SN1 Reaction: Mechanism, Carbocation Stability, Racemisation and Exam Traps

The SN1 reaction (${\mathrm{S}}_{\mathrm{N}}1$: substitution, nucleophilic, unimolecular) happens in two steps. First the leaving group departs on its own, leaving a flat carbocation; then the nucleophile captures it from either face. Only the haloalkane takes part in the slow first step, so the rate depends on its concentration alone. SN1 is fastest for tertiary, benzylic and allylic halides, whose carbocations are most stable. When the carbon that ionises is the only stereocentre, an optically active substrate gives a racemic product. It is in the Class 12 NCERT chapter on haloalkanes and haloarenes and is tested in JEE Main, JEE Advanced and NEET.

1. Mechanism

A polar protic solvent helps the C–X bond to break unevenly: it hydrogen-bonds the leaving halide ion and surrounds the carbocation. The figure follows an optically active tertiary bromide through the reaction.

1. Ionisation (slow). The C–Br bond breaks heterolytically: both electrons go with bromine as $\mathrm{B}{\mathrm{r}}^{-}$, and the carbon becomes a carbocation. This step decides the rate.
2. Capture (fast). The carbocation is sp${}^2$ and flat, with an empty p orbital above and below the plane. Water attacks the two faces with equal chance.
3. Proton loss. The oxonium ion formed loses ${\mathrm{H}}^{+}$ to the solvent, giving the alcohol. The two products are mirror images in equal amounts, so the mixture does not rotate plane-polarised light.

2. What makes SN1 fast

SN1 is as fast as its carbocation is stable. Groups that push electrons towards the positive carbon, by hyperconjugation, by +I effect or by resonance, speed it up.

Stability of carbocations. $({\mathrm{C}}_6{\mathrm{H}}_5{)}_3{\mathrm{C}}^{+}$ $>$ $({\mathrm{C}}_6{\mathrm{H}}_5{)}_2\mathrm{C}{\mathrm{H}}^{+}$ $>$ ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}{\mathrm{H}}_2{}^{+}$ and 3° $>$ 2° $>$ 1° $>$ $\mathrm{C}{\mathrm{H}}_3{}^{+}$ among alkyl cations. A benzyl or allyl cation spreads its charge by resonance, so benzyl chloride, although primary, reacts much faster by SN1 than ethyl chloride.

Ions that cannot form. In aryl and vinyl halides the C–X bond has partial double-bond character (resonance), so it is short and strong, and the cation would put its charge on a carbon with high s-character (sp${}^2$ in the phenyl cation, sp in the vinyl cation), which holds it badly. A bridgehead carbon in a small rigid ring cannot become flat. These halides do not ionise.

3. Racemisation, rearrangement and the Lucas test

3.1 Racemisation

The ionising carbon loses its 3D arrangement when it becomes flat, and attack from the two faces gives both configurations. If the molecule has a second stereocentre that is not touched, the two products are not mirror images but diastereomers, usually formed in unequal amounts, so the mixture is optically active. Compare SN2, where the attacked carbon is always inverted (see the SN2 page).

3.2 Rearrangement

A carbocation can become more stable by moving a hydride or a methyl group from the next carbon (a 1,2-shift) before the nucleophile arrives, so the product can carry the new group on a different carbon. With HBr, 3-methylbutan-2-ol gives mainly 2-bromo-2-methylbutane: the 2° cation turns into a 3° cation by a 1,2-hydride shift.

3.3 The Lucas test

Lucas reagent (concentrated HCl with anhydrous $\mathrm{ZnC}{\mathrm{l}}_2$) turns an alcohol into an insoluble alkyl chloride, which makes the solution cloudy. Tertiary and secondary alcohols react by SN1 through a carbocation, so tertiary alcohols turn cloudy at once at room temperature and secondary ones in about five minutes. Primary alcohols do not react at room temperature, except benzyl and allyl alcohols, whose resonance-stabilised cations form at once.

4. Worked examples

5. Exam traps