11. Radical Reactions

Write the steps for formation of tetrachloromethane (CCl₄) from the reaction of methane with Cl₂ + hv.

A possible alternative mechanism to that shown in Problem 47 for the monochlorination of methane involves the following propagation steps:

How do you know that the reaction does not take place by this mechanism?

If cyclopentane reacts with more than one equivalent of Cl₂ at a high temperature, how many dichlorocyclopentanes would you expect to obtain as products?

Show how you would prepare cyclopentene from each compound.

c. cyclopentane (not by dehydrogenation)

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.

a.

a. Propose a mechanism for the free-radical chlorination of ethane,

a. Draw the structure of the transition state for the second propagation step in the chlorination of methane.

Show whether the transition state is product-like or reactant-like and which of the two partial bonds is stronger.

When healthy, Earth’s stratosphere contains a low concentration of ozone (O₃) that absorbs potentially harmful ­ultraviolet (UV) radiation by the cycle shown at right.

Chlorofluorocarbon refrigerants, such as Freon 12 (CF₂Cl₂), are stable in the lower atmosphere, but in the stratosphere they absorb high-energy UV radiation to generate chlorine radicals.

The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically. The following reactions are all known to be exothermic (except the one requiring light) and to have high rate constants. Propose two mechanisms to explain how a small number of chlorine ­radicals can destroy large numbers of ozone molecules. Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?

Using the BDEs in Table 4-2 (page 167), 

(c) Suggest two reasons why iodine does not react well with methane.

Alkoxy radicals (R—O•) are generally more stable than alkyl (R•) radicals. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a ­hydrogen atom from tert-butyl alcohol, (CH₃)₃COH. Explain why tert-butyl alcohol works as an antiknock additive for gasoline

Using cyclohexane as one of your starting materials, show how you would synthesize the following compounds.

(a)

When exactly 1 mole of methane is mixed with exactly 1 mole of chlorine and light is shone on the mixture, a ­chlorination reaction occurs. The products are found to contain substantial amounts of di-, tri-, and tetrachloromethane, as well as ­unreacted methane.

b. How would you run this reaction to get a good conversion of methane to CH₃Cl? Of methane to CCl₄?

Write the propagation steps leading to the formation of dichloromethane (CH₂Cl₂) from chloromethane.

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.

b.

b. Calculate ΔH° for each step in this reaction.

c. Calculate the overall value of ΔH° for this reaction.