What are the important differences between chlorination and bromination?

Chlorination is faster than bromination.

Chlorination yields a mixture of products, while bromination usually yields only one major product.

Chlorination is slower than bromination.

Chlorination usually yields only one major product, while bromination yields a mixture of products.

What is the reactivity-selectivity principle?

Less reactive reagents are more selective.

More reactive reagents are more selective.

Describe the 2-step method for converting an alkane to an alkene.

Radical halogenation with Br2 followed by elimination of HBr.

Oxidation with ROOR followed by elimination of OH.

Allylic bromination with NBS followed by elimination of HBr.

Radical addition of HBr followed by substitution of Br-.

A radical halogenation reaction at a stereogenic center results in ________________ .

a racemic mixture of enantiomers

no change in the stereochemistry of the products.

Radical Reactions Organic Chemistry Test! Quiz

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Questions: 10 | Attempts: 1,964 | Updated: Mar 22, 2023

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Questions and Answers

1.

Which radical is most stable?
- A.
  
  ĊH3
- B.
  
  R-ĊH2
- C.
  
  R2-ĊH
- D.
  
  R3-Ċ
- E.
  
  CH2=CH-ĊH2
Correct Answer
E. CH2=CH-ĊH2

Explanation
An allyl radical is the most stable (even more stable than a 3° radical) because the "true" structure of the allyl radical is a hybrid of two resonance structures. The π bond and the unpaired electron are delocalized over the entire molecule which helps to stabilize the allyl radical. (pg 551-552)

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1
0
2.

What is the geometry of a carbon radical?
- A.
  
  Linear
- B.
  
  Tetrahedral
- C.
  
  Trigonal planar
- D.
  
  Bent
- E.
  
  Trigonal pyramidal
Correct Answer
C. Trigonal planar

Explanation
A carbon radical is trigonal planar, with 120° bond angles. (pg. 537)

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3.

What is the orbital hybridization of a carbon radical?
- A.
  
  P
- B.
  
  Sp³
- C.
  
  Sp²
- D.
  
  Sp
Correct Answer
C. Sp²

Explanation
A carbon radical is sp² hybridized, similar to carbocations. The unhybridized p orbital contains the unpaired electron and extends above and below the trigonal planar carbon. (pg. 537)

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4.

Fill in the blank correctly:The _______ the bond dissociation energy for a C-H bond, the more stable the resulting carbon radical.
- A.
  
  Lower
- B.
  
  Higher
Correct Answer
A. Lower

Explanation
Cleavage of the stronger 1° C-H bond to form a 1° carbon radical requires more energy than cleavage of the weaker 2° C-H bond to form a 2° carbon radical. This makes the 2° radical more stable because less energy is required for its formation. Cleavage of the WEAKER bond forms the MORE stable radical. (pg. 538)

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5.

Alkanes react with halogens in the presence of __________ to form alkyl halides.
- A.
  
  Light or heat
- B.
  
  Hv or Δ
- C.
  
  Peroxides
- D.
  
  O3
Correct Answer(s)
A. Light or heat
B. Hv or Δ

Explanation
In the presence of light (hv) or heat (Δ), alkanes react with halogens to form alkyl halides. The energy from light or heat is required to form the initial halide radical. (pg. 540)

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6.

What are the important differences between chlorination and bromination?
- A.
  
  Chlorination is faster than bromination.
- B.
  
  Chlorination is slower than bromination.
- C.
  
  Chlorination yields a mixture of products, while bromination usually yields only one major product.
- D.
  
  Chlorination usually yields only one major product, while bromination yields a mixture of products.
Correct Answer(s)
A. Chlorination is faster than bromination.
C. Chlorination yields a mixture of products, while bromination usually yields only one major product.

Explanation
Chlorination is faster than bromination; its rate-determining step is exothermic. Chlorination also yields a mixture of products because the rate-determining step is exothermic, and so all radicals are formed, leading to a mixture of products. Bromination has a slow, endothermic rate-determining step, and so it favors the formation of the most stable radical, leading to a single major product. (pg. 544-547)

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7.

What is the reactivity-selectivity principle?
- A.
  
  Less reactive reagents are more selective.
- B.
  
  More reactive reagents are more selective.
Correct Answer
A. Less reactive reagents are more selective.

Explanation
Less reactive reagents are more selective. In bromination (which is less reactive than chlorination), the major (and sometimes only) product results from cleavage of the WEAKEST C-H bond. (pg. 545)

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8.

Describe the 2-step method for converting an alkane to an alkene.
- A.
  
  Oxidation with ROOR followed by elimination of OH.
- B.
  
  Radical halogenation with Br2 followed by elimination of HBr.
- C.
  
  Allylic bromination with NBS followed by elimination of HBr.
- D.
  
  Radical addition of HBr followed by substitution of Br-.
Correct Answer
B. Radical halogenation with Br2 followed by elimination of HBr.

Explanation
Halogenation is a useful tool because it adds a functional group to an otherwise unreactive alkane, making an alkyl halide. This alkyl halide can then be converted to an alkene by elimination, or to alcohols or ethers by nucleophilic substitution. (pg. 547)

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9.

What is/are the monohalogenation product(s)?n-butane + chlorine + hv --> ______________
- A.
  
  1-chlorobutane
- B.
  
  (R)-2-chlorobutane
- C.
  
  (S)-2-chlorobutane
- D.
  
  (R,S)-2,3-dichlorobutane
- E.
  
  All of the above
Correct Answer(s)
A. 1-chlorobutane
B. (R)-2-chlorobutane
C. (S)-2-chlorobutane

Explanation
Monochlorination of n-butane forms two constitutional isomers by replacement of either a 1° or 2° hydrogen. Radical abstraction of the 2° hydrogen forms a planar radical that can react with Cl2 from either side, resulting in a racemic mixture of two enantiomers. (pg. 548)

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10.

A radical halogenation reaction at a stereogenic center results in ________________ .
- A.
  
  No change in the stereochemistry of the products.
- B.
  
  A racemic mixture of enantiomers
- C.
  
  A pair of diastereomers
- D.
  
  An achiral product
Correct Answer
B. A racemic mixture of enantiomers

Explanation
Abstraction of a hydrogen at a stereogenic center forms a trigonal planar sp² hybridized radical that is now achiral. This achiral radical then reacts with Cl2 from either side to form a new stereogenic center, resulting in an equal amount of two enantiomers -- a racemic mixture. (pg. 549)

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Current Version
Mar 22, 2023

Quiz Edited by
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Mar 05, 2009

Quiz Created by
Geekgirljess

Radical Reactions Organic Chemistry Test! Quiz

Which radical is most stable?

What is the geometry of a carbon radical?

What is the orbital hybridization of a carbon radical?

Fill in the blank correctly:The _______ the bond dissociation energy for a C-H bond, the more stable the resulting carbon radical.

Alkanes react with halogens in the presence of __________ to form alkyl halides.

What are the important differences between chlorination and bromination?

What is the reactivity-selectivity principle?

Describe the 2-step method for converting an alkane to an alkene.

What is/are the monohalogenation product(s)?n-butane + chlorine + hv --> ______________

A radical halogenation reaction at a stereogenic center results in ________________ .