= Solution
\b[The C–C bond anti to the activated N–O bond migrates.] This is <antiperiplanar migration in the Beckmann rearrangement>. Donation from that <sigma bond> into the developing <electron> deficiency at <nitrogen> requires the appropriate <antiperiplanar geometry>. Migration and leaving-group departure are coupled, giving a <nitrilium ion> rather than an equilibrating free <carbon> <carbocation>. B places the aromatic-side bond in the migrating orientation and therefore gives D; C places the spiro-side bond in that orientation and gives E. The two <product (chemistry)>s consequently record the <oxime> geometries. A rule based only on which group seems intrinsically more <electron> rich would miss this geometric constraint; retention of configuration in a migrating group is also consistent with concerted bond migration.
Back to article page
