= Solution
\b[B is 1,3-dibromo-2-hydroxyxanthone:] both <bromines> are on the hydroxy-substituted outer ring, at its two available positions ortho to OH. Define C2 as the OH-bearing <carbon>; C1 lies between it and the <carbonyl>-side ring junction, and C3 lies on its other side. These labels specify the substitution independently of the orientation of the printed drawing.
Phenolic <lone pair> donation strongly activates its ortho and para positions towards <electrophilic substitution>. The para position is a fused junction and has no replaceable <hydrogen>, leaving C1 and C3 as the two directly activated sites. C1 also benefits from the conjugative influence of the ring <ether> <oxygen>. Although other positions receive some activation from that <oxygen>, they lack the stronger local phenolic activation of the selected pair. Excess <bromine> can therefore replace both ortho <hydrogens>. Replacing two <hydrogens> in $\mathrm{C_{13}H_8O_3}$ gives \b[$\mathrm{C_{13}H_6Br_2O_3}$], consistent with the stated composition. Addition across an outer-ring <double bond> would sacrifice <aromaticity> and would not account for the <hydrogen> loss.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-1-products.png]
{title=Original solution structures of the dibrominated xanthone, sodium enolate D and hydrogen-bonded enol E}
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