Question

The propene reacts with \[HBr\] to form
(a) Ethane
(b) Hexane
(c) 1-bromo-propane
(d) 2-bromo propane

Answer 1

Hint: Propene is a member of alkene compound. It is very reactive due to the presence of an unsymmetrical double bond. Propene readily reacts with \[HBr\] and forms bromide compounds. The reaction of \[HBr\] with propene is an example of a nucleophilic addition reaction.

Complete Step by Step Answer:
Propene is an example of an unsymmetrical alkene in which three carbon atoms are present.
Propene readily reacts with \[HBr\]and forms 2-bromopropane (major product) and 1-bromopropane (minor product).

Image: Synthesis of 2-bromopropane

For the addition of \[HBr\]on an unsymmetrical alkene Marconikov rule is used to find a location at which the carbon atom of double bond the hydrogen and bromine atoms undergoes the addition.
According to the Marconikov rule the hydrogen atom of \[HBr\] prefers to bind the end of the carbon-carbon double bond which possesses the highest number of the hydrogen atom. Whereas the bromine atom prefers to bind the end of the carbon-carbon double bond which possesses the lowest number of hydrogen atoms.
Mechanism:
\[HBr\]is a polar molecule due to the electronegativity difference between hydrogen and bromine atoms.
Due to its polar nature, the \[HBr\]molecule can dissociate into \[{H^ + }\] (electrophile) and \[B{r^ - }\](nucleophile) when it approaches the alkene.
The electrophile (\[{H^ + }\]) can attack the double bond of propene and it can form a primary and secondary carbocation. Because secondary carbocation is more stable than primary carbocation. Therefore, formation of secondary carbocation is observed.
After the addition of electrophile (\[{H^ + }\]) the bromide ion or nucleophile (\[B{r^ - }\]) react with formed carbocation and forms the final product i.e., 2-bromopropane (as a major product).

Image: Mechanism of the addition of hydrobromic acid over an unsymmetrical alkene.
Therefore, from the above discussion, it is quite clear that option (d) will be the correct answer.

Note: When the addition of \[HBr\]to the unsymmetrical alkene takes place in the presence of peroxide then this reaction is called as anti-Markovnikov rule or peroxide effect or sometimes it is also known as the Kharasch effect. In such cases, the major product is derived from less stable carbocation.

Image: addition of hydrobromic acid on alkene in the presence of peroxide.