= Solution
\b[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 is
$$
\left[\mathrm{R{-}C1(\overset{\delta-}{O})\cdots C2H\cdots C3H(R')\cdots\overset{\delta+}{OH_2}}\right]^\ddagger.
$$
Here 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.
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