Q1(c). Neighbouring nitrogen: aziridinium ion

Introduction. Both compounds are secondary chlorides on a six-membered ring, and in both the C–Cl bond is drawn axial. The difference is the ring nitrogen of (I), two carbons away from C–Cl: its lone pair acts as an internal nucleophile (neighbouring-group participation, anchimeric assistance).

Scheme: The ring N displaces chloride from the back side (intramolecular SN2); water opens the strained three-membered ring.

1. In (I) the lone pair on N lies anti-periplanar to the axial C3–Cl bond, behind C-3. As Cl⁻ leaves, N bonds to C-3 from the back: an intramolecular SN2 step that forms a bicyclic aziridinium ion (1-phenyl-1-azoniabicyclo[3.1.0]hexane). This ionisation is the rate-determining step.
2. The internal attack has a large entropy advantage: the nucleophile is already held next to the reaction centre, so no bimolecular collision is needed. The transition state is reached far more easily than unassisted ionisation, which explains the factor of 10⁴.
3. (II) has no internal nucleophile. It must ionise to a secondary carbocation without help (SN1), or be attacked by weakly nucleophilic water (SN2); both are slow for a secondary ring chloride.
4. Water then opens the aziridinium ion: at C-3 it gives 1-phenylpiperidin-3-ol with retention (two inversions); at the ring CH₂ it gives the ring-contracted (1-phenylpyrrolidin-2-yl)methanol. A mixture of six- and five-membered products is the chemical evidence for the aziridinium intermediate.
5. The N-phenyl group lowers the nucleophilicity of N (lone pair partly delocalised into the ring), yet the participation still dominates; with N-alkyl groups the acceleration is even larger.

Conclusion. The ring nitrogen of (I) assists ionisation by forming an aziridinium ion (anchimeric assistance), so (I) is hydrolysed about 10⁴ times faster than cyclohexyl chloride (II), which has no neighbouring group.

Application. The same N-participation makes nitrogen mustards (RN(CH₂CH₂Cl)₂, e.g. cyclophosphamide metabolites) potent alkylating anticancer drugs: the aziridinium ion alkylates N-7 of guanine in DNA.