2-Phenylethyl tosylate in which the carbon bearing OTs is labelled with $^{13} C$ , i.e. $C X_{6} H X_{5} - CH X_{2} - X_{13} X_{2}^{213} CH X_{2} - OTs$ , is solvolysed in acetic acid. Assume that all of the product forms through the bridged phenonium ion (no direct SN2). What fraction of the product 2-phenylethyl acetate has the $^{13} C$ label on the carbon bonded to oxygen?

Answer: D

The phenyl ring bridges both carbons in the phenonium ion, which has a mirror plane that makes the two

CH X_{2}

groups equivalent. Acetate opens the ring at either carbon with equal probability, so the label ends up on the

CH X_{2} - OAc

carbon in 50 % of the product and on the benzylic carbon in the other 50 %.