Arrange the following in the decreasing order of basic strength in gas phase:

(A) $C_2H_5NH_2$
(B) $(C_2H_5)_2NH$
(C) $(C_2H_5)_3N$
(D) $NH_3$

Choose the correct answer from the options given below:

(C), (B), (A), (D)

The correct answer is Option (4) → (C), (B), (A), (D)

Basic strength depends on the availability of the lone pair of electrons on nitrogen to accept a proton.

In the gas phase, solvation effects are absent. Therefore, the basicity mainly depends on the electron donating inductive (+I) effect of alkyl groups.

Alkyl groups increase electron density on nitrogen and increase basicity. More alkyl groups attached to nitrogen lead to stronger electron donation.

Thus the order of basic strength becomes: Tertiary amine > Secondary amine > Primary amine > Ammonia

Applying to the given compounds

(C) (C₂H₅)₃N → tertiary amine

(B) (C₂H₅)₂NH → secondary amine

(A) C₂H₅NH₂ → primary amine

(D) NH₃ → no alkyl group

Decreasing order of basic strength

(C₂H₅)₃N > (C₂H₅)₂NH > C₂H₅NH₂ > NH₃

Thus, (C), (B), (A), (D)