Amines

General characteristics of amines

Amines are organic chemical compounds that are derivatives of ammonia, i.e. a molecule made of one nitrogen atom and three attached hydrogen atoms. These atoms are replaced by organic radicals. Depending on whether one, two or all of the hydrogen atoms in the ammonia molecule have been substituted, primary, secondary and tertiary amines are distinguished respectively. Each of the attached radicals can be aliphatic (as a straight or branched carbon chain) or aromatic (as a ring). Thus, amines can be additionally divided into aliphatic, aromatic and aliphatic-aromatic, i.e. mixed.

When creating the names of amines, the type and number of alkyl substituents are always mentioned, and the word “amine” is added at the end, e.g. ethylamine, methyldiethylamine. Another nomenclature system derives names from the compound in which one or more hydrogen atoms have been substituted with an amino group (taking into account the place of their substitution and the number of amino groups). Common names are also commonly used, e.g. Aniline (phenylamine).

Amines – methods of production

Amines can be produced by the following methods:

• The action of ammonia on haloalkyls. The reaction between an alkyl chloride, bromide or iodide and ammonia forms a hydrochloride (hydrobromide and hydroiodide, respectively) of the amine. It then reacts with another ammonia molecule. As a result, an amine and an ammonium compound are formed.
• The reduction of the corresponding nitro compounds. Metals such as tin, zinc or iron are used for this purpose. As a result of the reaction, they are oxidized in an acidic or alkaline environment. The reduction of aliphatic amines takes place in one step, while aromatic amines can be reduced differently, depending on the reaction conditions. This process is used for the industrial production of amines.
• The reduction of nitrobenzene. Aniline hydrochloride is formed, which in the next step reacts with sodium hydroxide. This way, aniline is obtained on a laboratory scale.
• The reaction of a carbonyl compound (containing a carbonyl group =C=O in the molecule) with ammonium methanoate.

Properties of aliphatic amines

At room temperature, lower aliphatic amines are gases and higher aliphatic mines are liquids or solids. They dissolve well in water. This property decreases as the order of the amines increases. This is due to the formation of a much larger number of hydrogen bonds between the molecules of primary aliphatic amines, compared to secondary and tertiary ones. The result of hydrogen bonds is also the high tendency of primary and secondary amines to associate. As a result, their boiling points are higher than those of saturated hydrocarbons with the same number of carbon atoms in the molecule. Some of the aliphatic amines have physical and chemical properties similar to those of ammonia. Likewise, they have an alkaline pH. They also react with acids, resulting in the formation of salts (attaching an acid molecule to an amine molecule). A large variety of features, depending on the order of aliphatic amines, allows their identification by means of specific reactions, as well as their separation from each other.

The best known aliphatic amine is methylamine, having the following formula: CH₃NH₂. It is a flammable, poisonous gas with a fishy smell. Due to its properties, it must be stored in liquefied form or as a salt, e.g. methylamine hydrochloride. It is very well soluble in water or benzene and slightly less in ethanol. Despite its toxicity, methylamine is an important component in the production of medicines, pesticides and dyes.

Properties of aromatic amines

Amines containing one or more aromatic rings in their structure are usually liquids or solids. They are highly poisonous and are characterized by an unpleasant smell. They are poorly soluble in water. In comparison to aliphatic amines, they do not show good alkaline properties. The presence of an amino group (first type substituent) makes them prone to react with other chemical compounds, because it facilitates the substitution of hydrogen atoms in the ortho- and para- position. In addition, the amino group is susceptible to oxidizing agents. It is oxidized already under the influence of weak oxidizing agents. Aromatic amines form salts with strong acids,and the salts are much less stable in comparison to the salts of aliphatic amines.

The best-known aromatic amine is phenylamine, commonly known as aniline. It is a highly poisonous compound. Its molecule is built of a benzene ring in which one of the hydrogen atoms has been substituted with an amino group. Aniline occurs in the form of an oily, colourless liquid with the characteristic smell of rotten fish. In atmospheric conditions, aniline turns brown. This effect results from the ongoing oxidation process by atmospheric oxygen. The solubility of aniline in water is low, but the compound dissolves well in ethers, alcohols and benzene.

Application of aliphatic and aromatic amines

• Amines are additives to solvents and removers.
• Methylamine is an important component in the production of pharmaceuticals.
• Tertiary amines are used as hardeners for synthetic resins, which are then used as adhesives, varnishes or insulating materials.
• Aniline is one of the components used in the production of explosives.
• Ethylamine is used in the synthesis of herbicidal compounds, also known as herbicides.
• One of the most important processes in the production of tires is their vulcanization, where, among others, diethylamine is used.