Question

Complete the following reactions:
A.\[C{H_3} - CH = C{H_2} + HBr\xrightarrow{{peroxide}}?\]
B.\[C{H_2} = CH - C{H_3} + HBr \to ?\]

Answer 1

Hint: Alkenes are unsaturated compounds consisting of a double bond and undergo addition reactions. The addition of a molecule to the alkenes in different ways. In the presence of peroxide the negative part of the molecule is attacked on the carbon containing more hydrogen atoms.

Complete answer:
Hydrogen bromide is a molecule consisting of hydrogen and bromine. The bromine is the negative part and the hydrogen is the positive part, it is due to the difference in electronegativity.
The reagent can be dissociated as follows:
\[HBr \to {H^ + } + B{r^ - }\]
In presence of peroxides the negative part of the reagent is attacked on the carbon in the alkenes with more number of hydrogen atoms. The positive part of the reagent is attacked on the carbon in the alkenes with less number of hydrogen atoms.
The following reaction takes place as an example of anti-markovnikov rule.
\[C{H_3} - CH = C{H_2} + HBr\xrightarrow{{peroxide}}C{H_3} - C{H_2} - C{H_2}Br\]
In the absence of peroxide the negative part of the reagent is attacked on the carbon in the alkenes with less number of hydrogen atoms. The positive part of the reagent is attacked on the carbon in the alkenes with greater number of hydrogen atoms.
The following reaction takes place as an example of anti-markovnikov rule.
\[C{H_2} = CH - C{H_3} + HBr \to C{H_3} - CH\left( {Br} \right) - C{H_3}\]

Note:
The presence of peroxide in the addition of alkenes makes the reaction to follow a free radical mechanism. Whenever there is a presence of peroxides the product in the addition of reagent to the alkenes takes place via anti-markovnikov rule which is opposite to the markovnikov rule.