Propose a mechanism for the following reaction. (Hint: The rate of the reaction is much slower if the nitrogen atom is replaced by CH.)

Carbonic anhydrase is an enzyme that catalyzes the conversion of carbon dioxide to bicarbonate ion. It is a metalloenzyme, with Zn²⁺ coordinated at the active site by three histidine side chains. Propose a mechanism for the reaction.

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

b. Both trans reactants form the same racemic mixture.

Co²⁺ catalyzes the hydrolysis of the lactam shown here. Propose a mechanism for the metal-ion catalyzed reaction.

The rate constant for the uncatalyzed reaction of two molecules of glycine ethyl ester to form glycylglycine ethyl ester is 0.6 M^-1 s^-1. In the presence of Co²⁺, the rate constant is 1.5 × 10⁶ M^-1 s^-1. What rate enhancement does the catalyst provide?

The deuterium kinetic isotope effect (k_H2O/k_D2O) for the hydrolysis of aspirin is 2.2. What does this tell you about the kind of catalysis exerted by the ortho-carboxyl substituent? (Hint: It is easier to break an O–H bond than an O–D bond.)

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

a. Both cis reactants form an optically active trans product, but each cis reactant forms a different trans product.