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Chapter 4: Q32P (page 233)
Propionitrile(
)is deprotonated by very strong bases. Write resonance forms to show the stabilization of the carbanion that results.
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When healthy, Earth鈥檚 stratosphere contains a low concentration of ozone (O3)that absorbs potentially harmful ultraviolet (UV) radiation by the cycle shown at right.
Chlorofluorocarbon refrigerants, such as Freon 12 (CF2Cl2), are stable in the lower atmosphere, but in the stratosphere, they absorb high-energy UV radiation to generate chlorine radicals.
The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically. The following reactions are all known to be exothermic (except the one requiring light) and to have high-rate constants. Propose two mechanisms to explain how a small number of chlorine radicals can destroy large numbers of ozone molecules. Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?
Under certain conditions, the bromination of cyclohexene follows an unusual rate law:
(a) What is the kinetic order with respect to cyclohexene?
(b) What is the kinetic order with respect to bromine?
(c) What is the kinetic order overall?
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螖贬0values 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-Clbond-dissociation enthalpy is 211 kJ/molor 50 kcal/mol).
When dichloromethane is treated with strong NaOH , an intermediate is generated that reacts like a carbene. Draw the structure of this reactive intermediate, and propose a mechanism for its formation.
(a) When n-heptaneburns in a gasoline engine, the combustion process takes place too quickly. The explosive detonation makes a noise called knocking. When 2,2,4 trimethylpentane (isooctane) is burned, combustion takes place in a slower, more controlled manner. Combustion is a free-radical chain reaction, and its rate depends on the reactivity of the free-radical intermediates. Explain why isooctane has less tendency to knock than doesn-heptane
(b) Alkoxy radicals (R-O)are generally more stable than alkyl (R) radicals. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a hydrogen atom from tert-butyl alcohol,(CH3)3COH. Explain whytert-butyl alcohol works as an antiknock additive for gasoline.
(c) Use the information in Table 4-2 (page 203) to explain why toluene (PhCH3) has a very high-octane rating of 111. Write an equation to show how toluene reacts with an alkyl free radical to give a relatively stable radical.
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