A Guidebook to Mechanism in Organic Chemistry

Types of Reaction

Nucleophiles *

HO®, RO®, RS®, Hal®, HS0 3 e, QCN, R CfeC®, eCH(COjEt),

x / '

(XLI)

RMgBr, RLi, LiAlH., *f^\jQ®

*

Where a^eagent is starred^the star indicates the atom that accepts
electrons from, or donates electrons to, the substrate as the case may
be. It rapidly becomes apparent that no clear distinction can be made
between what constitutes a reagent and what a substrate, for though
HNOs, eOH, etc., are normally<hought of as reagents, the diethyl
malonate carbanion (XLI) could, at will, be either reagent or sub­
strate, when reacted%ith, for example, an alkyl halide. The reaction
of the former on the latter is a nucleophilic attack, while that of the
"fatter on the former would be looked upon as an electrophilic attack;
but from the standpoint of whichever reactant a reaction itself is
viewed, its essential nature is not for a moment in doubt.
It should not be forgotten, however, that reaetions involving free
radicals as the reactive entities are also known. These are muab less
susceptible to variations in electron density in the substrate than are
reactions involving polaj intermediates, but they are greatly affected
by the addition of small traces of substances that either liberate or
remove radicals'. They are considered in detail below (p. 240).

* TYPES OF REACTION

Within this classification there are essentially four kinds of reaction

which organic compounds can undergo:

(a) Displacement (or substitution) reactions.

(b) Addition reactions.

(c) Elimination reactions.

(d) Rearrangements.

In (a) it-is displacement from carbon that is normally referred to but

the atom displaced can be either hydrogen or another atom or group.

In electrophilic substitution reactions it is often hydrogen that is