Sn1 Versus Sn2 Competition

The rate-determining step of an SN1 reaction is heterolytic ionisation of the carbon-halogen bond, which generates a carbocation and a halide ion in the transition state. A highly polar protic solvent like water solvates and stabilises both of these developing charges far better than a less polar mixed solvent, lowering the energy of the ionisation transition state and accelerating the SN1 path. Moving from aqueous ethanol to pure water therefore increases solvent polarity and shifts a borderline secondary substrate toward SN1. The SN2 nucleophile-concentration option is wrong because increased solvation of charges, not concentration, dominates here, and stronger solvation actually hampers the SN2 nucleophile. The claim that water lowers the SN2 backside-attack barrier is incorrect since SN2 is disfavoured by strongly solvating protic media. The reduced-polarity SN2 option is internally contradictory because water is more polar, not less. The secondary substrate is significant here because it lies at the SN1/SN2 borderline, so changing the solvent is enough to tip the balance, whereas a clearly primary or clearly tertiary halide would stay committed to one mechanism. This illustrates the general principle that protic solvents of high dielectric constant accelerate reactions whose rate-determining step creates separated charges. This matches the NCERT discussion of solvent effects. Sanity check: more polar protic solvent stabilises ions, so the ion-forming SN1 route is favoured.