Specifically the substituent constant for
meta- and for
para-substituents in benzene derivatives as defined by Hammett on the basis of the
ionization constant of a substituted benzoic acid in water at
,
i.e.
), where
is the
ionization constant of a
m- or
p-substituted benzoic acid and
that of benzoic acid itself. The term is also used as a collective description for
related electronic substituent constants based on other standard reaction series,
of which,
,
and
are typical; also constants which represent dissected electronic effects such as
and
. For this purpose it might be better always to spell out the term in full, i.e. as
'
Hammett sigma constant', and restrict
-constants to the scale of substituent constants which is based on benzoic acid. A large positive
-value implies high electron-withdrawing
power by inductive and/or
resonance effect, relative to H; a large negative
-value implies high electron-releasing
power relative to H.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1171
