6. Thermodynamics and Kinetics

Based on the Hammond postulate, which of the following is true about an endothermic reaction?

Deuterium (D) is the hydrogen isotope of mass number 2, with a proton and a neutron in its nucleus. The chemistry of deuterium is nearly identical to the chemistry of hydrogen, except that the C―D bond is slightly stronger than the C―H bond by 5.0 kJ/mol (1.2 kcal/ mol). Reaction rates tend to be slower when a C―D bond (as opposed to a C―H bond) is broken in a rate-limiting step. This effect, called a kinetic isotope effect, is clearly seen in the chlorination of methane. Methane undergoes free-radical chlorination 12 times as fast as tetradeuteriomethane (CD4) Faster: CH4 + Cl⋅ —> CH3Cl + HCl relative rate= 12 Slower: CD4 + Cl⋅ —> CD3Cl + DCl relative rate= 1 c. Consider the thermodynamics of the chlorination of methane and the chlorination of ethane, and use the Hammond postulate to explain why one of these reactions has a much larger isotope effect than the other.

Deuterium (D) is the hydrogen isotope of mass number 2, with a proton and a neutron in its nucleus. The chemistry of deuterium is nearly identical to the chemistry of hydrogen, except that the C―D bond is slightly stronger than the C―H bond by 5.0 kJ/mol (1.2 kcal/ mol). Reaction rates tend to be slower when a C―D bond (as opposed to a C―H bond) is broken in a rate-limiting step. This effect, called a kinetic isotope effect, is clearly seen in the chlorination of methane. Methane undergoes free-radical chlorination 12 times as fast as tetradeuteriomethane (CD4) Faster: CH4 + Cl⋅ —> CH3Cl + HCl relative rate= 12 Slower: CD4 + Cl⋅ —> CD3Cl + DCl relative rate= 1 c. Consider the thermodynamics of the chlorination of methane and the chlorination of ethane, and use the Hammond postulate to explain why one of these reactions has a much larger isotope effect than the other.

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction: Cyclohexane + trace Br2 hv—> 3-bromocyclohexene + HBr b. Draw the structure of the rate-limiting transition state. c. Use Hammond's postulate to predict which intermediate most closely resembles this transition state.

Assuming that ∆H° = -15kcal/mol for the reaction in Assessment 5.31(b), show the transition state for the forward and reverse reactions.