What Happens During a Nucleophilic Substitution Reaction?

In a nucleophilic substitution reaction, a nucleophile donates an electron pair to an electron-deficient atom, forming a new covalent bond while another bond breaks and releases a leaving group. The nucleophile therefore replaces the leaving group.

This principle is assessed at both SL and HL in Reactivity 3.4, although detailed mechanisms for reactions of halogenoalkanes are additional higher level content.

The Mechanism

A nucleophile is an electron-rich species that forms a bond by donating both bonding electrons. It may have a negative charge, such as $\text{OH}^{-}$, or contain a lone pair, such as $\text{NH}_3$.

In a halogenoalkane, the halogen is more electronegative than carbon, so the carbon-halogen bond is polar:

$$\text{C}^{\delta+}-\text{X}^{\delta-}$$

The partially positive carbon is the electrophilic centre. The nucleophile donates an electron pair to this carbon, while the carbon-halogen bond undergoes heterolytic fission: both bonding electrons move to the halogen, which becomes the leaving group.

The overall reaction can be represented as:

$$\text{Nu}^{-} + \text{R-X} \rightarrow \text{R-Nu} + \text{X}^{-}$$

For example, hydroxide ions can replace chlorine:

$$\text{CH}_3\text{Cl} + \text{OH}^{-} \rightarrow \text{CH}_3\text{OH} + \text{Cl}^{-}$$

The reaction is called substitution because one atom or group is exchanged for another; the carbon framework is retained. This differs from addition, where atoms are added across a multiple bond without replacing a leaving group. This distinction helps classify organic reactions.

A common misconception is that the nucleophile donates only one electron. It donates a complete electron pair, producing a new covalent bond.

Exam Technique

For an explain question, identify the nucleophile, electrophilic centre, and leaving group, then describe both bond formation and heterolytic bond breaking. At HL, curly arrows must begin at a lone pair or bond and end where the electron pair moves.