Page 181 - Mechanism and Theory in Organic Chemistry

Chapter 4

BIMOLECULAR

SUBSTITUTION

REACTIONS

A large number of organic reactions, differing often in mechanism and in the
nature of the attacking reagent, are overall substitution reactions on carbon in
which Y replaces X. Equation 4.1, which, in order to be as general as possible,
ignores charges, bonding electrons, and substituent groups, is a schematized
representation of these displacements.
Y+ C-X+C-Y +X (4.1)
Nucleopliilic aliphatic substitution is the displacement from saturated carbon of a
group with its bonding electrons by a group with an extra pair of electrons
(Equation 4.2).

R R
I I
Y: + R-C-X + Y-C-R + X:
I I
R R
BI ABa ABI Ba

Since both the entering group (or nucleophile) and the leaving group are Lewis
bases, Equation (4.2) is an example of a Lewis acid-base reaction in which
one base replaces another in the Lewis acid-base adduct. Reactions corresponding
to Equation 4.2 fall mainly into four charge types.l In these the Lewis bases are
either uncharged or carry a single negative ~harge.~

' E. D. Hughes and C. K. Ingold, J. Chcm. Sot., 244 (1935).
a Some few nucleophiles such as &OS2- carry a double negative charge.