Fundamental Concepts in Organic Reaction Mechanism

To actually use the idea of a mechanism, this unit builds up a small toolkit of fundamental concepts that recur in every organic reaction from here on.

A mechanism is the theoretical, step-by-step pathway that describes exactly what changes occur at each stage of a chemical transformation. Rather than memorising each reaction as an isolated fact, an organic reaction becomes understandable -- and predictable -- once you track two things simultaneously as the reaction proceeds:

1. The direction of electron flow. Bonds break and form because electron pairs (or, less often, single electrons) move from one location to another within or between molecules.
2. The type of intermediate formed along the way -- whether the reaction passes through a free radical, a carbocation, a carbanion, or some other reactive species.

Curved-arrow notation. The convention used throughout this unit (and organic chemistry generally) to show electron movement is the curved arrow:

• A full curved arrow (a complete, double-barbed arrowhead) represents the movement of a pair of electrons. It always starts at the source of the electron pair -- a lone pair or an existing bond -- and its head points to the atom (or the new bond position) that receives the electrons.
• A half-headed (fish-hook) arrow, introduced in the next section, instead represents the movement of a single electron, used specifically for homolytic (radical) processes. …