Question

What are alkyl halides? How is n-propyl bromide prepared from propylene?

Answer 1

Hint: Hydrogens present in hydrocarbons can be replaced to form different classes of hydrocarbons.Use of peroxide in the reaction, leads to the formation of less substituted product. Synthesis of n-propyl bromide from propylene is an addition reaction.

Complete step by step answer:
Alkyl halides are hydrocarbons in which one or more than one hydrogen atom is replaced by halogen (chlorine, fluorine, iodine, bromine)atoms. These are also known as halogeno alkanes. Depending on the degree of substitution at the carbon atom carrying the halogen, alkyl halides are classified into primary, secondary and tertiary alkyl halides.
Propylene is an alkene hydrocarbon containing three carbon atoms, and a double bond. N-propyl bromide is synthesized by propylene by the use of HBr in presence of peroxide. Peroxide is used to get a less substituted alkyl halide.
Reaction of propylene with HBr is an addition reaction in which bromine and hydrogen are added over the doubly bonded carbon atoms. In presence of peroxide, hydrogen is added on more substituted carbon atom and bromine atom is added on less substituted carbon atom. This mechanism proceeds through Anti-markovnikov's addition.

In presence of peroxide, reaction proceeds through a free radical mechanism. Free radical forms as an intermediate.During the reaction, there will be formation of two intermediates, one with primary free radical and other with secondary free radical. Since secondary free radical is more stable than primary free radical ,intermediate with secondary free radical gives the end product in major.

Note: In absence peroxide, reaction would have proceed through formation of carbocation and the resulting product would be 2-bromo propane, which is an isomer of n-propyl bromide.