Radical Halogenation of Alkanes
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Questions and Answers

What happens to radical formation when chlorine is involved?

  • It is highly unstable.
  • It does not occur.
  • It is endothermic.
  • It is exothermic. (correct)

According to Hammond’s Postulate, what does the transition state resemble?

  • The lower energy species.
  • The higher energy species. (correct)
  • An intermediate product.
  • The starting material.

Which statement about bromine in radical reactions is true?

  • Bromine radicals show no selectivity.
  • Bromine radicals create a racemic mixture. (correct)
  • Bromine is perfect for all reactions.
  • Bromine radicals do not form stable intermediates.

What is the expected outcome in a radical addition reaction involving alkenes and peroxides?

<p>Preference for the less substituted halide. (A)</p> Signup and view all the answers

Which of the following statements about fluorine and iodine radicals is correct?

<p>Fluorine radicals react too violently to be useful. (A)</p> Signup and view all the answers

Why do radicals resist rearrangement compared to carbocations?

<p>Radicals have half-filled p-orbitals. (D)</p> Signup and view all the answers

What can be inferred about the radical addition of HBr compared to HCl and HI?

<p>HBr shows preference for Markovnikov addition without peroxide. (B)</p> Signup and view all the answers

What is the result of removing a hydrogen atom from an asymmetric center in a radical reaction?

<p>Production of a pair of enantiomers. (C)</p> Signup and view all the answers

What is the ratio of the predicted formation of 2-chlorobutane to 1-chlorobutane based on the relative reactivity?

<p>3.8:1 (A)</p> Signup and view all the answers

Which factor primarily influences the distribution of products in radical chlorination reactions?

<p>The probability of hydrogen collision and reactivity (A)</p> Signup and view all the answers

In radical chlorination, how many equivalent hydrogen atoms are there in butane that can react?

<p>6 methyl and 4 methylene (A)</p> Signup and view all the answers

What property makes bromine radicals less reactive than chlorine radicals?

<p>Weaker H-Bromine bond (A)</p> Signup and view all the answers

How many unique products can result from the monochlorination of methylcyclohexane, excluding stereoisomers?

<p>3 (C)</p> Signup and view all the answers

Given the reactivity values of 1°, 2°, and 3°, which is the least abundant product from the monochlorination of methylcyclohexane?

<p>Primary product (C)</p> Signup and view all the answers

What is the expected percentage yield of 1-chlorobutane compared to 2-chlorobutane based on calculations?

<p>21% 1-chlorobutane, 79% 2-chlorobutane (A)</p> Signup and view all the answers

What is the primary factor that distinguishes the ability of alkanes to undergo radical chlorination versus bromination?

<p>Reactivity of the radical formed (B)</p> Signup and view all the answers

Study Notes

Radical Chlorination of Alkanes

  • There are six ways to form 1-chlorobutane and four ways to form 2-chlorobutane.
  • The relative reactivity of primary, secondary and tertiary hydrogens to radical chlorination is 1:3.8:5.

Radical Stability and Rate of Formation

  • Radicals are not as strongly stabilized by hyperconjugation as carbocations.

Regiochemistry of Radical Chlorination

  • The relative reactivity of a particular hydrogen and the probability that hydrogen collides and reacts determines the regiochemistry of a radical halogenation reaction.

Radical Bromination

  • Bromine radicals are less reactive than chlorine radicals because the H-Br bond (88 kcal/mol) is weaker than the H-Cl bond (103 kcal/mol) formed in chlorination.
  • This leads to greater radical selectivity in the formation of the more stable radical.
  • The reactivity-selectivity principle states that radical formation is exothermic with chlorine and endothermic with bromine.
  • The Hammond’s Postulate: The transition state more strongly resembles the higher energy species (starting material or product).

Stereochemistry of a Radical Substitution Reaction

  • A new asymmetric center is formed, and the product is a racemic mixture.

Utility of Peroxides: Anti-Markovnikov Addition

  • Haloacids add to alkenes to give the more substituted halide.
  • In the presence of a peroxide, the less substituted halide is preferred.
  • The peroxide is cleaved in the first initiation step, and the resulting alkoxy radicals abstract hydrogen from H-Br to yield Br radicals, which propagate the chain reaction.
  • This is known as the anti-Markovnikov addition of HBr to alkenes

Stereochemistry of a Radical Addition Reaction

  • A new asymmetric center is formed, and the product is a racemic mixture.

Radicals Resist Rearrangement

  • Carbocations readily rearrange. Radicals, with their half-filled p-orbitals, rarely exhibit migration of hydrogen or ordinary alkyl groups.

Radical Addition of HBr is Unique

  • HCl and HI exhibit Markovnikov addition with or without peroxide.

Bromine Radicals Hit the Sweet Spot

  • Bromine radicals are reactive enough to make the reaction happen, while still being selective enough to form the more thermodynamically stable radical, making it unique.

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Description

This quiz covers the key concepts of radical chlorination and bromination of alkanes, including the formation of 1-chlorobutane and 2-chlorobutane. It delves into the stability of radicals, regiochemistry, and the principles governing the reactivity and selectivity of chlorine and bromine radicals. Test your understanding of these important organic chemistry principles.

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