Question

explain why addition of HCl to CH₂ = CH — CH₃ takes place according to the general mechanism while HBr adds in an unexpected manner.​

Answer 1

As HCl Because of greater bond strength ( 430 kJ mol⁻¹) cannot break symmetrically, so addition of HCl to CH₂ = CH — CH₃ takes place according to the general mechanism of Marknownikov's rule.

Marknownikow's rule is not applicable in case of addition of HBr to propene. the addition of HBr to propene in the presence of organic peroxide takes place contrary to Marknownikov's rule.

This abnormal addition or unexpected addition of HBr to propene in the presence of peroxide is called Peroxide effect or Anti-Marknownikov's rule , e.g.,

$\bigg\lgroup$ refer to Attachment {ii} $\bigg\rgroup$

Mechanism : the reaction is explained on the basis of free radical mechanism as shown below:

$\bigg\lgroup$refer to Attachment {iii} $\bigg\rgroup$