Question 1

Which statement best explains why the major product of propane chlorination is 2-chloropropane, even though there are more primary hydrogens than secondary hydrogens in propane?

Accepted Answer

The activation energy for abstraction of a secondary hydrogen is lower, making the formation of the 2° radical more favorable.

Question 2

Given the bond dissociation enthalpies for the formation of free radicals, which radical is the most stable?

Accepted Answer

Tertiary radical ($$\mathrm{R_3C^\bullet})$$

Question 3

In the free-radical chlorination of isobutane, there are nine primary hydrogens and one tertiary hydrogen. If the relative reactivity of tertiary hydrogens is 5.5 times that of primary hydrogens, what is the expected ratio of tertiary to primary product?

Accepted Answer

$$\dfrac{5.5}{9.0 + 5.5}$$

Question 4

Which of the following best describes the selectivity of bromination compared to chlorination in the free-radical halogenation of propane?

Accepted Answer

Bromination is more selective than chlorination.

Question 5

What is the main reason bromination of propane is much slower than chlorination?

Accepted Answer

The abstraction of hydrogen by bromine is endothermic, requiring more energy.

Question 6

During the chlorination of propane, what is the total energy change for the reaction at the secondary position?

Accepted Answer

$$-34\ \mathrm{kJ/mol}$$

Question 7

Which hydrogen atom in propane is abstracted more frequently during chlorination, and why?

Accepted Answer

Secondary hydrogen, because the secondary radical and its transition state are lower in energy.

Question 8

If the activation energy for abstraction of a secondary hydrogen in propane is 13 kJ/mol lower than for a primary hydrogen, what effect does this have on the product distribution?

Accepted Answer

It favors the formation of secondary products.

Question 9

In the bromination of propane, the product ratio of 2-bromopropane to 1-bromopropane is 97:3. What does this indicate about the reactivity of secondary hydrogens compared to primary hydrogens?

Accepted Answer

Secondary hydrogens are 97 times as reactive as primary hydrogens.

Question 10

Which of the following best explains why tertiary free radicals are more stable than secondary or primary free radicals?

Accepted Answer

Tertiary radicals are stabilized by hyperconjugation from more alkyl substituents.

Question 11

Which equation correctly represents the bond dissociation enthalpy for the formation of a secondary radical from propane?

Accepted Answer

$$\mathrm{CH_3CH_2CH_3} \rightarrow \mathrm{CH_3CH^\bullet CH_3$} + \mathrm{H^\bullet}$$

Question 12

What is the bond dissociation enthalpy for the formation of a tertiary radical, according to the provided data?

Accepted Answer

$$381\ \mathrm{kJ/mol}$$

Question 13

Which of the following statements about the energy diagram for propane chlorination is correct?

Accepted Answer

The activation energy for formation of the 2° radical is lower than for the 1° radical.

Question 14

Why is bromination of propane more selective than chlorination?

Accepted Answer

The transition state for bromination resembles the products more closely, favoring the more stable radical.

Question 15

If the energy released in forming the H–Cl bond is $$-431\ \mathrm{kJ/mol}$$ and the energy required to break the CH$$_3CH$_2CH$_3$$–H bond is $$+410\ \mathrm{kJ/mol}$$, what is the total energy change for the reaction at the primary position?

Accepted Answer

$$-21\ \mathrm{kJ/mol}$$

Question 16

Which of the following is NOT a reason why free radicals are more stable when more highly substituted?

Accepted Answer

Presence of electron-withdrawing groups.

Question 17

In the mechanism of chlorination of propane, what is the role of the chlorine molecule in the initiation step?

Accepted Answer

It undergoes homolytic cleavage to form two chlorine radicals.

Question 18

Which product is favored in the bromination of propane, and what is the approximate product ratio?

Accepted Answer

2-bromopropane, 97:3

Question 19

Which factor is most responsible for the difference in selectivity between chlorination and bromination of alkanes?

Accepted Answer

The difference in bond dissociation energies of H–Cl and H–Br.

Free Radical Halogenation of Alkanes: Mechanisms, Selectivity, and Energetics

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