Electrophilic Addition Mechanism

As we can see in organic chemistry, an electrophilic addition reaction occurs when a double or triple bond in a chemical molecule is broken, resulting in the production of two new bonds. The creation of an electrophile X+ that forms a covalent bonding with an electron-rich, unsaturated C=C bond is the driving force behind this reaction. During the creation of the C-X bond, the positive charge on X is passed to the carbon-carbon bond, thus generating a carbocation.

The intermediate’s positively charged charge joins with an electron-rich species to produce the second covalent bond in the second step of an electrophilic addition. The nucleophilic assault method used in an SN1 reaction is used in the second phase. The nature of the electrophile and the positively charged intermediate is not always evident, and it varies depending on the reactants and reaction circumstances. Regioselectivity is critical in all asymmetric addition reactions to carbon, and it is frequently determined by Markovnikov’s rule. Anti-Markovnikov additions are produced by organoborane chemicals.

Instead of an addition reaction, electrophilic attack on an aromatic system results in electrophilic aromatic substitution. Double bonds contribute lone pair electrons to an electrophile during electrophilic addition processes (Electron-loving, electron poor, a Lewis base). There are many different types of electrophilic additions, however this section will focus on hydrogen halide additions (HX). Many of the basic concepts addressed will apply to electrophilic addition processes in the future.

Mechanism

Electrophilic Attack

The two π electrons from the double bond attack the H in the HBr electrophile in the very first step of the mechanism, as shown by the curved arrow. Among the hydrogen from HBr and a carbon from the double bond, the two π electrons form a C-H sigma bond. The electrons from the H-X bond transfer onto the halogen at the same time, forming a halide anion. One of the carbons becomes an electron deficient carbocation intermediate when pi electrons are removed from the double bond. The positive charge is held in an unhybridized p orbital on this carbon, which is sp2 hybridised.

Nucleophilic attack by halide anion

The nucleophilic halide anion can now accept an electron pair from the generated carbocation, which can now behave as an electrophile. To make the neutral alkyl halide product of electrophilic addition, the electron pair forms an X-C sigma bond. In this reaction, all halides (HBr, HCl, HI, HF) can participate and add on in the same way. Although different halides react at different rates, this is owing to the weakening of the H-X bond as X grows larger (low orbital overlap).

Reaction Labelled Diagram

This two-step electrophilic addition mechanism’s energy diagram is illustrated below. The transition state for each mechanistic step is represented by two peaks on the energy diagram. The valley between the peaks indicates the high-energy carbocation reaction intermediate. The first phase of the mechanism (E1) is the rate-determining step because the energy of activation for the first step (E1) is substantially higher than the second (E1). In the first step of the mechanism, both the alkene and the hydrogen halide are reactants; this electrophilic addition is a second order reaction, and the rate law expression can be written rate = k[Alkene][HX]. Furthermore, any structural characteristic  that helps stabilise the transition state between the reactants and the carbocation intermediate reduces E1 and thus leads to an increase in the reaction rate. However, the alkyl halide product of this reaction is much stable than the reactants making the reaction exothermic.

Free Radical Mechanism 

The majority of hydrogen halides (hydrogen chloride, hydrogen bromide, and so on) react with alkenes via an electrophilic addition process. Hydrogen bromide, on the other hand, contributes by a different process in the presence of organic peroxides. A free radical chain reaction occurs when organic peroxides are present.

Chain Initiation

The chain is started when an oxygen-oxygen connection in the organic peroxide breaks, releasing free radicals.

                            R-O-O-R 🡪 R-O• + •O-R

To form bromine radicals, these free radicals take a hydrogen atom from a hydrogen bromide molecule.

                           R-O• + H-Br 🡪 R-O-H + •Br

Chain Propagation

A bromine radical can join to the ethene via using one of the electrons present in the π bond. This leads to the formation of a new radical with the single electron on the other carbon atom. 

                              CH2=CH2 + •Br 🡪 •CH2-CH2-Br

To continue the reaction, that radical combines with another HBr molecule to make bromoethane and another bromine radical.

                   •CH2-CH2-Br + HBr 🡪 CH3-CH2-Br + •Br

Chain Termination

Two free radicals eventually collide and form a molecule of some type. Because no new free radicals are generated, the process comes to a halt.

Conclusion

For functional group interconversions, electrophilic addition of HX to alkenes is a helpful reaction. A proton assault on the double bond initiates the reaction, which results in the production of a carbocation. The Markovnicov rule governs these reactions, indicating that the product is produced from the most stable carbocation. When this response is carried out, a number of practical issues occur.

In a polar medium like concentrated HBr, most alkenes are insoluble, and water in the reaction media causes the alkene’s hydration reaction to compete with the primary reaction. The use of a phase-transfer catalyst, that help improve the contact between the HBr and the alkene, solves this problem.