MECHANISM INVOLVING THE OPENING OF EPOXIDES

Do you know that reactions of epoxides follow S_N²   mechanism due to their steric effect?. Epoxides are also known as cyclic ether. Epoxides are organic compounds that can be obtained by the reaction of alkenes with per acids such as per acetic acid, per benzoic acid etc as shown below;

Preparation of epoxides

NOTE:  CH₃COOH can be replaced by oxygen but at minimal quantity. The reaction above is known as epoxidation reaction.

To get a proper understanding of this reaction we want to discuss, students needs to farmiliarize themselves with S_N²   mechanism. It is paramount to know that the reaction of epoxides involving chain opening is of two types namely;

1. acid catalyzed
2. base catalyzed

It is also important to know that the knowledge of homolytic and heterolytic bond cleavage is needed for proper understanding of this reaction mechanism we are about to study.

MECHANISM OF THE REACTION

Now we want to discuss the mechanism of the reaction below so that students will have a clearer understanding of ring opening reactions of epoxides. Taking the reaction below as a focus point.

Incomplete epoxide reaction equation

How can we predict the product of this reaction?

Observing the epoxide above closely, we can see that there is steric hinderance. We should also know that the rection above is an acid catalyzed reaction.
Fistly, the NaOH undergoes heterolytic bond cleavage so as to produce OH^- which is the nucleophile. the H₃O⁺ also undergoes heterolytic bond cleavage to produce H+ in the rection system as shown below;

Heterolytic cleavage of sodium hydroxide

Heterolytic cleavage of hydronium ion

secondly, since the oxygen connecting the two carbon atom is a very electronegative atom, it will inductively withdraw electron from the carbon that is more substituted as shown below, so the carbon is loosely bonded to the oxygen as illustrated below; the oxygen then carries a partial negetive charge while the carbon carries a partial positive charge as illustrated in the overall reaction equation (see below).

Opening epoxide ring

Students should always note that there is no formation of carboncation in S_N²   mechanism,what we have is a transition state where the oxygen atom is loosely bonded to the more substituted carbon and the hydroxyl ion (nucleophile) attack at the same time, making the reaction a second order reaction. so we can say that both the nucleophile and epoxide are involved in the rate determining step.

Thirdly, the OH^- (i.e the nucleophile) then attacks the more substituted atom at the opposite side of the ring and finally push the oxygen away as shown below;

Attack of the epoxide by hydroxyl ion (nucleophile)

we can also observe from the reaction above that there is inversion of configuration due to backside attack (that is as the nucleophile attacks, the  CH₃ bonded to the carbon in concern moves up). This inversion is taking place due to steric hinderance (denoting S_N²   mechanism). we should also put in mind that one of the basic characteristics of S_N²   reaction is inversion of configuration.

Lastly, the H+ from the hydronium ion then attacks the negatively charged oxygen to complete the product as illustrated below;

Attack by hydrogen ion 

In conclusion..........
for a reaction to follow S_N² mechanism, a good nucleophile is needed to push the leaving away, so as to increase the rate of the reaction at rate determining step. another condition to consider is the kind of solvent to be used so as to increase rate of reaction.

Therefore, for S_N² mechanism, a polar solvent will favour it due to the type of bond cleavage (i.e Heterolytic bond cleavage) that occur.

    OVERALL REACTION

It is very important to know that one of the major difference between  S_N¹ and S_N²   reaction is that,  S_N¹ reaction involves the formation of carbocation which can still rearrange (either by hydride shift or hyperconjugation) to give a more suitable product while in S_N²  reaction, a transition state is observed.

STUDY QUESTIONS

predict the product of the following chemical reaction by showing the step by step mechanism.


        1.

       2.

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