Chemistry Class 11 NCERT Solutions: Chapter 13 Hydrocarbons Part 9

Q: 16. Addition of Equation to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

Answer

Addition of Equation to propene is an example of an electrophilic substitution reaction. Hydrogen bromide provides an electrophile, Equation . This electrophile attacks the double bond to form Equation and Equation carbocations as shown:

Q 16 Ans 1 Structure of Peimary and Secondary Carbocation

Q 16 Ans 1 Structure of Peimary and Secondary Carbocation

Q 16 Ans 1 Structure of Peimary and Secondary Carbocation

Secondary carbocations are more stable than primary carbocations. Hence, the former predominates since it will form at a faster rate. Thus, in the next step, Equation attacks the carbocation to form 2 – bromopropane as the major product.

Q 16 Ans 2 Structure of Bromopropane

Q 16 Ans 2 Structure of Bromopropane

Q 16 Ans 2 Structure of Bromopropane

This reaction follows Markovnikov’s rule where the negative part of the addendum is attached to the carbon atom having a lesser number of hydrogen atoms. In the presence of benzoyl peroxide, an addition reaction takes place anti to Markovnikov’s rule. The reaction follows a free radical chain mechanism as:

Q 16 Ans 3 Structure of Free Radical Chain Mechanism

Q 16 Ans 3 Structure of Free Radical Chain Mechanism

Q 16 Ans 3 Structure of Free Radical Chain Mechanism

Q 16 Ans 4 Structure of Free Radical Chain Mechanism

Q 16 Ans 4 Structure of Free Radical Chain Mechanism

Q 16 Ans 4 Structure of Free Radical Chain Mechanism

Secondary free radicals are more stable than primary radicals. Hence, the former predominates since it forms at a faster rate. Thus, 1 – bromopropane is obtained as the major product.

Q 16 Ans 4 Structure of 1- Bromopropane Major Product

Q 16 Ans 4 Structure of 1- Bromopropane Major Product

Q 16 Ans 4 Structure of 1- Bromopropane Major Product

In the presence of peroxide, Equation free radical acts as an electrophile. Hence, two different products are obtained on addition of Equation to propene in the absence and presence of peroxide.

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