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How many of the following species are unstable (antiaromatic or highly reactive) at room temperature? (i) cyclobutadiene (ii) pentalene (fused bicyclic 5-membered ring system) (iii) cyclopentane (iv) cyclopentadienone (v) benzene (vi) naphthalene (vii) borole (B-H containing five-membered ring)

1. 4
2. 1
3. 2
4. 3

Correct answer: 4

Solution

Cyclobutadiene (4 pi, antiaromatic), pentalene (8 pi, antiaromatic), cyclopentadienone (cross-conjugated, behaves antiaromatic/very reactive), and borole (4 pi, antiaromatic) are all unstable. Cyclopentane is a stable saturated ring, while benzene and naphthalene are aromatic and stable.

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