Chemistry – Organic chemistry | e-Consult

Reaction Mechanism and Product Prediction:

The reaction is a free radical substitution reaction. The mechanism involves three steps:

1. Initiation: Homolytic cleavage of HBr by heat or light to generate bromine radicals (Br•).
   HBr --[heat/light]--> H• + Br•

2. Propagation:

   • Abstraction of a hydrogen atom from the alkane by a bromine radical, forming a carbon radical.
   CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂ + Br• --> CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH• + HBr

3. The carbon radical reacts with HBr to form the alkyl bromide and regenerate a bromine radical.
CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH• + HBr --> CH₃CH₂CH₂CH₂CH₂CH₂CH₂Br + Br•
4. Termination: Combination of two radicals to form a stable molecule.
   Br• + Br• --> Br₂; CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH• + Br• --> CH₃CH₂CH₂CH₂CH₂CH₂CH₂Br

Major Organic Product(s):

The major product is the 1-bromo-octane. However, because the reaction is free radical, there is also some substitution at other carbon atoms. This leads to a mixture of products, including 2-bromo-octane, 3-bromo-octane, etc. The ratio of products depends on the reaction conditions (temperature, concentration of reactants).

Regioselectivity Explanation:

The regioselectivity is determined by the stability of the radical intermediate. Primary radicals are generally less stable than secondary radicals, which are less stable than tertiary radicals. Therefore, the substitution occurs preferentially at the primary carbon, resulting in the formation of 1-bromo-octane as the major product. While other substitution products are formed, they are present in lower amounts.