Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 2 / B10 / a / ii / Solution

The C–O-forming transition state has a partial bond from water oxygen to C3, positive charge developing on that oxygen, and negative charge delocalizing toward the carbonyl oxygen. A compact drawing isHere the two C–C dotted connections denote redistribution of pi bonding, not cleavage of either carbon–carbon sigma bond. In a bond-order drawing retain the full sigma skeleton: C1–O is changing from double toward single, C1–C2 from single toward double, C2–C3 from double toward single, and C3–O from absent toward single. The incoming water approaches a face of the approximately planar conjugated unit and C3 begins to become tetrahedral. The total electric charge remains zero. If proton transfer to another water occurs concertedly, include a partially broken incoming-water O–H bond and a partially formed relay O–H bond; that is a solvent-assisted variant, not a different regioselectivity. The original mechanism schematic identifies these bond changes without depicting a falsely isolated free hydroxide reagent.