is characteristic of the reagent (i.e. a measure of its nucleophilicity) and
is characteristic of the substrate (i.e. a measure of its sensitivity to the nucleophilicity of the reagent). A scale of
values is based on the rate coefficients
for the reaction of methyl bromide with nucleophiles in water at
,
being defined as 1.00 for these reactions and
being defined as 0.00 for the hydrolysis of methyl bromide. (Other scales have been devised.)
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1169