Preparation from alkanes

Some commercially important haloalkanes are prepared from alkanes by reaction with the halogen (usually Cl₂ or Br₂) in the presence of ultraviolet (UV) light.

In this reaction

• the hydrocarbon skeleton stays intact and
• if the hydrocarbon is present in large excess, only one C-H bond in the reactant alkane is converted to a C-X (X = Cl, Br) bond in the product.
  
  

  CH3CH2–H + Cl–Cl

  CH3CH2–Cl + H–Cl

  
  
• if the halogen is present in excess, polyhalogenated products or a mixture of products results.

For alkanes with more than three carbons, there is more than one type of hydrogen.  Thus there are two possible products, and mixtures of organic products are formed as for propane below.  It should be noted that the two products are not formed in equal amounts.  The reasons for this will become clear in future studies of chemistry.  At this stage, it is only important to recognize that the number of monohaloalkanes possible for a given alkane depends on the number of different types of hydrogens present.

Cl2 light
CH3CH2CH3		CH3CH2CH2Cl

A word of caution about the way in which organic reactions are commonly written:

Commonly the focus is on the organic reactants and products.