University:
The University Of Western Ontario
Course:
Biology 2290F | Scientific Method in Biology
Academic year:

2024
Views:

112

Pages:

2
Author:

Kyuwon J.

Markovnikov’s Rule and Hydrohalogenation of Alkenes 1. Markovnikov’s Rule • States that in the addition of HX (HCl, HBr, HI) to an alkene, the hydrogen (H) attaches to the carbon with more hydrogen atoms, and the halide (X) attaches to the more substituted carbon. • Follows carbocation stability: + + – Primary (R CH+ 2 ) < Secondary (R2 CH ) < Tertiary (R3 C ) – More stable carbocations form faster and lead to major products. • Used to predict major products in alkene addition reactions. 2. Hydrohalogenation Reaction Mechanism • Reaction: CH3 CH CH2 + HCl → majorproduct • Step 1: Protonation (H+ from HX adds to one of the double-bonded carbons) – Forms a carbocation intermediate (most stable form). • Step 2: Nucleophilic Attack (X− attaches to the carbocation). • Major product follows Markovnikov’s Rule. 3. Carbocation Stability Order • Tertiary (R3 C+ ) > Secondary (R2 CH+ ) > Primary (RCH+ 2 ) > Methyl (CH+ ) 3 • More substituted carbocations are stabilized by: – Hyperconjugation – Inductive effects – Resonance (in allyl and benzyl systems) 4. Anti-Markovnikov Addition (Peroxide Effect) • If peroxides (ROOR) are present, the reaction follows Anti-Markovnikov’s Rule. • HBr addition occurs via a free radical mechanism. • The Br attaches to the less substituted carbon instead of the more substituted one. • Only observed with HBr, not HCl or HI. 1 5. Predicting Major Products • Example: CH3 CH CH2 + HBr – Without peroxides: 2-Bromopropane (Markovnikov product). – With peroxides: 1-Bromopropane (Anti-Markovnikov product). • Example: CH3 CH CH CH3 + HCl – Carbocation forms at the more stable secondary carbon. – Major product: 2-Chlorobutane. 2