6. Thermodynamics and Kinetics

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).

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:

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.

For each of the following acid–base reactions, (iv) draw the transition state, paying close attention to the degree of bond forming and breaking present in the transition state. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)

Is the structure of the transition state in the following reaction coordinate diagrams more similar to the structure of the reactant or to the structure of the product?

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Which is the most likely transition state for the reactions shown? Explain your answer. [Note the difference in the size of the partial charges or partial unpaired electrons.]

Which would you expect to be more selective for carbocation formation, the electrophilic addition of HF or HBr to 2-methylbut-2-ene? Explain your answer.

Show the transition state of each step of the following alkene addition reaction. Be sure to indicate whether the transition state in each is reactant-like or product-like.

The most stable intermediate forms first. Explain this statement by showing a reaction coordinate diagram for the formation of a 3° carbocation over a 2° carbocation in the following alkene addition 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).

a. Draw the transition state for the rate-limiting step of each of these reactions, showing how a bond to hydrogen or ­deuterium is being broken in this step.

Using the bond-dissociation energies in Table 5.6,

(b) Will the transition state be reactant-like or product-like? Explain your answer.