Q1(b). Crossover experiments

Answer in brief

A crossover experiment uses doubly labelled partners: crossed products ⇒ intermolecular; none ⇒ intramolecular (or a caged pair).

Key points

• Design. Two substrates that differ only in a label on both halves (A–B and A*–B*) react together in one flask. The labels must not change the rate.
• Reading. Crossed products (A–B*', A*–B') mean the fragments were separate in solution: intermolecular. Their absence means intramolecular.
• Fries rearrangement: with a mixture of two aryl esters (or an ester with added anisole), crossed hydroxy-ketones are found under many conditions, so at least part of the acyl transfer is intermolecular (a free acylium ion).
• Claisen rearrangement: two differently labelled allyl aryl ethers heated together give no crossed products: a concerted [3,3] shift (intramolecular).