Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 2 / B9 / b / i

Fluoride favors geminal disubstitution: after the first substitution, the second F replaces the other chloride at that same phosphorus, giving a $\mathrm{P}{\mathrm{F}}_2$ unit and two unchanged $\mathrm{PC}{\mathrm{l}}_2$ units. The first highly electronegative F withdraws electron density inductively, maintaining or increasing the electrophilicity of its phosphorus and activating the neighboring P–Cl bond toward another nucleophile. The small fluoride ion also has little steric hindrance to attack there. In this context geminal substitution means both incoming substituents on one phosphorus; it does not describe the relative positions of different ring nitrogen atoms. Electronic activation at the substituted phosphorus competes with effects elsewhere in the ring, so the argument predicts the preferred pathway rather than an unconditional exclusive product.