Amines are organic compounds formed from ammonia (NH3 by the replacement of one or more hydrogen atoms by hydrocarbon groups (unlike the amides where the hydrogen in ammonia is replaced by organic acid groups). Primary amines have one hydrogen atom replaced by a hydrocarbon group, secondary amines have two hydrogen atoms replaced, and tertiary amines have all three hydrogen atoms replaced. In 1850 German chemist August von Hofmann (1818-1892) discovered that heating alkyl halides with an alcoholic solution of ammonia produces a mixture of the three types of amine. This process was modified in 1890 by Hinsberg to produce separate products, based on the different reactivity and solubilities of the three forms. This was a much easier method of obtaining the required chemicals than collecting them from herring brine, where they occur naturally.
Primary amines also are manufactured by a reaction known as the Hofmann degradation which takes amides as the starting point. The amide is heated with bromine, and the resulting mixture is then treated with excess sodium hydroxide to give the required primary amine.Reduction of amides will also yield amines, as will reduction of alkyl cyanides and nitroalkenes. Secondary amines are manufactured by heating a primary amine with an alkyl halide, although care must be used to ensure that excess alkyl halide is not used. Tertiary amines are manufactured by heating an alcoholic solution of ammonia with excess alkyl halide.
The simpler amines are generally gases or liquids at temperatures just above the freezing point of water. Weak hydrogen bonding elevates their boiling points. The amines are soluble in water and they are also weak bases when in solution. The reactions of amines are very similar to those of ammonia.Combustion yields water, carbon dioxide, and nitrogen. Reaction with nitrous acid is used to distinguish between primary, secondary, and tertiary amines. When a primary amines is reacted with nitrous acid, nitrogen is produced as well as a non-gaseous product which is soluble in water (the product varies depending on the amine under test but alcohols are common). Secondary amines react with nitrous acid to give nitroso compounds, pale yellow oils that are insoluble in water. Tertiary amines dissolve in nitrous acid without evolving any gas. In general, amines are recognized by their solubility in hydrochloric acid coupled with their ammonia aroma.
The simplest amine is monomethylamine (commonly referred to as methylamine). It has the formula CH3NH2 and is a colorless, flammable gas that is soluble in water. It is manufactured by heating methanal with ammonium chloride. Methylamine is used extensively in the manufacture of herbicides and fungicides.
The simplest secondary amine is dimethylamine. It has the chemical formula CH3NHCH3 and is a colorless, flammable liquid that is made by reacting a hot solution of sodium hydroxide with nitrosodimethylaniline. This compound also is used as a fungicide and herbicide, but it is chiefly used in the manufacture of other compounds such as solvents and rocket fuels.
The simplest tertiary amine is trimethylamine. It has a chemical formula of (CH3)3N and is also a colorless, flammable liquid. It is used as a starting point in the manufacture of methanal.
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