Question

Reaction of $HBr$ with propene in the presence of peroxide gives:
A. $3-$ bromopropane
B. Allyl bromide
C. $n-$ propyl bromide
D. Isopropyl bromide

Answer 1

Hint: The following reaction follows anti markovnikov rule .anti markovnikov addition reaction is a reaction that takes place when an electrophile hydrogen halide reacts with alkene and alkyne, hydrogen of hydrogen halide will become bonded to the carbon that has least number of hydrogen atoms in an alkyne and alkene. This reaction uses peroxide as a catalyst.

Complete step by step answer:
When $HBr$ reacts with propene in the presence of peroxide as catalyst it results in the formation of $n-$ propyl bromide. This is an anti-markovnikov addition reaction.
$C{{H}_{3}}CH=C{{H}_{2}}+HBr\xrightarrow{peroxide}C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Br$
Let us now see the mechanism of this reaction:
Step $1$ = Chain initiation step
When an oxygen-oxygen bond of peroxide breaks, it gives free radicals
$ROOR\to R{{O}^{\bullet }}+R{{O}^{\bullet }}$
When these free radicals react with hydrogen bromide, it extracts hydrogen from hydrogen bromide and gives bromine radical.
$R{{O}^{\bullet }}+HBr\to ROH+B{{r}^{\bullet }}$
Step $2$ = Chain propagation step
Now, when a bromine radical joined with propene, bromine radical will attach on carbon that has more number of hydrogen atoms and the secondary radical is formed. This secondary radical formed is more stable therefore bromine radical is attached on carbon that has more number of hydrogen atoms.
After that, the new radical will react with $HBr$ that results in the formation of bromoethane and one more bromine radical.
$C{{H}_{3}}C{{H}_{2}}=C{{H}_{2}}+B{{r}^{\bullet }}{{\to }^{\bullet }}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}Br$
$^{\bullet }C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}Br+HBr\to C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Br+B{{r}^{\bullet }}$
Step $3$ = chain termination step
Two radicals hit each other and form a molecule eventually. This process stops as there are no new radicals formed.

So, the correct answer is Option C .

Note: alkenes considered to be in the group of unsaturated hydrocarbons where alkene must contain one double bond. In the presence of peroxide, the regioselectivity for the addition reaction of other electrophiles are not altered. This reaction mechanism has 3 steps that is initiation, propagation and termination.