Question

Choose the correct order of basicity of the following. \[ (a)\ (C_2H_5)_2NH \qquad (b)\ C_2H_5NH_2 \qquad (c)\ NH_3 \qquad (d)\ C_6H_5NH_2 \]

• A. \(a\gt c\gt d\gt b\)
• B. \(a\gt b\gt c\gt d\)
• C. \(d\gt a\gt c\gt b\)
• D. \(d\gt c\gt b\gt a\)

Hint:

Alkyl groups increase basicity through the \(+I\) effect, whereas resonance decreases basicity. \[ \text{Secondary amine} \gt \text{Primary amine} \gt NH_3 \gt \text{Aniline} \] because the lone pair of aniline is delocalized into the benzene ring.

Answer 1

The Correct Option is B

Basicity of an amine comes down to how freely available the lone pair on nitrogen is to grab a proton, and two effects push in opposite directions here, alkyl groups feed electron density onto nitrogen through the plus I effect, raising basicity, while conjugation with a benzene ring pulls the lone pair away into the ring, lowering basicity.

Comparing the aliphatic amines first, diethylamine has two ethyl groups donating electron density, ethylamine has only one, and ammonia has none, so purely from the electron donating effect, diethylamine is the most basic of the three and ammonia the least, giving diethylamine greater than ethylamine greater than ammonia.

Aniline is the outlier, its nitrogen lone pair is pulled into resonance with the aromatic ring, spreading it out and making it far less available for protonation than even plain ammonia, so aniline drops to the very bottom of the list.

Putting both comparisons together gives diethylamine greater than ethylamine greater than ammonia greater than aniline, that is a greater than b greater than c greater than d.

So the correct choice is option (2).