Example 6.3

(II) From alkenes (i) Addition of hydrogen halides : An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. Propene yields two products, however only one predominates as per Markovnikov’s rule. (Unit , Class XI) (ii) Addition of halogens: In the laboratory, addition of bromine in CCl to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic -dibromides, which are colourless (Unit , Class XI).

Alkyl iodides are often prepared by the reaction of alkyl chlorides/ bromides with NaI in dry acetone. This reaction is known as Finkelstein reaction. NaCl or NaBr thus formed is precipitated in dry acetone. It facilitates the forward reaction according to Le Chatelier’s Principle.

The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as