Which Statement About Cyclooctatetraene Is Not True? A Tub-Shaped 8π Ring

Answer

Option (c) is the statement that is not true. The other three describe cyclooctatetraene as it actually is.

Check the electron count first

Cyclooctatetraene, $\mathrm{C_8H_8}$, is an eight-membered ring carrying four double bonds. If those four $\pi$ bonds were delocalised around a flat ring, the count would be $4 \times 2 = 8$ $\pi$ electrons. Eight is $4n$ with $n = 2$, not $4n + 2$, so a planar fully conjugated octagon would be an antiaromatic arrangement rather than a closed bonding shell. The molecule avoids that outcome.

(a) Two types of C–C bond — true

Crystallographic structures give two distinct distances around the ring: about 147 pm for the formally single bonds and about 134 pm for the formally double bonds. A delocalised aromatic ring would show a single averaged length instead; benzene, for comparison, has all six C–C bonds at about 139 pm. Two lengths mean the double bonds stay where they are drawn.

(b) Nonplanar ring, no ring-wide resonance — true

The ring puckers into a tub shape. In that geometry neighbouring p orbitals are no longer parallel, so they cannot overlap continuously around the ring and the $\pi$ system breaks into four separate double bonds. The spectral signature matches: a single sharp $^1\mathrm{H}$ NMR line at 5.78 $\delta$, which is an alkene-like shift rather than the downfield ring-current shift an aromatic ring produces.

(c) Extensive resonance with identical ring bonds — not true

This statement contradicts (a) and (b) directly. Extended resonance would delocalise all eight $\pi$ electrons and equalise the ring bond lengths. Neither happens: the lengths differ, the p orbitals do not overlap continuously, and the reactivity is that of an open-chain polyene rather than an aromatic ring.

(d) Non-aromatic and tub-shaped — true

The criteria for aromaticity require a planar, cyclic, fully conjugated system with $4n + 2$ $\pi$ electrons. The tub geometry fails the planarity requirement, so the molecule is classified as non-aromatic.

Two endings to avoid

“Non-aromatic” is not the same as “planar and antiaromatic.” The flat octagon is the arrangement the molecule actively avoids; the tub is what it adopts instead. And “non-aromatic” is not “unreactive” either — the tub keeps four ordinary double bonds, so the molecule still behaves as an unsaturated polyene and adds halogens across those bonds.