The chemistry of amines - Crunch Chemistry

Amines are examples of organic nitrogen compounds and we can think of them as substituted ammonia molecules since they have very similar chemical properties – they may be soluble in water and they can behave as a base, a ligand or a nucleophile.

Primary amines have a nitrogen atom bonded to one alkyl (or aryl) group and two hydrogen atoms.

Secondary amines have a nitrogen atom bonded to two alkyl (or aryl) groups and one hydrogen atom.

Tertiary amines have a nitrogen atom bonded to three alkyl (or aryl) groups.

And diamines are named with the amine as a side group.

Amines behaving as bases

The nitrogen atom in an amine has a lone pair of electrons which it can use to accept a proton (the definition of a base) by making a dative covalent / coordinate bond.

Amines react with acids to form salts.

e.g. CH₃NH₂ + HCl ⇾ CH₃NH₃⁺Cl^– (the salt is methylammonium chloride)

In turns out that amines are stronger bases than ammonia, with the exception of phenylamine which is a very weak base. Alkyl groups are electron donating as they exert a positive inductive effect. The result is that the lone pair of electrons on nitrogen is more available for making a dative covalent bond to a proton and the cation formed is stabilised in amines compared with ammonia.

In contrast, the lone pair of electrons on the nitrogen in phenylamine is less available for making dative covalent bonds compared with ammonia. The lone pair of electrons is in a p orbital which overlaps with the p orbitals that make up the delocalised ring of electrons (∏ bonding system) in the benzene and so the lone pair of electrons are also delocalised.

It is worth noting that although phenylamine is a very weak base, it is considerably more reactive than benzene because the extra electron density in the benzene ring makes it more open to attack by electrophiles.

Amines behaving as ligands

Both amines and diamines will behave as ligands donating the lone pair of electrons on the nitrogen atoms to a central metal ion.

Amines behaving as nucleophiles

The reaction between a halogenoalkane and excess hot ethanolic ammonia under pressure (in a sealed tube) is a nucleophilic substitution reaction that forms a primary amine.

NH₃ + CH₃CH₂Br ⇾ CH₃CH₂NH₂ + HBr

The excess ammonia reacts with the HBr formed preventing the HBr from behaving as a nucleophile and reversing the reaction.

NH₃ + HBr ⇾ NH₄Br

In a similar way, the reaction between a halogenoalkane and a primary amine acting as the nucleophile will result in the formation of a secondary amine.

CH₃NH₂ + CH₃Br ⇾ CH₃NHCH₃ + HBr

Other methods of preparing an amine

reduction of a nitrile

We can make a nitrile by refluxing a halogenoalkane with potassium cyanide in ethanol (the halogen is replaced by the nitrile group, CN, which is also a useful way to add another carbon to the alkyl chain).

The nitrile is reduced to an amine by either reacting with hydrogen in the presence of a nickel catalyst or by using a powerful reducing agent such as lithium tetrahydridoaluminate, LiAlH₄, in dry ether.

CH₃CH₂CN + 4[H] ⇾ CH₃CH₂CH₂NH₂

Note that we can use the shorthand [H] for the reducing agent just as we have often used [O] for an oxidising agent.

reduction of an amide

Once again, the reducing agent is lithium tetrahydridoaluminate, LiAlH₄, in dry ether.

The preparation and key reactions of phenylamine are discussed in the context of making an azo dye.

Practice questions

Methylamine reacts with ethanoyl chloride to produce a secondary amide, N-methylacetamide, and a salt, methylammonium chloride.

CH₃COCl + 2CH₃NH₂ ⇾ CH₃CONHCH₃ + CH₃NH₃⁺Cl^–

Explain how methylamine is behaving as both a base and a nucleophile in this reaction. You may include an illustration in your answer,

Propylamine reacts with water to produce a solution with a pH > 7. Write an equation for the reaction and explain why the solution is not neutral.

(a) Complete the diagram below to show the mechanism for the reaction between butylamine and 2-bromopropane.

(b) Name the mechanism.

Butylamine can be formed from either a halogenoalkane or a nitrile.

(a) Write an equation to show the formation of butylamine from a halogenoakane, giving the reagents and conditions required.

(b) Write an equation to show the formation of butylamine from a nitrile, giving the reagents required.

Answers

Methylamine is behaving as a nucleophile when it uses the lone pair of electrons n the nitrogen atom to attack the 𝛿+ C atom of the carbonyl group in the acyl chloride. The lone pair is donated to form a new covalent bond between the carbon and the nitrogen atoms.

The by-product of the reaction is hydrogen chloride which reacts with a second molecule of methylamine in an acid-base reaction. Methylamine is behaving as a base because it accepts a proton / H⁺ from HCl.

NOTE: You can find the details of this mechanism and read up on the reactions of acyl chlorides here 😊.