CBSE 2025, Set 2

Question

**Read the passage and answer:**

Amines have a lone pair of electrons on nitrogen atom due to which they behave as Lewis base. Greater the value of K$_b$ or smaller the value of pK$_b$, stronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. But it does not occur in a regular manner as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in aqueous solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron releasing groups such as –CH₃, –OCH₃, –NH₂ etc., increase the basicity while electron-withdrawing substituents such as –NO₂, –CN, halogens etc., decrease the basicity of amines.

(a) Arrange the following in the increasing order of their basic character. Give reason:

![Three anilines: aniline, 2,4-dinitroaniline, 4-methylaniline](https://www.edvaya.com/images/chemistry/cbse/2025/set2/q29a-anilines-basic-order.webp)

(b) Why is pK$_b$ of aniline more than that of methylamine?

Edvaya · Accepted Answer

The correct answer is Subjective. **(a) Increasing order of basic character:** $$\underbrace{	ext{2,4-Dinitroaniline}}_{	ext{least basic}} methylamine (aniline is less basic):** **Higher pKb = weaker base.** Aniline is a much weaker base than methylamine because: In aniline, the lone pair on nitrogen is **delocalised into the benzene ring** through resonance, making it less available for accepting a proton. In methylamine (CH₃NH₂), the lone pair is localised on N and the +I effect of CH₃ increases electron density on N. Therefore, methylamine is a much stronger base than aniline.

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Chemistry > Organic Chemistry > Amines

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