reacted with excess, hot ethanolic ammonia under pressure a primary amine is formed

Formation of primary amine

Reduction of nitriles

• Nitriles contain a -CN functional group which can be reduced to an -NH₂ group
• The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH₄ in dry ether can be used to form a primary amine

Nitriles can be reduced with LiAlH₄ or H₂ and Ni catalyst