Indian Forest Service (Mains), Chemistry optional · 2016 · Paper II
Q1(a). Aromatic, antiaromatic or non-aromatic
Question
Classify the following compounds as aromatic, antiaromatic and non-aromatic. Explain your answer.
[structures given in the paper]
Answer in brief
A antiaromatic; B non-aromatic (folded); C non-aromatic (non-planar); D aromatic .
Key points
- B would be antiaromatic only if forced flat; the real molecule is bent, like cyclo-octatetraene (tub), and behaves as a non-aromatic diene.
- C: bridging C-1 and C-6 by CH₂ removes the inner H atoms; 1,6-methano[10]annulene is planar enough to be aromatic.
- D is very unstable (an empty sp² orbital, no π overlap with the charge), but its instability is not antiaromaticity: the 6π ring is unchanged.
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.
CoversHückel testantiaromaticnon-aromaticsp² orbitalaromatic
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