Read the following passage and answer questions.

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 $-CH_3$ group, there is no staric hinderance to H-bonding. In case the alkyl group is bigger than $-CH_3$ group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from $-CH_3$ to $-C_2H_5$ 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 medium.

The most basic amine in gaseous medium is:

$(CH_3)_3N$

The correct answer is Option (3) → $(CH_3)_3N$
In the gaseous medium, solvation does not occur, so the basic strength depends mainly on the +I (electron-donating) effect of alkyl groups.

Alkyl groups push electron density toward the nitrogen atom, increasing the availability of the lone pair for protonation. The greater the number of alkyl groups attached to nitrogen, the stronger the +I effect and the higher the basicity.

Basicity Order in Gas Phase

Tertiary > Secondary > Primary > NH₃

(CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃

• (CH₃)₃N has three methyl groups → strongest +I effect → most basic.

• (CH₃)₂NH has two methyl groups → next.

• CH₃NH₂ has one methyl group.

• NH₃ has no alkyl groups → least basic.

Therefore, the most basic amine in the gaseous medium is (CH₃)₃N.