A Guidebook to Mechanism in Organic Chemistry

The Breaking and Forming of Bonds

H

A

H

H

the p atomic orbitals on C 2 and C 3 are no longer parallel, cannot

. therefore overlap, and delocalisation is thus prevented. Some overlap
will still take place if the orbitals are not exactly parallel, but over­
lapping, with its consequent stabilisation, decreases fairly rapidly as
the parallel position is departed from. Examples where delocalisation,
with consequent stabilisation, is actually prevented by steric factors
aitftiiscussed subsequently (p. 22).
The delocalisation that is so effective in promoting the stability of
aromatic compounds results when there are no partially occupied
orbitals of the same energy. The complete filling of such orbitals can
be shown to occur with 2+4n IT electrons, and (m electrons («= 1) is
the arrangement that occurs by far the most commonly in aromatic
compounds. lOw electrons (n—2) are present in naphthalene
(delocalisation energy, 6r kcal/mole) and 147r electrons (n = 3) in
adfnracene and phenanthrene (delocalisation energies, 84 and 91
kcal/mole, respectively) and though these substances are not mono­
cyclic like benzene, the introduction of the trans-annular bonds that
makes them bi- and tri-cyclic, respectively, seems to cause relatively
little perturbation so far as delocalisation of the n electrons over the
cyclic group of ten or fourteen carbon atoms is concerned.

A covalent bond between two atoms can essentially be broken in the

-following ways:

THE BREAKING AND FORMING OF BONDS

R:X-»-R:e+X®

R® + :Xe