6. Thermodynamics and Kinetics

a. Propose a mechanism for the following reaction:

b. Use the bond-dissociation enthalpies given in Table 4-2 (page 167) to calculate the value of ΔH° for each step shown in your mechanism. (The BDE for CH₂=CHCH₂―Br is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ΔH° for the reaction. Are these values consistent with a rapid free-radical chain reaction?

Keto–enol tautomerism is a reaction we discuss in detail in Chapter 19. Estimate the equilibrium constant of this reaction (BDE for C―C π bond = 65 kcal/mol ; for C―O π bond = 85 kcal/mol).

Predict the sign and magnitude of ∆H^oin kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.

Iodination of alkanes using iodine (I₂) is usually an unfavorable reaction. (See Problem 4-17 , for example.) Tetraiodomethane (CI₄) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔH for each of the steps in your proposed mechanism.

The following bond-dissociation energies maybe helpful:

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

a. CH₃—CH₃ + I₂ → CH₃CH₂I + HI

The bromination of methane proceeds through the following steps:

d. Compute the overall value of ΔH° for the bromination

a. Using bond-dissociation enthalpies from Table 4-2. (page 167), calculate the heat of reaction for each step in the free-radical bromination of methane

b. Calculate the overall heat of the reaction.

•CH₃ + HCl → CH₄ + Cl•

b. What is the activation energy for this reverse reaction?

c. What is the heat of reaction (ΔH°) for this reverse reaction?

a. Using the BDEs in Table 4-2 (page 167), compute the value of ΔH° for each step in the iodination of methane.

b. Compute the overall value of ΔH° for iodination.

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

b. CH₃CH₂Cl + HI → CH₃CH₂I + HCl

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

c. (CH₃)₃C—OH + HCl → (CH₃)₃C—Cl + H₂O