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 correct decreasing order of basic strength of amines in aqueous solution.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C2H5)2NH , (C2H5)3N , C2H5NH2 , NH3
The passage states that inductive effects, solvation effects and steric effects affect the basic strength of amines. An amine is a base because the lone pair on nitrogen accepts a proton. In water the three effects pull in different directions:
Inductive effect: alkyl groups push electrons towards nitrogen, so the lone pair is more available. More alkyl groups give a stronger effect: 3° > 2° > 1° > NH3.
Solvation: the ammonium ion formed is stabilised by hydrogen bonds to water through its N–H hydrogens. More N–H hydrogens give better stabilisation: 1° > 2° > 3°.
Steric hindrance: the ethyl group is bigger than methyl, so in the ethyl amines bulky groups crowd the nitrogen and hinder this hydrogen bonding, most of all in the 3° amine.
The balance of these effects gives the place of each compound:
(C2H5)2NH — first (strongest). Diethylamine (2°) has two electron-releasing ethyl groups and still one N–H for hydrogen bonding with water. This gives the best balance of the inductive effect and solvation (pKb 3.00).
(C2H5)3N — second. Triethylamine (3°) has the largest inductive effect, but its ion has only one N–H and the three ethyl groups hinder solvation, so it falls below the 2° amine (pKb 3.25).
C2H5NH2 — third. Ethylamine (1°) is solvated best, but it has only one ethyl group, so its lone pair is the least enriched of the three amines (pKb 3.29).
NH3 — last (weakest). Ammonia has no alkyl group at all, so it has no +I effect; all three amines are stronger bases (pKb 4.75).
NCERT (2026-27 edition):
Basic Character of Amines – “Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order. Secondly, when the alkyl group is small, like –CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:
(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3
(CH3)2NH > CH3NH2 > (CH3)3N > NH3”