Indian Forest Service (Mains), Chemistry optional · 2016 · Paper II
Q1(e). Basicity of pyrrole, triethylamine and pyridine
Question
Arrange the following in decreasing order of basicity. Give explanation to justify your answer.
[structures given in the paper]
Answer in brief
B (Et₃N) > C (pyridine) > A (pyrrole) .
Key points
- B, Et₃N: lone pair in an sp³ orbital (25 % s), pushed up by the +I effect of three ethyl groups; the cation is well stabilised.
- C, pyridine: lone pair in an sp² orbital in the ring plane, not part of the sextet; 33 % s character holds it closer to N, so it is less basic than Et₃N.
- A, pyrrole: the N lone pair is one of the six π electrons; protonation (it occurs at C-2, not N) destroys aromaticity. Pyrrole is practically non-basic.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversnitrogen lone pairsp³ orbitalsp² orbital
All parts of Question 1
- 1(a) Aromatic, antiaromatic or non-aromatic5 marks
- 1(b) Crossover experiments5 marks
- 1(c) Quinuclidine against triethylamine with ethyl iodide5 marks
- 1(d) Order of carbocation stability5 marks
- 1(e) Basicity of pyrrole, triethylamine and pyridine5 marks
- 1(f) Saytzeff and Hofmann eliminations5 marks
- 1(g) 6,6-Dimethylfulvene, phenyllithium and acetaldehyde5 marks
- 1(h) Pinacol rearrangement with ring expansion5 marks