6. Thermodynamics and Kinetics

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(c)

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(d) I• vs •OH

In the following reactions, identify the bonds formed and the bonds broken.

(a)

In the following reactions, identify the bonds formed and the bonds broken.

(c)

Teflon-coated frying pans routinely endure temperatures that would cause polyethylene or polypropylene to oxidize and decompose. Decomposition of polyethylene is initiated by free-radical abstraction of a hydrogen atom by O₂. Bond-dissociation energies of C—H bonds are about 400 kJ/mol, and C—F bonds are about 460 kJ/mol. The BDE of the H—OO bond is about 192 kJ/mol, and the F—OO bond is about 63 kJ/mol. Show why Teflon (Figure 7-5) is much more resistant to oxidation than polyethylene is.

Use the bond-dissociation enthalpies in Table 4-2 (page 167) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies for formation of the two radicals.

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 ΔH° 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 reaction. (The I―Cl bond-dissociation enthalpy is 211 kJ/mol or 50 kcal/mol.)

Tributyltin hydride (Bu₃SnH) is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. ­Free-radical initiators promote this reaction, and free-radical inhibitors are known to slow or stop it. Your job is to develop a mechanism, using the following reaction as the example.

The following bond-dissociation enthalpies may be helpful: 

b. Calculate values of ΔH for your proposed steps to show that they are energetically feasible. (Hint: A trace of Br₂ and light suggests it’s there only as an initiator, to create Br• radicals. Then decide which atom can be abstracted most favorably from the starting materials by the Br• radical. That should complete the initiation. Finally, decide what energetically favored propagation steps will accomplish the reaction.)

Which of the following would be expected to give a hotter flame during combustion? Explain.

Without concerning yourself with the mechanism of the reaction, calculate the equilibrium constant for the following equilibrium processes. (Assume T = 298 K.)

(a)

Calculate ∆H° for the following equilibrium processes.

(a)

Calculate ∆H° for the following equilibrium processes.

(d)

Calculate the ∆H° value for the following reaction:

To this point, hydrogenation has always been an exothermic process. Using the numbers from Fig­ure 21.6, calculate ∆H^o_hydr for each step of the reduction of benzene.

∆H₁ + ∆H₂ + ∆H₃ = ∆H_tot = ―49.5 kcal /mol (―208 kJ/mol)