Chemistry Workshops

for Organic Chemistry

Chapter 7 Workshop Problems
Alkyl Halides and Radical Reactions

1. Consider the bromination of 3-methylpentane.

a. Write out all of the different monobromination products (including stereoisomers) that may be obtained. Name each product.

b. Which of these isomers are chiral and which are achiral?

c. Which pairs of isomers are enantiomers?

d. Using the dissociation energies for primary, secondary, and tertiary C-H bonds of 98, 95, and 92 kcal/mol, arrange the products in the order that reflects the relative rates of formation. Briefly explain your reasoning for the ordering you propose (a calculation might be appropriate).

2. Styrene can readily be polymerized by either strong acid or a radical initiator as shown below. Give a reasonable mechanism for each process (use curved arrows to show movement of electron pairs or "fish hooks" to show movement of single electrons). You need to show just a few of the polymer chain lengthening steps. Be sure to explain why the polymer has a regular structure with the phenyl group on every other carbon in the polymer backbone.

3. Consider the chlorination of methane. The initiation step involves the dissociation of Cl2 using either light or heat. Alternatively, tetramethyl lead ((CH3)4Pb) can be added to initiate this same reaction at a lower temperature. The Pb-C bond energy in (CH3)4Pb is 49 kcal/mol.

a. Show the initiation and propagation steps for the chlorination of CH4 using (CH3)4Pb with CH4 and Cl2. Explain why lower temperatures are needed for the reaction with (CH3)4Pb than with Cl2 as the initiator.

b. An alternative to the accepted mechanism for the chlorination of methane is given below.

This mechanism does not operate to any significant extent under normal conditions. Calculate the enthalpies of each step and suggest a reason why this mechanism cannot compete with the accepted one.

c. When a small amount of iodine is added to a mixture of chlorine and methane, it prevents chlorination from occurring. Therefore, iodine is a free-radical inhibitor for this reaction. Calculate delta H values for the possible reactions of iodine with species present in the chlorination of methane, and use these values to explain why iodine inhibits the reactions.