When the alcohol A undergoes acid-catalyzed dehydration, two isomeric alkenes are formed: B and C (see Fig. P15.67a). The relative percentage of each alkene formed is shown as a function of time in Fig. P15.67b. The composition of the alkene mixture at very long times is the equilibrium composition. Furthermore, if either alkene is subjected to the conditions of the reaction, the equilibrium mixture of alkenes is obtained.

(a) Is the dehydration a kinetically controlled or thermodynamically controlled reaction? Explain.

(b) Give a structural reason why compound C is favored at equilibrium.

(c) Suggest one reason why alkene B is formed more rapidly.

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Ph OH A (a) percent of each alkene in mixture 100 90 80 70 60 50 30 20 10 //////////------- CH3 p-toluenesulfonic acid toluene 20 40 60 time (min) Ph B 80 -CH3 alkene C alkene B 100 Ph C 120 -CH3 + H₂O (b) Figure P15.67 The relative amounts of alkene products B and C in Problem 15.67 as a function of time