Electrophilic and Nucleophilic Substitution in Aromatic SystemsMore forms can also be written in each case in which the positive
charge is now delocalised over the other ring, leading to a total of
seven forms for the a-intermediate as against sue for the /?-, but the
above, in which the second ring retains intact, fully delocalised n
orbitals, are probably the most important and the contrast, between
two contributing forms in the one case and one in the other, corres
pondingly more marked.
The sulphonatioa^jf naphthalene is found to lead to almost
compJit? a-substitutifln at 80° but to approximately 85 per cent
/J-substitution at 160°. This is due to the fact that the rate of /?-
substitution is essentially negligible below ca. 110° but a-substitution,
although very rapid, is reversible and as the /J-sulphonic acid is
thermodynamically more stable than the a- (primarily due to the large
S0 3 H group occupying a less hindered position in the former),
kinetic ot rate control of product (-»•<») at low temperatures gives
place to thermodynamic control of product (->-j3-) at higher tempera
tures (cf. p. 220). That this is the real explanation is confirmed by
heating the a-sulphonic acid with H 2 S0 4 when an ^ mixture contain
ing largely the j3-acid is obtained, the detailed evidence being against
a mere /w/romolecular rearrangement having taken place. • ;
The possibility of the charge becoming more widely delocalised
in the naphthalene intermediate, as compared with benzene, would
lead us to expect more ready electrophilic attack on naphthalene which
is indeed observed. J>
Pyridine (XXIV), like benzene, has six w electrons (one being
supplied by nitrogen) in delocalised w orbitals but, unlike benzene,
the orbitals will be deformed by being attracted towards the nitrogen
state froma-substitution than that from 0- attack (cf. benzene with an
o/p-directive substituent):