Mechanism of Electrophilic Addition

Introduction

An electrophilic addition reaction occurs when an electrophile attacks a substrate, with the end result being the addition of one or more relatively simple molecules across a multiple bond. Alkenes have an electron-rich double bond because they have four electrons instead of two in a single bond. The pi electrons are also positioned above and below the double bond, making them easier to access for reactions. Double bonds can easily give lone pair electrons to act as a nucleophile in general (nucleus-loving, electron rich, a Lewis acid). Double bonds contribute lone pair electrons to an electrophile during electrophilic addition processes (Electron-loving, electron-poor, a Lewis base). 

The additional reactions are common in compounds with many bonds. By adding to multiple bonds, the alkene (=), alkynes (≡), C=O, CN react. As a result of the reaction, the number of groups linked to the substrate increases while the unsaturation of the substrate decreases. There are four types of addition reactions: 

1. electrophilic, 
2. nucleophilic, 
3. free radical concerted, 
4. and heterophilic (Cyclic)

Electrophilic Addition Reaction

• In organic chemistry, an electrophilic addition reaction occurs when a double or triple bond in a chemical compound is broken, resulting in the production of two new bonds.
• The formation of an electrophile X+ that forms a covalent bond with an electron-rich, unsaturated C=C bond is the driving force behind this reaction. During the formation of the C-X bond, the positive charge on X is transferred to the carbon-carbon bond, generating a carbocation.
• 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 intermediate’s positively charged charge combines with an electron-rich species to form the second covalent bond in the second step of an electrophilic addition. 
• The nucleophilic attack method used in an SN1 reaction is used in the second step. The nature of the electrophile and the positively charged intermediate is not always evident, and it varies depending on the reactants and reaction conditions.

General Reaction for electrophilic addition

• The alkene’s pi bond is broken in this reaction, resulting in two single sigma bonds. One of these sigma bonds is attached to the H and the other to the X of the hydrogen halide, as indicated in the chemical mechanism. With HBr and HCl, this reaction works nicely. HI can also be employed, however it is commonly produced during the reaction when potassium iodide (KI) is combined with phosphoric acid (H3PO4).

Addition to symmetrical alkenes

• With the hydrogen halides, all alkenes undergo addition reactions. One of the carbon atoms in the double bond is joined by a hydrogen atom, while the other is joined by a halogen atom.

Mechanism

Step 1: Electrophilic Attack

The two pi electrons from the double bond attack the H in the HBr electrophile in the first step of the reaction, as represented by the curved arrow. Between the hydrogen from HBr and a carbon from the double bond, the two pi 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.

Note

The following hints may help you comprehend these reactions: 

• Identify the electrophile present in the reagent mixture 
• The regioselectivity is dictated by the electrophile’s first addition to the alkene. 
• The reaction is not stereoselective if it occurs via a planar carbocation. 
• Syn addition occurs when two new bonds from the same species form at the same moment. 
• Anti addition occurs when two new bonds establish at different periods from distinct species.

Conclusion

Electrophilic addition reactions are a type of reaction that allows C=C and C≡C to be interconverted into a variety of functional groups. In terms of concept, adding is the inverse of elimination. What exactly does “electrophilic addition” mean? Electrophiles (E+) are electron-poor species that react with electron-rich species (the C=C). The term “addition” refers to the combining of two systems into a single entity.