Preparing Amines & Explaining their Basicity (AQA A Level Chemistry): Video

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The two go together because an amine is ammonia but with a hydrogen atom replaced with another group, and that group and number of hydrogens substituted decides both how the amine is prepared and how strong a base it is.

Amines can be thought of as derivatives of ammonia, in which one or more of the hydrogens is replaced by an alkyl or an aryl group. If the group is an alkyl group the amine is aliphatic; if it is an aryl group the amine is aromatic. That same group also sets the basicity: an alkyl group donates electron density to the nitrogen and makes the amine a stronger base, while a benzene ring draws the lone pair into itself and makes the amine weaker.

We start with what an amine is and how amines are classified. Then the two routes to a primary aliphatic amine, and the four-step route to an aromatic one. Then why amines act as bases at all, and what makes one stronger than another.

Amines are derivatives of ammonia, in which one or more of the hydrogens is replaced by an alkyl or aryl group. The number of substituted hydrogens is the basis for classifying them as primary, secondary or tertiary.

That classification is not the same as in alcohols and haloalkanes, where primary, secondary and tertiary are based on the substituents on the carbon atom rather than the nitrogen atom.

If the R group is an alkyl group, such as methyl or ethyl, the amine is aliphatic. If it is an aryl group, a benzene ring or phenyl, it is aromatic. Aliphatic and aromatic amines share similar chemical reactions, but the aryl group can strongly influence the chemistry and reactivity of the amine group.

Primary aliphatic amines can be prepared in two ways.

The first is the reaction of a halogenoalkane with ammonia. This is a nucleophilic substitution reaction, in which the nitrogen lone pair in ammonia acts as a nucleophile and replaces the halogen. The halogenoalkane is reacted with excess hot ethanolic ammonia under pressure to give a primary amine. Without excess ammonia, a mixture of secondary and tertiary amines and a quaternary ammonium salt is formed, so it is the excess ammonia that maximises the primary amine.

The second is the reduction of a nitrile. Nitriles contain a CN group, which can be reduced to an -NH2 group, either by passing the nitrile vapour and hydrogen gas over a nickel catalyst, or by using LiAlH4 in dry ether.

Both routes put an alkyl group on the nitrogen, which is what makes these amines aliphatic.

Phenylamine is a benzene ring with an amine group attached, and it is produced in a four-step synthesis.

Benzene is nitrated with concentrated nitric acid and concentrated sulfuric acid at 25 to 60 degrees Celsius to form nitrobenzene. The nitrobenzene is reduced with hot tin and concentrated hydrochloric acid under reflux, giving an acidic mixture. Sodium hydroxide is added to that mixture to form phenylamine, and the phenylamine is separated from the reaction mixture by steam distillation.

Aromatic amines, prepared by the reduction of nitro compounds, are used in the manufacture of dyes. The group on the nitrogen here is an aryl group, which is what makes this amine aromatic.

The nitrogen atom in ammonia and in amine molecules can accept a proton. It donates its lone pair of electrons to the proton and forms a dative bond, and that is what makes amines bases. Amines react with hydrochloric acid to form amine salts, such as ethylammonium chloride. Like ammonium salts, amine salts are soluble ionic compounds, so they are water soluble when the alkyl group is small and become less so as the carbon chain increases.

The strength of an amine as a base depends on how readily that lone pair attracts a proton. Alkyl groups are electron releasing so donate electron density to the nitrogen, so the lone pair on the nitrogen becomes more available and the basicity increases. A benzene ring does the opposite: the lone pair on the nitrogen is delocalised into the ring, and becomes less available, so the basicity decreases. Ethylamine, with its electron-donating ethyl group, is more basic than phenylamine, which has an electron-withdrawing benzene ring.

When describing and comparing basicity you must say where the lone pair that becomes more or less available is; it is on the nitrogen.

Amines have a fishy smell, which disappears as soon as you add an acid, because you are making a non-volatile salt. Decomposing fish give off amines, which are produced by the bacterial decomposition of proteins.

An amine is ammonia with one or more hydrogens replaced by an alkyl or an aryl group, and the number replaced classifies it as primary, secondary or tertiary.

Primary aliphatic amines come from a halogenoalkane with excess ethanolic ammonia, or from reducing a nitrile. Aromatic amines come from nitrating benzene and then reducing the nitrobenzene.

Amines are bases because the nitrogen lone pair accepts a proton. An alkyl group makes that lone pair more available and the amine more basic; a benzene ring makes it less available and the amine less basic.

The group on the nitrogen sets both the route and the strength.

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