Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
From Alcohols: Alcohols can be converted to haloalkanes using (Lucas reagent), , , or . The reaction with thionyl chloride () is preferred because the by-products ( and ) are gases and escape easily, leaving pure alkyl chloride.
From Hydrocarbons (Addition of to Alkenes): Addition of to unsymmetrical alkenes follows Markovnikov's rule, where the negative part of the addendum adds to the carbon with fewer hydrogen atoms. In the presence of peroxides, (not or ) follows the Anti-Markovnikov rule (Kharasch effect).
Halogen Exchange: This is used to prepare iodoalkanes and fluoroalkanes which are difficult to prepare by direct halogenation. The Finkelstein reaction uses in dry acetone to produce . The Swarts reaction uses metallic fluorides like , , or to produce .
From Diazonium Salts (Sandmeyer's Reaction): Primary aromatic amines react with at to form benzene diazonium chloride. When treated with or , the diazonium group is replaced by or .
Free Radical Halogenation: Alkanes react with or in the presence of UV light or heat. This method usually results in a complex mixture of mono-, di-, and polyhaloalkanes, making it less suitable for laboratory synthesis of pure compounds.
📐Formulae
💡Examples
Problem 1:
Identify the major product formed when propene () reacts with in the presence of benzoyl peroxide.
Solution:
(1-bromopropane)
Explanation:
In the presence of organic peroxides, the addition of to an unsymmetrical alkene follows the Anti-Markovnikov rule. The atom attaches to the primary carbon (the one with more hydrogen atoms) via a free radical mechanism.
Problem 2:
Write the chemical equation for the preparation of chloroethane from ethanol using thionyl chloride.
Solution:
Explanation:
This is Darzen's process. It is the best method for preparing alkyl chlorides because the gaseous by-products and escape from the reaction mixture.
Problem 3:
How is iodobenzene prepared from benzene diazonium chloride?
Solution:
Explanation:
Unlike the preparation of chlorobenzene or bromobenzene, the preparation of iodobenzene does not require a cuprous halide catalyst. Simply shaking the diazonium salt solution with potassium iodide () is sufficient.