Before diving into the four named mechanisms, it pays to build the vocabulary that all of them share, because once you truly understand these four terms, the mechanisms almost predict themselves. Every polar organic reaction is a story of electron-rich meeting electron-poor, and these terms just name the players.

A nucleophile ("nucleus-loving") is electron-rich — it has a lone pair or a pi bond it is willing to donate, and it seeks out positive charge. Anions (like OH⁻, CN⁻, RO⁻) and neutral molecules with lone pairs (water, amines) are nucleophiles. An electrophile ("electron-loving") is electron-poor — it has a partial or full positive charge and wants electrons. In this chapter the electrophile is typically a carbon bearing a leaving group, because that carbon is made δ+ by the electronegative group attached to it. The reaction is simply the nucleophile donating its electrons to the electrophilic carbon.

The leaving group is the piece that departs, taking a bonding pair of electrons with it. The single most important property of a good leaving group is that it is a weak base — because a weak base is stable and content once it leaves, holding the electron pair comfortably. This connects directly back to acid–base chemistry (Track 4): the best leaving groups are the conjugate bases of strong acids. Halides (I⁻, Br⁻, Cl⁻) are good leaving groups; hydroxide (OH⁻) and amide (NH₂⁻) are terrible ones because they are strong, unstable bases. This is why you can't just kick –OH off an alcohol directly — you first have to convert it into something that leaves willingly.

With those pieces named, there are two fundamentally different things a nucleophile/base can do to a substrate carrying a leaving group:

Substitution — the nucleophile attacks the carbon and replaces the leaving group. One group swaps for another; the carbon skeleton is unchanged. Abbreviated S.
Elimination — instead of attacking carbon, the reagent acts as a base and removes a proton from the carbon adjacent to the leaving group; the leaving group departs, and a new pi bond (double bond) forms between those two carbons. Abbreviated E.

The great challenge of this chapter — and a favorite exam theme — is that substitution and elimination compete: the same reagent can often do either, and you must predict which dominates. The outcome hinges on four factors that recur through the next three tracks: the structure of the substrate (1°/2°/3°), the strength of the nucleophile/base, the nature of the leaving group, and the solvent. Keep those four dials in mind; the whole SN1/SN2/E1/E2 decision reduces to reading them — the same four dials that governed the mechanisms in the last two tracks.

"Who Wins"