Amines
Amines are important class of organic compounds which are basic in nature. They are generally derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl / aryl group(s). In nature, they are generally found in proteins, vitamins, alkaloids and hormones. Two biologically active compounds namely adrenaline and ephedrine contain secondary amino group and are used to increase blood pressure.
Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridized and the geometry of amine is pyramidal. Due to the presence of unshared pair of electrons, the angle C-N-E (where E is C or H) is less than 109.5°.
Inductive effects, solvation effects, steric effects etc. affect the basic strength of amines. Amines are classified as primary (1°), secondary (2°), and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. Amines are engaged in intermolecular hydrogen bonding. Aliphatic and aromatic primary amines show positive carbylamine reaction. Amines can be prepared from various compounds like reduction of nitro, nitriles, amides etc. compounds. A colourless crystalline solid benzene diazonium chloride can be obtained from aniline by reacting with sodium nitrite and hydrochloric acid at 273-278 K.
Identify the tertiary amine (3°) from the following compounds.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → N,N-Dimethylaniline
The passage states that amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in the ammonia molecule. So only the groups on the nitrogen atom count:
Primary (1°): R–NH2 or Ar–NH2; one H of NH3 replaced, two N–H left.
Secondary (2°): R–NH–R′; two H replaced, one N–H left.
Tertiary (3°): all three H replaced, so N carries three carbon groups and no N–H.
In N,N-dimethylaniline, \( \ce{C6H5N(CH3)2} \), the prefix N,N shows that both methyl groups are on the nitrogen. With the phenyl group, the nitrogen carries three carbon groups and no hydrogen, so it is a tertiary amine.
Why the other options are incorrect
Option (1) Ethylenediamine — Incorrect. \( \ce{H2N-CH2-CH2-NH2} \) has two –NH2 groups, each nitrogen with one carbon and two hydrogens. It is a primary diamine.
Option (3) p-toluidine — Incorrect. In 4-methylaniline the methyl group is on the ring, not on nitrogen. The nitrogen is still –NH2, so it is a primary aromatic amine.
Option (4) 2,4,6-tribromoaniline — Incorrect. The three bromine atoms are on the ring. The nitrogen keeps both hydrogens, so it is a primary amine.
NCERT (2026-27 edition):
Classification – “Amines are classified as primary (1°), secondary (2°) and tertiary (3°) 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 R or Ar , we get RNH2 or ArNH2, a primary amine (1°). If two hydrogen atoms of ammonia or one hydrogen atom of R-NH2 are replaced by another alkyl/aryl(R’) group, … You get 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.”
Table 9.1 (part):
