5.5.2 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^-

Amine basicity

• The nitrogen atom in ammonia and amine molecules can accept a proton (H⁺ ion)
• They can therefore act as Bronsted-Lowry bases in aqueous solutions by donating its lone pair of electrons to a proton and form a dative bond

The nitrogen atom in ammonia and amines can donate its lone pair of electrons to form a bond with a proton and therefore act as a base

• The strength of amines depends on the ability of the lone pair of electrons on the nitrogen atom to accept a proton and form a dative covalent bond
• The more readily a proton is attracted, the stronger the base is
• Factors that may affect the basicity of amines include:
  • Positive inductive effect - Some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available and therefore increasing the amine’s basicity
  • Delocalisation - The presence of aromatic rings such as the benzene ring causes the lone pair of electrons on the nitrogen atom to be delocalised into the benzene ring
  • The lone pair becomes less available to form a dative covalent bond with ammonia and hence decreases the amine’s basicity
• Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so make it a stronger base
• Secondary amines are stronger bases than primary amines because they have more alkyl groups that are substituted onto the nitrogen atom in place of hydrogen atoms
  • Therefore more electron density is pushed onto the nitrogen atom (as the inductive effect of alkyl groups is greater than that of hydrogen atoms)
    

Base strength of aromatic amines

• Primary aromatic amines such as phenylamine do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring
• This means the nitrogen is less able to accept protons
• Ethylamine (which has an electron-donating ethyl group) is more basic than phenylamine (which has an electron-withdrawing benzene ring)

Ethylamine is more basic than phenylamine due to electron donating ethyl group which increases electron density on the nitrogen and makes it more attractive to protons

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⁺ + 4H₂0

• 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