Addition of HBr to propene yields 2-brom | Class 11 Chemistry Chapter Hydrocarbons, Hydrocarbons NCERT Solutions

Current NCERT chapter

Academic session 2026-27, Chapter 9 in the current curriculum.

Welcome to the NCERT Solutions for Class 11 Chemistry - Chapter Hydrocarbons. This page offers a step-by-step solution to the specific question from Exercise 1, Question 16:

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

. With detailed answers and explanations for each chapter, students can strengthen their understanding and prepare confidently for exams. Ideal for CBSE and other board students, this resource will simplify your study experience.

Question 16:

Addition of HBr 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 HBr to propene is an example of an electrophilic substitution reaction.

Hydrogen bromide provides an electrophile H+. This electrophile attacks the double bond to form 1° and 2° carbocations.

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

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:

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.

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


Study Tips for Answering NCERT Questions:

NCERT questions are designed to test your understanding of the concepts and theories discussed in the chapter. Here are some tips to help you answer NCERT questions effectively:

  • Read the question carefully and focus on the core concept being asked.
  • Reference examples and data from the chapter when answering questions about Hydrocarbons.
  • Review previous year question papers to get an idea of how such questions may be framed in exams.
  • Practice answering questions within the time limit to improve your speed and accuracy.
  • Discuss your answers with your teachers or peers to get feedback and improve your understanding.

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Comments

  • Asmatullah
  • Apr 03, 2019

Why the HCl,HF and HI don't show peroxide effect


  • Asmatullah
  • Apr 03, 2019

Why the HCl,HF and HI don't show peroxide effect


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Welcome to the NCERT Solutions for Class 11 Chemistry - Chapter . This page offers a step-by-step solution to the specific question from Excercise 1 , Question 16: Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the sam....

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