Question

What is the expected order of basic strength of different compounds from following (in gaseous phase)?

• A. $R_{3}N<R_{2}NH<R-NH_{2}<NH_{3}$
• B. $NH_{3}<R-NH_{2}<R_{2}NH<R_{3}N$
• C. $R_{2}NH<R_{3}N<R-NH_{2}<NH_{3}$
• D. $NH_{3}<R_{3}N<R_{2}NH<R-NH_{2}$

Hint:

Gas Phase $\rightarrow$ Only Induction matters $\rightarrow$ $3^\circ > 2^\circ > 1^\circ > \text{NH}_3$.
Aqueous Phase $\rightarrow$ Solvation and Sterics ruin the fun! $\rightarrow$ Generally $2^\circ$ wins, followed by $1^\circ$ or $3^\circ$ depending on the specific alkyl group (e.g., Methyl is $2 > 1 > 3$, Ethyl is $2 > 3 > 1$). Always check the phase!

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Basic strength of amines depends on the availability of the lone pair of electrons on the nitrogen atom.

Step 2: Formula Application:
In the gaseous phase, the only factor that matters is the inductive effect (+I effect) of the alkyl groups ($R$).

Step 3: Explanation:
Alkyl groups are electron-donating. More alkyl groups increase the electron density on Nitrogen, making it a better electron donor (base). Therefore, the order is: Tertiary ($3^{\circ}$) > Secondary ($2^{\circ}$) > Primary ($1^{\circ}$) > Ammonia.

Step 4: Final Answer:
The order is $NH_3 < R-NH_2 < R_2NH < R_3N$.