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.