Electrophilic addition reactions

Electrophilic addition reactions Definition 

• Electrophilic addition reactions are a crucial class of organic reactions where electrophiles add across the electron-rich π-bonds of alkenes.
• This process is important for the functionalization of alkenes, converting them into more complex molecules.

General Mechanism of Electrophilic Addition to Alkenes

1. 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.

2. Nucleophilic Attack:

   • A nucleophile (X⁻) then attacks the carbocation intermediate, leading to the formation of the final addition product.

Factors Influencing Electrophilic Addition reactions

• Factors Influencing Electrophilic Addition

1. Stability of Carbocation

   • More stable carbocations form more readily, favoring pathways that lead to these intermediates.
   • Carbocation stability order: 3°>2°>1°>Methyl

2. Steric Effects

   • Less sterically hindered alkenes react faster since electrophiles can more easily access the double bond.

3. Electronic Effects

   • Electron-donating groups (EDG) increase nucleophilicity, enhancing reactivity.
   • Electron-withdrawing groups (EWG) decrease nucleophilicity, slowing the reaction.

4. Solvent and Temperature

   • These factors affect reaction rate and product distribution.

Example: Hydrohalogenation of Alkenes

• Reaction: Addition of hydrogen halides (HX) to alkenes, forming haloalkanes.
• Follows Markovnikov’s Rule:
  • The hydrogen (H⁺) attaches to the less substituted carbon.
  • The halide (X⁻) attaches to the more substituted carbon.

Reaction:

• Propene + Hydrogen Bromide → 2-Bromopropane

• $\mathit{CH_3CH} = \mathit{CH_2} + \mathit{HBr} \rightarrow \mathit{CH_3CHBrCH_3}$

• Step 1: Electrophilic Attack

  • π-electrons of propene attack H⁺ from HBr, forming a carbocation at the more substituted carbon.

  • $\mathrm{CH_3CH=CH_2 \rightarrow CH_3CH^+CH_3}$

• Step 2: Nucleophilic Attack

  • Br⁻ attacks the carbocation, forming 2-bromopropane.

  • $\mathrm{CH_3CH^+CH_3 + Br^- \rightarrow CH_3CHBrCH_3}$

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