How can I explain the mechanism of acid-catalyzed epoxidation of alkenes with peroxy acids?

Question

Ava Andrews · Accepted Answer

The reaction involves a cyclic concerted mechanism.

The Prilezhaev reaction is the reaction of an alkene with a peroxy acid such as m-chloroperoxybenzoic acid, MCPBA, to form an epoxide.

Electron-rich alkenes are more reactive.

The charge distribution in a peroxy acid puts a δ⁺ charge on the O atom of the OH group. This makes it an electrophilic atom that can add to alkenes.

The reaction proceeds via a concerted mechanism — all the steps happen at once.

We will describe them as if they were happening one after the other.

Step 1. The electrons in the π bond attack the electrophilic O atom. This forms a C-O bond and leaves a positive charge on the other C atom of the alkene.

Step 2. The O-O bond breaks, and the electrons move to form the new C=O group.

Step 3. The electrons from the old C=O group move to form an O-H bond with the H atom of the old OH group.

Step 4. The electrons from the old O-H bond move to form a C-O bond with the positively charged C atom of the alkene.

Here's a video on the epoxidation of alkenes by MCPBA.

Jacob Anderson · Answer

undefined The mechanism of acid-catalyzed epoxidation of alkenes with peroxy acids involves several steps:

1. Protonation: The peroxy acid (such as peracetic acid) is protonated by the acid catalyst (typically a strong acid like sulfuric acid), forming a protonated peroxy acid.

2. Electrophilic Attack: The protonated peroxy acid acts as an electrophile, attacking the double bond of the alkene. This leads to the formation of a cyclic intermediate called an epoxide.

3. Deprotonation: The acidic proton from the protonated peroxy acid is transferred back to the acid catalyst, regenerating the catalyst and releasing a protonated epoxide intermediate.

4. Rearrangement (if necessary): In some cases, rearrangement of the intermediate may occur to yield a more stable epoxide product.

5. Deprotonation: The protonated epoxide intermediate is deprotonated by a base (often water or the conjugate base of the acid catalyst), resulting in the formation of the final epoxide product.

Overall, the acid-catalyzed epoxidation of alkenes with peroxy acids involves the generation of an electrophilic species from the peroxy acid, which reacts with the alkene to form the epoxide intermediate. Deprotonation steps then lead to the formation of the final epoxide product.