Question

Write the structure of the alkyl halide obtained by the action of hydrogen bromide on pent-1-ene .
A.In presence of peroxide
B.In absence of peroxide

Answer 1

Hint: To solve this we need to know the Markovnikov rule and Anti Markovnikov rule . In the presence of peroxide the Anti Markovnikov rule will follow and if peroxide is not present then the Markovnikov rule will follow.

Complete step by step answer:
In the presence of peroxide
Anti Markovnikov’s rule will follow . The formula of peroxide is ${H_2}{O_2}$.
Anti Markovnikov’s rule is defined as when we add hydrogen halide$\left( {HX} \right)$ to an alkene or alkyne , then the hydrogen of protic acid will form bond with the carbon having less number of hydrogen and the electronegative part means the halide $\left( X \right)$part will form bond with the carbon atom having greater Number of hydrogen .This rule will follow only in the presence of peroxide .
$C{H_2} = CH - C{H_2} - C{H_2} - C{H_3}\xrightarrow[{{H_2}{O_2}}]{{HBr}}BrC{H_2} - C{H_2} - C{H_2} - C{H_2} - C{H_3}$
So , the reaction above shows us that the electronegative part which is a bromine forming bond with the carbon having more number of hydrogen which is ${c_1}$. The hydrogen is forming a bond with the ${c_2}$ carbon . The resultant structure will be $1 - Bromopentane$
b. In the absence of peroxide : Markovnikov’s rule will follow .
Markovnikov rule is defined as when we add any hydrogen halide (protic acid) to an alkene or alkyne , then the hydrogen of hydrogen halide will bonded to the carbon with greater number of hydrogen and the electronegative part which is the halide $\left( X \right)$ part will bonded with the carbon with more number of alkyl substituents. In this reaction no peroxide will be present.
$C{H_2} = CH - C{H_2} - C{H_2} - C{H_3}\xrightarrow{{HBr}}C{H_3} - CHBr - C{H_2} - C{H_2} - C{H_3}$
Now , here the Bromine which is electronegative will form a bond with the carbon having lesser number of hydrogen or we can also state that the carbon ${c_2}$ having more number of carboxyl substituents . the hydrogen is forming a bond with ${c_1}$. The resultant structure will be $2 - bromopentane$

Note:Markovnikov’s rule is an empirical rule which deals with the regioselectivity of electrophilic addition to alkenes and alkynes. It tells us how the alkyl halide will react in the presence of peroxide or in the absence of peroxide . It deals how the alkyl halide adds to the double bonded carbons and forms the stable compound.