Deuterium (D) is the hydrogen isotope of mass number 2. Deuterium shows almost identical chemistry to that of hydrogen, except that C−D bond is slightly stronger than the C−H bond by 5.0 kJ/mol, so that when a C−D bond is broken in a rate-limiting step, the reaction rate tends to be slower (as opposed to a C−H bond), an effect called a kinetic isotope effect.

When the chlorination of methane was compared to that of propane, experimental results showed that methane has a much larger isotope effect than propane. Consider the thermodynamics of the chlorination of methane and the chlorination of propane and explain this phenomenon using the Hammond postulate.

• A. The rate-limiting step in the chlorination of methane is endothermic, while in the chlorination of propane it is exothermic. Therefore, according to the Hammond's Postulate, substitution of H atoms in methane molecule with D produces a greater change in its transition state energy than the same change in the propane molecule produces in its transition state energy. This means that the deuterium will be abstracted more slowly in the chlorination of methane than in propane, resulting in a slower overall reaction rate in the chlorination of methane.
• B. The rate-limiting step in the chlorination of methane is endothermic, while in the chlorination of propane it is exothermic. Therefore, according to the Hammond's Postulate, substitution of H atoms in methane molecule with D produces a greater change in its transition state energy than the same change in the propane molecule produces in its transition state energy. This means that the deuterium will be abstracted more slowly in the chlorination of methane than in propane, resulting in a faster overall reaction rate in the chlorination of methane.
• C. The rate-limiting step in the chlorination of methane is endothermic, while in the chlorination of propane it is exothermic. Therefore, according to the Hammond's Postulate, substitution of H atoms in methane molecule with D produces a smaller change in its transition state energy than the same change in the propane molecule produces in its transition state energy. This means that the deuterium will be abstracted faster in the chlorination of methane than in propane, resulting in a faster overall reaction rate in the chlorination of methane.
• D. The rate-limiting step in the chlorination of methane is endothermic, while in the chlorination of propane it is exothermic. Therefore, according to the Hammond's Postulate, substitution of H atoms in methane molecule with D produces a smaller change in its transition state energy than the same change in the propane molecule produces in its transition state energy. This means that the deuterium will be abstracted more slowly in the chlorination of methane than in propane, resulting in a slower overall reaction rate in the chlorination of methane.

Answer: (A)