4.46 Chemical properties, preparation, and uses of amines

1. Basic properties

In amine molecules, the nitrogen atom has a lone electron pair, so amines can add a hydrogen cation H+:

or

That is, just like ammonia, amines exhibit basic properties.

Thus, when methylamine dissolves in water, a reversible reaction takes place:

It can be seen that when reacting with water, a methylamine molecule adds a hydrogen cation. This forms the methylammonium ion CH3NH3+ and the hydroxide ion OH–. The presence of OH– ions causes the alkaline reaction of an aqueous methylamine solution, so a solution of methylamine in water turns phenolphthalein crimson.

The basic properties of amines are also manifested in reactions with acids. For example, when methylamine reacts with hydrochloric acid, a salt is formed — methylammonium chloride:

Like all salts, methylammonium chloride has an ionic structure. It consists of the methylammonium cation and the chloride anion. Methylammonium chloride is a solid substance that is highly soluble in water.

or

These reactions consist in the addition of a hydrogen cation H+ to a methylamine molecule.

orAmine salts are solid substances that are highly soluble in water. Alkalis displace amines from their salts:

You encountered a similar reaction when studying inorganic chemistry, when considering the displacement of ammonia from ammonium salts by alkalis.

Structural formula of aniline:Because of the influence of the benzene ring on the amino group, some properties of aniline differ from the chemical properties of saturated amines. On the other hand, the amino group in the aniline molecule influences the properties of the benzene ring.

Reactions of the amino group

Because of the influence of the benzene ring on the amino group, the basic properties of aniline are weaker than those of saturated amines. Thus, unlike saturated amines, an aqueous solution of aniline does not change the color of indicators. At the same time, like saturated amines, aniline forms salts when reacting with acids:or

Substitution reactions in the benzene ring

In the aniline molecule, the amino group affects the properties of the benzene ring, facilitating substitution of hydrogen atoms. Thus, when a solution of aniline is mixed with bromine water, the water is observed to decolorize and a white precipitate forms. The equation for the reaction taking place:As a result of the reaction, three hydrogen atoms of the benzene ring are replaced by bromine atoms, forming 2,4,6-tribromoaniline. Note that substitution of a hydrogen atom of the benzene ring by a bromine atom is also possible for benzene, but the reaction conditions are much harsher than for aniline. Recall that a similar effect on the benzene ring is exerted by the hydroxyl group in a phenol molecule (§ 27).

Amines can be obtained from nitro compounds, which you became familiar with when studying the nitration of benzene (§ 20). The essence of the reaction for obtaining amines from nitro compounds becomes clear if we compare the formulas of nitro compounds and amines:

As can be seen, to obtain an amine from a nitro compound, the oxygen atoms in the nitro compound molecule must be "replaced" with hydrogen atoms, that is, the substance must be reduced. Let us give the scheme of such a reaction:

The scheme shown does not indicate which substance is used to reduce the nitro compound; the reducing agent here is conventionally denoted [H].

Let us consider which substances are used in practice to reduce nitro compounds.

Reduction of nitro compounds with hydrogen in the presence of a catalyst:

Reduction of nitro compounds with hydrogen at the moment of evolution. To obtain aniline by this method, nitrobenzene, concentrated hydrochloric acid, and iron filings are placed in a flask. Iron reacts with hydrochloric acid to form hydrogen, which is initially evolved in atomic form:

Atomic hydrogen has high reactivity and, without having time to combine into less active H2 molecules, reduces the nitro group of the nitrobenzene molecule:

The overall equation for the reduction of nitrobenzene by hydrogen at the moment of evolution:

Aniline is displaced from the resulting salt by the action of an alkali:

Thus, the process of obtaining aniline by reducing nitrobenzene with hydrogen at the moment of evolution can be represented by the scheme:

Interesting to know

The reaction for obtaining amines by reduction of nitro compounds bears the name of the outstanding Russian chemist Nikolai Nikolaevich Zinin (1812–1880). He not only made a significant contribution to the development of organic chemistry, but also created a brilliant school of Russian organic chemists. His students included Alexander Mikhailovich Butlerov, the founder of the theory of the structure of organic compounds, and Alexander Porfiryevich Borodin, an outstanding chemist and composer — the author of the opera «Prince Igor».

You can learn about other methods of obtaining amines by following the link in the QR code.

Amines are widely used in organic synthesis. They serve as starting materials for the production of dyes, pharmaceuticals, and many other substances.

1. Organic substance A was dissolved in water. A few drops of phenolphthalein were added to the resulting solution. As a result, the solution turned crimson. Indicate the formula of substance A.

Give the equation for the reaction that occurs when substance A dissolves in water.

2. Write the equation for the reaction between ethylamine (2 mol) and a solution of sulfuric acid (1 mol). Name the product of the reaction.

3. How can aniline be obtained using benzene as the starting substance? Write the equations for the corresponding reactions.

4. Nitrobenzene, hydrobromic acid (excess), and iron filings were placed in a flask. This produced an organic substance X, in which the mass fraction of bromine is 46.0 %.