There are three main 'reactive species' in organic chemistry;

1. The Nucleophile: In general, a reactive species with an available pair of electrons. In practice, this splits neatly into two categories - neutral species with a lone pair (e.g. H₂O where the Oxygen has two lone pairs) and species with a negative charge (anionic) (e.g. OH^-, the hydroxide ion).

2. The Electrophile: In general, a reactive species that can receive a pair of electrons. Again, in practice this splits into two types - positively charged species (cationic) (e.g. H⁺ ) and species that can eject a negatively charged ion (this will be explained more fully when applied to Nucleophilic Substitution).

3. The Radical: Simply a species with a single unpaired electron. Radicals are highly reactive, but their mechanisms are slightly different to those of nucleophiles and electrophiles, so they have a section of their own. (See Radicals)

To understand organic mechanisms, you need to understand the notation used to write them. This notation primarily involves arrows to show the movement of electrons.

Curly Arrows: In order to clarify the mechanisms of organic reactions, so-called 'Curly Arrows' are used.

These arrows show the movement of a pair of electrons from one place to another, and will be used almost throughout organic chemistry. These arrows must be given a clear starting point and destination, otherwise the mechanism will look ambiguous.

A generic curly arrow

A subset of the curly arrow notation, are the '1/2 hook' arrows used in radical reactions to show the movement of a single electron. For more detail on these, see the Radicals section.

A generic '1/2 hook' arrow

Resonance Arrows: These are double headed arrows placed between resonant species to show their equivalence.

e.g. Resonance in benzene