Q47E - Organic Chemistry (Mcmurry)

Question

Write the mechanism of the hydrolysis of cis-5,6-epoxydecane by reaction with aqueous acid. What is the stereochemistry of the product, assuming normal backside S_N2 attack?

Step-by-Step Solution

Answer

The stereochemistry of the product of acid hydrolysis of cis-5,6-epoxydecane is a racemic mixture of R,R and S,S diols.

Step 1: Acid catalyzed hydrolysis

Acid hydrolysis is chemical process in which a protic acid is used to catalyze the cleavage of chemical bond via nucleophilic substitution reaction, with the addition of element of water. Epoxides are susceptible to hydrolysis in the presence of acid catalyst since the epoxide ring has great angle strain because it has three members in the ring.

Acid-catalyzed hydrolysis of epoxides produce diols.

Step 2: Protonation

Protonation is defined as the addition of a proton to an atom, molecule or ion. It is different from hydrogenation because in protonation a change in charge of protonated species occurs, while the charge remain unaffected during hydrogenation.For example formation of ammonium group $\text{N}{{\text{H}}_4}^{+}$from ammonia ${\text{NH}}_3$

Step 3: Mechanism of the reaction

In the first step, there is protonation of epoxide oxygen, the epoxide oxygen will be protonated by the acid to form hydroxyl carbocation.

In second step, there is attack of water molecule on carbon a and carbon b in two pathways as shown below in the mechanism.

At last, the loss of proton results in the formation of racemic mixture product of acid hydrolysis.

Result

The stereochemistry of the product of acid hydrolysis of cis-5,6-epoxydecane is a racemic mixture of R,R and S,S diols as shown above in the figure.