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.
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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.
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