Reactivity: How and Why - Organic Chemistry Concepts: An EFL Approach - page 71

In this introduction to concepts, we do not discuss rearrangements in
any detail. It is enough to cover the range of addition, elimination, and
substitution, because they represent the majority of organic reactions.
For these three reaction types, the focus can be either on those involving
polar or ionic reactive species. Examples with radicals are less common
and are not needed for a general understanding of the basic principles.

5.5 REACTION MECHANISM: THE PATH FROM
REACTANT TO PRODUCT
For an organic chemist, a reaction mechanism is a detailed description of the way
bonds are broken and made as a reaction goes from starting materials to products. If
they are not part of the changes, solvents or inorganic additives are usually ignored.

A mechanism is a proposal that shows electron movements which explain all
bond breaking and making. For example, see Figure 5.9 . The main information
that a mechanism should show includes:

any reactive intermediates or transition states that occur; n

curly arrows to show all electron movements;

n

information about relative reaction rates in stepwise reactions;

n

details of any relevant stereochemical features.

n

FIGURE 5.9
A mechanism for an addition reaction.

The difference between a reactive intermediate and a transition state can be
explained as follows. A reactive intermediate is a real species in a reaction and it
can be measured. This can help to prove a suggested mechanism.

A transition state is a reasonable proposal of a species that can occur in a reac-
tion mechanism. However, this species cannot be proven by measurement.
Transition states can be suggested at any point along the reaction path which
a mechanism follows from one step to the next. Important transition states are
often at the points of highest energy during the progress of a reaction. See exam-
ples in Section 5.6 .