Organic Chemistry

132 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

rate of a reaction number of collisions

per unit of time

=×fraction with

sufficient energy

× fraction with

proper orientation

PROBLEM 18

a. Which of the reactions in Figure 3.4 has a thermodynamically stable product?

b. Which of the reactions in Figure 3.4 has the most kinetically stable product?

c. Which of the reactions in Figure 3.4 has the least kinetically stable product?

PROBLEM 19

Draw a reaction coordinate diagram for a reaction in which

a. the product is thermodynamically unstable and kinetically unstable.

b. the product is thermodynamically unstable and kinetically stable.

The rate of a chemical reaction is the speed at which the reacting substances are

used up or the speed at which the products are formed. The rate of a reaction depends

on the following factors:

1.The number of collisions that take place between the reacting molecules in a

given period of time.The greater the number of collisions, the faster is the

reaction.

2.The fraction of the collisions that occur with sufficient energy to get the reacting

molecules over the energy barrier.If the free energy of activation is small, more

collisions will lead to reaction than if the free energy of activation is large.

3.The fraction of the collisions that occur with the proper orientation.For exam-

ple, 2-butene and HBr will react only if the molecules collide with the hydrogen

of HBr approaching the bond of 2-butene. If collision occurs with the hydro-

gen approaching a methyl group of 2-butene, no reaction will take place, regard-

less of the energy of the collision.

Increasing the concentration of the reactants increases the rate of a reaction because

it increases the number of collisions that occur in a given period of time. Increasing the

temperature at which the reaction is carried out also increases the rate of a reaction

because it increases both the frequency of collisions (molecules that are moving faster

collide more frequently) and the number of collisions that have sufficient energy to get

the reacting molecules over the energy barrier.

For a reaction in which a single reactant molecule A is converted into a product

molecule B, the rate of the reaction is proportional to the concentration of A. If the

concentration of A is doubled, the rate of the reaction will double; if the concentration

of A is tripled, the rate of the reaction will triple; and so on. Because the rate of this re-

action is proportional to the concentration of only onereactant, it is called a first-

order reaction.

We can replace the proportionality symbol with an equals sign if we use a pro-

portionality constant k, which is called a rate constant. The rate constant of a first-

order reaction is called a first-order rate constant.

A reaction whose rate depends on the concentrations of tworeactants is called a

second-order reaction. If the concentration of either A or B is doubled, the rate of the

rate = k[A]

(r)

AB

rate ∝ [A]

p

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