Electrophilic addition reactions are a fundamental class of reactions in organic chemistry, where electrophiles are attracted to and add across the electron-rich π-bonds of alkenes.
This process is pivotal in the functionalization of alkenes, converting them into a wide variety of more complex molecules.
General Mechanism of Electrophilic Addition to Alkenes
Electrophilic Attack:
The alkene, acting as a nucleophile due to its electron-rich double bond, reacts with an electrophile (E⁺).
This results in the formation of a carbocation intermediate, typically at the more substituted carbon (following Markovnikov's rule) due to its greater stability.
Nucleophilic Attack:
A nucleophile (X⁻) then quickly attacks the positively charged carbocation intermediate, leading to the formation of the addition product.
Factors Influencing Electrophilic Addition
1.Stability of Carbocation:
More stable carbocations form more readily, favoring reactions that lead to such intermediates.
2.Steric Effects:
Less sterically hindered alkenes are more reactive as they allow better access to the electrophile.
3.Electronic Effects:
Electron-donating groups on the alkene increase nucleophilicity, enhancing reactivity.
Electron-withdrawing groups decrease nucleophilicity, slowing the reaction.
4.Solvent and Temperature:
These can affect the reaction rate and the product distribution.
Example: Hydrohalogenation of Alkenes
Hydrohalogenation is a classic example of electrophilic addition where a hydrogen halide (HX) adds to an alkene, forming a haloalkane.
The reaction follows Markovnikov's rule, where the hydrogen atom bonds to the less substituted carbon of the double bond, and the halogen bonds to the more substituted carbon.
Chemical Reaction:
Propene + Hydrogen Bromide → 2-Bromopropane
Equation:
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃
Electrophilic Attack: The π-electrons of propene attack the hydrogen of HBr, forming a carbocation intermediate on the more substituted carbon (the 2-position).
CH₃CH=CH₂ → CH₃CH⁺CH₃
Nucleophilic Attack: The bromide ion (Br⁻) from HBr attacks the carbocation, resulting in the formation of 2-bromopropane.
CH₃CH⁺CH₃ + Br⁻ → CH₃CHBrCH₃