The SN1 Mechanism: Unimolecular Nucleophilic Substitution

The $S_N1$ mechanism (Substitution, Nucleophilic, Unimolecular) dominates for tertiary, and for benzylic or allylic, haloalkanes reacting under conditions of a weak nucleophile and a polar protic solvent. Unlike $S_N2$, it proceeds in two distinct steps.

Step 1 (slow, rate-determining): ionisation. The $\text{C--X}$ bond breaks heterolytically without any assistance from the nucleophile, the halogen departing as $\text{X}^-$ and leaving behind a carbocation -- a planar, $sp^2$-hybridised carbon with an empty $p$ orbital exposed equally on both faces of the former stereocentre. Because this step involves breaking a bond with no compensating bond formation, it has a high activation energy and is intrinsically slow; it alone determines the overall rate.

Step 2 (fast): nucleophilic capture. The nucleophile, which may be a comparatively weak one such as water or an alcohol, attacks the flat carbocation. Since the empty $p$ orbital is equally accessible from either face, attack occurs from both faces with roughly equal probability.

Kinetics. Because the nucleophile plays no part in the slow, rate-determining first step, the observed rate depends only on the concentration of the substrate: $\text{rate} = k[\text{RX}]$, first order overall and independent of nucleophile concentration -- doubling the nucleophile's concentration has essentially no effect on the rate.

Stereochemistry -- racemization. Starting from a single, optically pure enantiomer, capture of the planar carbocation from both faces with near-equal probability produces a roughly 50:50 mixture of both configurations at that carbon -- that is, the product is largely racemic, and the original optical activity is mostly lost. (In practice a slight excess of the inverted product is often observed, because the departing $\text{X}^-$ can briefly shield one face as an ion pair before fully separating, but the dominant, textbook-level outcome is racemization.) …