Bonding in Carbon CompoundsIt should, perhaps, be mentioned that such delocalisation can only
occur when all the atoms in the diene are essentially in the same plane.
For in other positions, (e.g. XIV, p. 13), possible owing to rotation
about the central C—C bond, the n atomic orbitals on carbon atoms
2 and 3 would' not be parallel and could thus not overlap at all
effectively. The effect of the delocalisation that actually takes place is
thus to impose considerable restriction on rotation about the central
C—C bond, observed as a preferred orientation of the compound.(v) Benzene and aromaticiry
A somewhat similar state oNTffairs occurs with benzene. The known
planar structure of the molecule implies sp^2 hybridisation, with p- atomic orbitals, at right angles to the plane of the nucleus, on each of
the six carbon atoms (VI): •
(VIII) (VI) (V©Overlapping could, of co%rse, take place 1:2, 3:4, 5:6, or 1:6, 5:4,
3d? leading to formulations corresponding to the Kekule structures
&* (e.g. VII) but,, in fact, delocalisation takes place as with butadiene,
though to a very much greater extent, leading to a cyclic tr orbital
embracing all six carbon atoms of the ring. Other orbitals in addition
to the above are required to accommodate the total of six electrons
(cf. p. 1), but the net result is annular rings of negative charge above
and below the plane of the nucleus (VIII).
Support for this view is provided by the fact that all the carbon-
carbon bond lengths in benzene are the same, i.e. all the bonds are of
exactly the same character, all being somewhere in between double
and single bonds as is revealed by their length, 1 • 39 A. The degree of
'multiplicity' of a bond is usually expressed as the bond order, which
is one for a single, two for a double and three for a triple bond. The
relation between bond order and bond length is exemplified by a
curve of the type