Amines

Derivatives of ammonia in which one or more of the hydrogen atoms in the ammonia molecule have been replaced by alkyl or aryl groups

• Ammonia is a pyramidal molecule with bond angles of approximately 107 degrees. • This is due to the repulsion caused by the long pair on the nitrogen atom. • Amines keep this basic shape

• Amines are polar molecules • Primary amines can hydrogen bond to one another using their -NH2 groups (in the same way as alcohols with their -OH groups). • However, as nitrogen is less electronegative than oxygen, the hydrogen bonds are not as strong as those in alcohols. • This means that the boiling points of amines are lower than those of comparable alcohols. • Shorter chain amines are gases at room temperature whilst those with slightly longer chains are volatile liquids.

• Primary amines with chain lengths up to about four carbon atoms are very soluble in both water and alcohols because they form hydrogen bonds with these solvents. • Most amines are also soluble in less polar solvents.

• Amines have a lone pair of electrons which is important to how it reacts. • The lone pair may be used to form a bond with: 1) A H+ ion, when the amine is acting as a base 2) An electron-deficient carbon atom, when the amine is acting as a nucleophile.

• Amines can accept a proton (H+ ion). This means that they are Brønsted-Lowry bases.

• Amines react with acids to form salts. • The products are ionic compounds that will crystallise as the water evaporates.

By the addition of a strong bases such as sodium hydroxide that removes the proton from the salt and regenerates the insoluble amine.

How readily it accepts a proton

• How many alkyl groups are attached to the nitrogen atom • This is because alkyl groups have a inductive effect which increases base strength • The more alkyl groups present, the stronger the base is.

• Alkyl groups release electrons away from the alkyl groups and towards the nitrogen atom. • This is called the inductive effect. • The inductive effect of the alkyl group increases the electron density on the nitrogen atom and therefore makes it a better electron pair donor.

They are less soluble in water

• Aryl groups withdraw electrons from the nitrogen atom because the lone pair of electrons overlaps with the delocalised system on the benzene ring. • This makes the nitrogen as weaker electron pair donor and therefore less attractive to protons • This means arylamines are weaker bases than other amines and also ammonia

The lone pair of electrons from an amine will attack positively charged carbon atoms, meaning it will act as a nucleophile.

• Primary aliphatic amines are produced when halogenoalkanes are reacted with ammonia • This is mucleohilic substitution by NH2.

• The primary amine is also a nucleophile and this will react with the halogenoalkane to produce a secondary amine. • The secondary amine will then react as a nucleophile to give a tertiary amine which will in turn react to produce a quarternary ammonium salt. • This means that the nucleophilic substitution of ammonia and a halogenoalkane will produce a mixture of primary, secondary and tertiary amines and a quarternary ammonium salt.

• A mixture of primary, secondary and tertiary amines and a quarternary ammonium salt is produced. • Therefore it is not a very efficient way of preparing an amine, though the products may be separated by fractional distillation. • A large excess ammonia gives a better yield of primary amine.

An amine that only has alkyl and hydrogen- not aryl groups

In the first stage it acts as a nucleophile and in the second stage it acts as a base.

• The reduction of nitriles. • Cyanide ions are used to form the nitrile by nucleophilic substitution. • This nitrile is then reduced.

Step 1) Halogenoalkanes react with the cyanide ions in aqeous ethanol. The cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile Step 2) Nitriles contain the functional group C-(triple bond)-N. They can be reduced to primary amines with a nickel hydrogen catalyst.

A purer product is produced because only the primary amine can be formed.

1) Benzene is reacted with a mixture of concentrated nitric and concentrated sulfuric acid which produces nitrobenzene. 2) Nitrobenzene is reduced to phenylamine, using tin and hydrochloric acid as the reducing agent.

• The tin and hydrochloric acid react to form hydrogen, which reduces the nitrobenzene by removing oxygen atoms of the NO2 group and replacing them with hydrogen atoms. • Since the reaction is carried out in hydrochloric acid, the salt (C6H5NH3)+ Cl- is formed. • Sodium hydroxide is added to liberate the free amine.