What is halogenation?

Halogenation refers to chemical reactions where halogen atoms (Cl, Br, I etc.) are added to a chemical compound. 

Halogenation of Alcohols

Among the substances used to halogenate alcohol are hydrogen halides, PCl3, PCl5, PBr3, PI3 and SOCl2

  • For hydrogen halides (HX), the general equation is: alcohol + hydrogen halide → halogenoalkane + water.
    \( \text{C}_2\text{H}_5\text{OH} + \text{HX} \rightarrow \text{C}_2\text{H}_5\text{X} + \text{H}_2\text{O} \)
  • Sulfur dichloride oxide (SOCl2) can also substitute a chlorine atom into an alcohol molecule. In this reaction, the by-products HCl and SO2 are both gases and escape from the reaction mixture.
    \( \text{C}_2\text{H}_5\text{OH} + \text{SOCl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{HCl} + \text{SO}_2 \)
  • Phosphorus Pentachloride (PCl5) is also often used. However, as POCl3 is a liquid, this is less preferred as it is difficult to separate POCl3 out.
    \( \text{CH}_3\text{CH}_2\text{OH} + \text{PCl}_5 \rightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{POCl}_3 + \text{HCl} \)
  • Phosphorus Trichloride (PCl3) is a very efficient reagent as none of the Cl molecules are wasted in byproducts. However, as H3PO3 is a liquid, it faces the same problem as PCl5. 
    \( \text{PCl}_3 + 3\text{CH}_3\text{CH}_2\text{OH} \rightarrow 3\text{CH}_3\text{CH}_2\text{Cl} \rightarrow \text{H}_3\text{PO}_3 \)
  • Phosphorus Tribromide (PBr3)and Phosphorus Triiodide (PI3) are also used in the same way. 
    \( \text{PI}_3 + 3\text{CH}_3\text{CH}_2\text{OH} \rightarrow 3\text{CH}_3\text{CH}_2\text{I} \rightarrow \text{H}_3\text{PO}_3 \)
    \( \text{PBr}_3 + 3\text{CH}_3\text{CH}_2\text{OH} \rightarrow 3\text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{H}_3\text{PO}_3 \)
Mobirise Website Builder
Halogenation of Carboxylic Acids
Carboxylic acids react in the preparation of acyl chlorides. Acyl chlorides (RCOCl) are made by reacting carboxylic acids with PCl5, PCl3, or SOCl2.

We will only cover acyl chlorides, but note that acyl bromides and iodides can be formed using similar reactions.
  • For SOCl2, the reaction is as follows:
    \( \text{CH}_3\text{COOH} + \text{SOCl}_2 \rightarrow \text{CH}_3\text{COCl} + \text{SO}_2 + \text{HCl} \)

  • For PCl3, the reaction is as follows:
    \( 3\text{CH}_3\text{COOH} + \text{PCl}_3 \rightarrow 3\text{CH}_3\text{COCl} + \text{H}_3\text{PO}_3 \)
  • For PCl5, the reaction is as follows:\( \text{CH}_3\text{COOH} + \text{PCl}_5 \rightarrow \text{CH}_3\text{COCl} + \text{POCl}_3 + \text{HCl} \)

Halogenation of alkanes

  • Chlorine atoms or bromine atoms can substitute for hydrogen atoms in alkanes in the presence of ultraviolet light, producing halogenoalkanes.
  • This is called a free-radical substitution reaction.
  • The overall reaction between alkanes and halogens, involves initiation, propagation, and termination steps. 
    \( \text{Cl}_2 \rightarrow 2\text{Cl} \cdot \)

    \(\text{Cl}\cdot+ \text{CH}_3\text{CH}_2\text{Cl} \rightarrow \text{CH}_3\text{CH}_2\cdot + \text{HCl} \)
    \( \text{CH}_3\text{CH}_2\cdot + \text{Cl}_2 \rightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{Cl}\cdot \)

    \( \text{CH}_3\text{CH}_2\cdot + \text{CH}_3\text{CH}_2\cdot \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)
    \( \text{CH}_3\text{CH}_2\cdot + \text{Cl}\cdot \rightarrow \text{CH}_3\text{CH}_2\text{Cl} \)
Mobirise Website Builder
Download PDF

© Copyright 2025 Lecturemate - All Rights Reserved