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Hydrohalogenation of Hydrocarbons

Key Concepts

  • Unsaturated hydrocarbons (alkenes and alkynes) react with hydrogen halides (HX)
        in addition reactions producing halogenated compounds

  • Markovnikov's Rule:
        In additions of HX to unsymmetrical alkenes, the H+ of HX goes to the
        double-bonded carbon that already has the greatest number of hydrogens

Animated Tutorial

Examples

Symmetrical Alkenes

Ethene (ethylene) is a symmetrical molecule.
  H
|
  H
|
 
  C = C  
  |
H
  |
H
 

Each carbon atom is covalently bonded to two hydrogen atoms.
Ethene reacts with hydrogen bromide to produce bromoethane.
ethene + hydrogen bromide bromoethane
C2H4 + HBr C2H5Br
  H
|
  H
|
 
  C = C  
  |
H
  |
H
 
+ H-Br
 H
|
 Br
|
 
H-C-C-H
 |
H
 |
H
 

Bromoethane is the only product possible for this reaction.

Unsymmetrical Alkenes

Propene is an unsymmetrical molecule.
  H    
|    
      H
|
 
  C(1) = C(2) - C -H
  |    
H    
  |    
H    
  |
H
 

C(1) is covalently bonded to two hydrogen atoms.
C(2) is covalently bonded to one hydrogen atom.
When propene reacts with a halogen halide such as hydrogen bromide, two structural isomers are possible products:

  • If the bromine atom bonds with C(1), the product is 1-bromopropane.

  • If the bromine atom bonds with C(2), the product is 2-bromopropane.

propene + hydrogen bromide 1-bromopropane + 2-bromopropane
  H
|
      H
|
 
  C = C - C -H
  |
H
  |
H
  |
H
 
+ H-Br
 Br
|
 H
|
 H
|
 
H-C-C-C-H
 |
H
 |
H
 |
H
 
+
 H
|
 Br
|
 H
|
 
H-C-C-C-H
 |
H
 |
H
 |
H
 

Markovnikov's Rule states that the hydrogen adds to the carbon atom of the double bond which is already bonded to more hydrogen atoms, that is, the hydrogen atom adds to C(1) and the bromine atom adds to C(2) so the major product of the reaction will be 2-bromopropane.
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Related AUS-e-TUTE Topics

Summary of Nomenclature

Naming Straight Chain Alkanes

Naming Branched Chain Alkanes

Naming Simple Alkenes

Naming Simple Alkynes

Naming Structural Isomers of Alkanes

Naming Structural Isomers of Alkenes

Naming Alkanols

Naming Haloalkanes

Functional Groups

Combustion of Hydrocarbons

Halogenation of Hydrocarbons

Hydration of Alkenes to Alkanols

Oxidation of Alkanols

Dehydration of Alkanols (alcohols)

Preparation and Naming of Esters (Esterification)

Substitution Reactions of Haloalkanes (alkyl halides)

Preparation and Naming of Polymers (Polymerization)

Properties and Uses of Alkanes

Properties and Uses of Ethene (ethylene)

Properties and Uses of Polythene (polyethylene)

 
 

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