Q43E - Organic Chemistry (Mcmurry)

Question

Treatment of trans-2-chlorocyclohexanol with NaOH yields 1,2-epoxycyclohexane,but reaction of the cis isomer under the same conditions yields cyclohexanone. Propose mechanisms for both reactions, and explain why the different results are obtained.

Step-by-Step Solution

Answer

The mechanism of trans-2-chlorocyclohexanol is shown below:

The mechanism of cis-2-chlorocyclohexanol is shown below:

Step 1: Anti-periplanar

In organic chemistry, anti-periplanar and syn-periplanar are a pair of terms used to describe chemical bond geometry of a molecule. The anti-periplanar confirmation is a periplanar confirmation in which the dihedral angle between two atoms or group of atom is between 150 and 180^o. Groups are anti-coplanar in anti-periplanar confirmation.

Step 2: Treatment of trans -2-chlorocyclohexanol with NaOH

The trans-2-chlorocyclohexanol has the –Cl and –OH in the trans orientation. The abstraction of proton from alcohol by base gives the alkoxy anion. This allow epoxide formation because the internal nucleophilic attack by the alkoxide ion to displace chloride ion occurs in the next step to yield the epoxide ion. 

The mechanism of the reaction is given below:

Step 3: Treatment of cis -2-chlorocyclohexanol with NaOH

The cis-2-chlorocyclohexanol don’t have –Cl and –OH in the trans orientation. Hence, epoxidation is not possible. The base picks up the proton that leads to the expulsion of chloride through E² elimination mechanism to yield an enol. The enol then tautomerize to keto group yielding cyclohexanone.

The mechanism of the reaction is given below:

                Mechanism of cis-2-chlorocyclohexanone

In the trans isomer, the-OH and –Cl are in the trans orientation that allow epoxide formation to occur whereas in cis isomer epoxidation can’t occur instead of E² elimination by base, because it produces an enol, which tautomerizes to cyclohexanone. Hence, different results are obtained.