11. Radical Reactions

Explain why the rate of bromination of methane decreases if HBr is added to the reaction mixture.

Predict the major products of the following alkane halogenation reactions. [The number of products shown ignores the formation of racemic mixtures.]

(b)

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

A student adds NBS to a solution of 1-methylcyclohexene and irradiates the mixture with a sunlamp until all the NBS has reacted. After a careful distillation, the product mixture contains two major products of formula C₇H₁₁Br.

a. Draw the resonance forms of the three possible allylic free radical intermediates.

What alkyl halide will be obtained in greatest yield? Ignore stereoisomers.

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.

Using the bond-dissociation energies in Table 5.6,

(a) predict whether or not an iodine radical would be selective for forming a single radical propane.

Predict the major product of the following bromination reactions.

(b)

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

b. Toluene reacts with bromine in the presence of light to give benzyl bromide. Propose a mechanism for this reaction.

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

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

Predict the product(s) of the following halogenation reactions. Only one equivalent of the halogen is used in each case. If the reaction proceeds selectively, indicate this by only drawing the major product. If the reaction is not selective, draw all possible products.

(e)

How many alkyl chlorides are obtained from monochlorination of the following alkanes? Disregard stereoisomers.

f.

For each compound, predict the major product of free-radical bromination. Remember that bromination is highly selective, and only the most stable radical will be formed.

(a) cyclohexane

(b) methylcyclopentane

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

c.

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