Question

Predict the major product (s) of the following reactions and explain their formation.

\[\ce{H3C - CH = CH2 ->[(Ph.CO.O)2][HBr]}\]

\[\ce{H3C - CH = CH2 ->[HBr]}\]

Answer 1

Addition of HBr to unsymmetrical alkenes follows Markonikov rule. It states that negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms.

Mechanism: Hydrogen bromide provides an electrophile, H⁺, which attacks the double bond to form carbocation as shown below:

The (b) is attacked by Br^– ion to form the product as follows:

Addition reaction of HBr to unsymmetrical alkenes in the presence of peroxide follows anti-Markovnikov rule.
Mechanism: Peroxide effect proceeds via free radical chain mechanism as given below:

(i) 

(ii) \[\ce{\overset{\bullet}{C6}H5 + H - Br ->[Homolysis] C6H6 + \overset{\bullet}{B}r}\]

(iii)

(iv) \[\ce{CH3 - \overset{\bullet}{C}H - CH2Br + H - Br ->[Homolysis] \underset{(major product)}{CH3 - CH2 - CH2Br + Br}}\]

\[\ce{H3C - CH = CH2 ->[(PhCOO)2][HBr] \underset{(Anti-Markovnikov addition)}{H3CCH2CH2Br}}\]

\[\begin{array}{cc}
\ce{H3C - CH = CH2 ->[HBr] H3C - CH - CH3}\\
\phantom{......................}|\\
\phantom{.......................}\ce{\underset{(Markovnikov addition)}{Br}}
\end{array}\]