Chemistry Structure and Nomenclature of Amines

Topics Covered :

● Introduction
● Amines
● Structure of Amines
● Classification of Amines
● Nomenclature of Amines

Introduction :

`color{green}(text(Definition ))` : Amines constitute an important class of organic compounds derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl/aryl group(s).

`=>` In nature, they occur among proteins, vitamins, alkaloids and hormones.

`=>` Synthetic examples include polymers, dyestuffs and drugs.

`=>` Two biologically active compounds, namely adrenaline and ephedrine, both containing secondary amino group, are used to increase blood pressure.

`=>` Novocain, a synthetic amino compound, is used as an anaesthetic in dentistry.

● Benadryl, a well known antihistaminic drug also contains tertiary amino group.

● Quaternary ammonium salts are used as surfactants.

● Diazonium salts are intermediates in the preparation of a variety of aromatic compounds including dyes.

Amines :

Amines can be considered as derivatives of ammonia, obtained by replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups. For example : See fig.

Structure of Amines :

`=>` Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons.

● Nitrogen orbitals in amines are therefore, `color{red}(sp^3)` hybridised and the geometry of amines is pyramidal.

● Each of the three `color{red}(sp^3)` hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines.

● The fourth orbital of nitrogen in all amines contains an unshared pair of electrons.

`=>` Due to the presence of unshared pair of electrons, the angle `color{red}(C–N–E)`, (where `color{red}(E)` is `color{red}(C)` or `color{red}(H)`) is less than `109.5°`; for instance, it is `108^o` in case of trimethylamine as shown in Fig. 13.1.

Classification :

`=>` Amines are classified as primary `(1^o)`, secondary `(2^o)` and tertiary `(3^o)` depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule.

● If one hydrogen atom of ammonia is replaced by `color{red}(R)` or `color{red}(Ar)`, we get `color{red}(RNH_2)` or `color{red}(ArNH_2)`, a primary amine `(1^o)`.

● If two hydrogen atoms of ammonia or one hydrogen atom of `color{red}(R-NH_2)` are replaced by another alkyl/aryl(`color{red}(R)’`) group, you get `color{red}(R-NHR)’`, secondary amine. The second alkyl/aryl group may be same or different.

● Replacement of another hydrogen atom by alkyl/aryl group leads to the formation of tertiary amine.

`=>` Amines are said to be ‘simple’ when all the alkyl or aryl groups are the same, and ‘mixed’ when they are different.

Nomenclature :

`color{green}(text(Common System ))` : In this, an aliphatic amine is named by prefixing alkyl group to amine, i.e., alkylamine as one word (e.g., methylamine).

`=>` In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is appended before the name of alkyl group.

`color{green}(text(IUPAC System ))` : In this, amines are named as alkanamines, derived by replacement of ‘e’ of alkane by the word amine.

● For example, `color{red}(CH_3NH_2)` is named as methanamine.

● In case, more than one amino group is present at different positions in the parent chain, their positions are specified by giving numbers to the carbon atoms bearing `color{red}(–NH_2)` groups and suitable prefix such as di, tri, etc. is attached to the amine. The letter ‘e’ of the suffix of the hydrocarbon part is retained. For example, `color{red}(H_2N–CH_2–CH_2–NH_2)` is named as ethane-1, 2-diamine.

`=>` In arylamines, `color{red}(–NH_2)` group is directly attached to the benzene ring. `color{red}(C_6H_5NH_2)` is the simplest example of arylamine.

● In common system, it is known as aniline.

● It is also an accepted IUPAC name.

● While naming arylamines according to IUPAC system, suffix ‘e’ of arene is replaced by ‘amine’.

● Thus in IUPAC system, `color{red}(C_6H_5–NH_2)` is named as benzenamine.

`=>` Common and IUPAC names of some alkylamines and arylamines are given in Table 13.1.