Radical Reactions Organic Chemistry Test! Quiz

1. What is the reactivity-selectivity principle?

Less reactive reagents are more selective.

More reactive reagents are more selective.

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)

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)

2. Fill in the blank correctly:

The _______ the bond dissociation energy for a C-H bond, the more stable the resulting carbon radical.

Lower

Higher

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)

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)

3. What is the geometry of a carbon radical?

Linear

Tetrahedral

Trigonal planar

Bent

Trigonal pyramidal

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

Explanation

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

4. What is the orbital hybridization of a carbon radical?

P

Sp³

Sp²

Sp

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)

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)

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

No change in the stereochemistry of the products.

A racemic mixture of enantiomers

A pair of diastereomers

An achiral product

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)

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)

6. Which radical is most stable?

ĊH3

R-ĊH2

R2-ĊH

R3-Ċ

CH2=CH-ĊH2

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)

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)

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

Oxidation with ROOR followed by elimination of OH.

Radical halogenation with Br2 followed by elimination of HBr.

Allylic bromination with NBS followed by elimination of HBr.

Radical addition of HBr followed by substitution of Br-.

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)

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)

8. What are the important differences between chlorination and bromination?

Chlorination is faster than bromination.

Chlorination is slower than bromination.

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

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

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)

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

Light or heat

Hv or Δ

Peroxides

O3

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)

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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10. What is/are the monohalogenation product(s)?

n-butane + chlorine + hv --> ______________

1-chlorobutane

(R)-2-chlorobutane

(S)-2-chlorobutane

(R,S)-2,3-dichlorobutane

All of the above

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)

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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