11. Radical Reactions

What are the product(s) of each of the following reactions? Disregard stereoisomers.

c.

Suggest a mechanism for the following reactions.

(a)

Free-radical chlorination of hexane gives very poor yields of 1-chlorohexane, while cyclohexane can be converted to chlorocyclohexane in good yield.

a. How do you account for this difference?

b. What ratio of reactants (cyclohexane and chlorine) would you use for the synthesis of chlorocyclohexane?

How could an industrial plant control the proportions of methane and chlorine to favor production of CCl₄? To favor production of CH₃Cl?

Peroxides are often added to free-radical reactions as initiators because the oxygen–oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of the O―O bond in hydrogen peroxide (H―O―O―H) is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide-initiated reaction of cyclopentane with chlorine. The BDE for HO―Cl is 210 kJ/mol (50 kcal/mol).

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.

a. Explain how a mixture is formed from this stoichiometric mixture of reactants, and propose mechanisms for the ­formation of these compounds from chloromethane.

Write a mechanism for the light-initiated reaction of cyclohexane with chlorine to give chlorocyclohexane. Label the ­initiation and propagation steps.

For each alkane,

3. which monobrominated derivatives could you form in good yield by free-radical bromination?

c. 2-methylpentane

d. 2,2,3,3-tetramethylbutane

For each alkane,

1. draw all the possible monochlorinated derivatives.

c. 2-methylpentane

d. 2,2,3,3-tetramethyl butane

The chlorination of pentane gives a mixture of three monochlorinated products.

a. Draw their structures.

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.

(a)

The radical fluorination of 2-methyl propane resulted in a 14:86 ratio of products.

(a) On the basis of this ratio, calculate the relative reactivity of 1° and 3° C―H bonds in the radical fluorination.

The radical fluorination of 2-methyl propane resulted in a 14:86 ratio of products.

(b) From the relative reactivity, calculate the difference in energy between the transition states of the first propagation steps leading to a 1° and 3° radical.