Amines: Preparation - Nomenclature - Properties

Amines

Amines are compounds which are formed by substitution of hydrogen atoms in ammonia NH3 by one or more alkyl groups. Therefore, amines are named according to the number of alkyl groups they contain. If an alkyl group replaces one hydrogen of ammonia, it is called as a primary amine RNH2 ; if two alkyl groups replace two hydrogen atoms, it is called as a secondary amine R2NH ; if three alkyl groups replace three hydrogen atoms, it is called as a tertiary amine R3N.

For example:

Nomenclature of Amines

a) Special (Common) Nomenclature

1- In this method, firstly, the alkyl group bonded to nitrogen atom is written then the word amine.

For example:

2- If there are two or three alkyl groups bonded to nitrogen atom, di- or tri- prefixes are added to the word amine.

3- If the alkyl groups are different, they are written in alphabetical order.

b) Amines are named according to IUPAC system as in the following steps:

1.      The longest carbon chain which carries the amine (-NH2) group is chosen and numbered. Amine group acts as the functional group.

2.     Previous rules are valid in naming amines.

Preparation of Amines

There are many methods to prepare amines. We will explain only two of them here.

1- Reaction of Ammonia and Alkyl Halide

Using ammonia, we can prepare three types of amines.

For this, as in the example below, alcoholic ammonia solution and alkyl halide are mixed and heated.

This is not a successful laboratory method as a mixture of primary, secondary and tertiary amines which are difficult to separate are prepared. We only prepare primary amines with this method.

2- Ammonia-Alcohol Reactions

Primary amines are prepared when ammonia and alcohol react with aluminum oxide as catalyst at 400°C. In this reaction, just like the reaction above, a mix- ture of primary, secondary and tertiary amines is obtained. But when a great amount of ammonia is used, we get mainly primary amine.

Physical Properties of amines

Molecular weights of amines are small and their odors are like ammonia’s. They are in gas form at room temperature. These compounds form hydrogen bonds due to their ionization property. Therefore, their boiling points are high- er than those of compounds which cannot ionize. Figure 1

Amines can dissolve in water as they can form hydrogen bonds with water molecules. Besides, they can dissolve in organic solvents such as benzene and ether.

Figure 1

a- Hydrogen bonds between water and amine molecules.

b- Hydrogen bonds between amine molecules

Chemical Properties of amines

Amines are accepted as bases as nitrogen atom has a pair of electrons which

can make bond with protons or Lewis acids.

This salt can be transformed into amine again when it reacts with a strong base.

Besides, amines form compounds known as amides when they react with acid chlorides.

See also

1.      Alcohols: nomenclature – synthesis - Properties

2.     Alkyl halides: nomenclature - synthesis - Properties

References

1.      K. J. Denniston c J. J.Topping c and R. L.Caretc “General Organic and Biochemistry”c Mc-Graw- Hillc New York   (2004).

2.     K.W. Whittenc R.E. Davis  and L. M. Peckc “General Chemistry” 7th ed. Holt Rinehart and Winstonc New York (2010).

3.     Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.

4.     Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). ISBN 978-93-85998-89-8.