Factors affecting Electron-availability in BondsThus the nitro group in nitrobenzene lowers the density of nega
tive charge over the nucleus, as compared with benzene itself: it is an
electron-witMrawing group, \f contrast to the negatively charged
oxygen atom in the phenoxide ion (XXI), which is an electron-
donating group. Because of the presence of an electron-withdrawing
group, nitrobenzene will be less readily attacked than benzene itself
by positive ions or electron-deficieUt reagents (oxidising agents such
as KMn0 4 , for example) which, as will be seen below (p. 101), are
.exactly the type of reagents involved in normal aromatic substitution
reactions. •
Mesomeric, like inductive, effects are permanent polarisations in
the ground state of the molecule and are therefore manifested in the
physical properties of the compounds in which they occur. The
essential difference between inductive and mesomeric effects is that
the former occur essentially in saturated groups or compound»j*he
latter in unsaturated and, especially, conjugated compounds. The
former involve the electrons in a bonds, the latter those in n bonds
^pd orbitals. Inductive effects are transmitted over only quite short
distances in saturated chains before dying away, whereas mesomeric
effects may be transmitted from one end to the other of quite large
molecules provided that conjugation (i.e. delocalised n orbitals) is
present through which they can proceed. Either effect influences the
behaviour of compounds in both essentially static and dynamic
situations: in both the position of equilibria and rates of reaction, in
the strength of acids and bases as much as in the reactivity of alkyl
halides or the relative ease of substitution of different aromatic
species.
(iii) Time-variable effects
Some workers have sought to distinguish between effects such as the
two considered above which are permanent polarisations manifested
in the ground state of the molecule and changed distributions of