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

2024
Views:

391

Pages:

2
Author:

Kyuwon J.

Halogenation and Halohydrin Formation of Alkenes 1. Introduction to Halogenation of Alkenes • Halogenation is the reaction where X2 (Cl2 , Br2 ) adds to an alkene. • Forms a vicinal dihalide (halogens on adjacent carbons). • Reaction is stereospecific and follows anti-addition. 2. Mechanism of Halogenation 1. Step 1: Formation of the Halonium Ion • The alkene’s π-electrons attack X2 , forming a cyclic halonium ion. • This prevents rearrangements. 2. Step 2: Nucleophilic Attack • The halide (X ) attacks from the opposite side (anti-addition). • Leads to the final trans product. 3. Example: Bromination of Ethene CH2 CH2 + Br2 → Br • Intermediate: Bromonium ion (Br CH2 CH+ 2 CH2 Br CH2 ). • Final Product: 1,2-Dibromoethane (anti-addition). 4. Stereochemistry of Halogenation • Halogenation is stereospecific. • Only the anti-product forms due to the halonium bridge. • Example: Bromination of cis- and trans-2-butene gives different products. 5. Halohydrin Formation (X2 + H2 O Addition) • If water is present, water competes with the halide ion. • Instead of a dihalide, an halohydrin forms. • Example: CH2 CH2 + Br2 + H2 O → Br 1 CH2 CH2 OH. 6. Mechanism of Halohydrin Formation 1. Step 1: Formation of the Halonium Ion (same as halogenation). 2. Step 2: Nucleophilic Attack by Water • H2 O, a stronger nucleophile, attacks instead of Br− . • Leads to Markovnikov regioselectivity: OH attaches to the more substituted carbon. 3. Step 3: Deprotonation • H2 O deprotonates to form the final halohydrin product. 7. Example: Bromohydrin Formation from Propene CH3 CH CH2 + Br2 + H2 O → CH3 CHOH CH2 Br • OH goes to the more substituted carbon. • Br goes to the less substituted carbon. 8. Applications of Halogenation and Halohydrin Formation • Halogenation is used in: – PVC production (vinyl chloride formation). – Pharmaceuticals (synthesis of halogenated drugs). • Halohydrins are important in: – Epoxide synthesis (conversion to oxiranes). – Disinfection (bromohydrins in water treatment). 2