Organic Chemistry

130 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

The bond dissociation energy is indicated by the special term Recall from

Section 2.3 that the barrier to rotation about the bond of ethene is In

other words, it takes to break the bond.

The value of for —calculated by subtracting the for the

bonds being formed from the for the bonds being broken—indicates that the ad-

dition of HBr to ethene is an exothermic reaction. But does this mean that the for

the reaction is also negative? In other words, is the reaction exergonic as well as

exothermic? Because has a significant negative value you can

assume that is also negative. If the value of were close to zero, you could no

longer assume that has the same sign as

Keep in mind that two assumptions are being made when using values to pre-

dict values of The first is that the entropy change in the reaction is small, caus-

ing to be close to zero and, therefore, the value of to be very close to the

value of the second is that the reaction is taking place in the gas phase.

When reactions are carried out in solution, which is the case for the vast majority of

organic reactions, the solvent molecules can interact with the reagents and with the

products. Polar solvent molecules cluster around a charge (either a full charge or a par-

tial charge) on a reactant or product, so that the negative poles of the solvent molecules

surround the positive charge and the positive poles of the solvent molecules surround

the negative charge. The interaction between a solvent and a species (a molecule or

ion) in solution is called solvation.

Solvation can have a large effect on both the and the of a reaction. For ex-

ample, in a reaction in which a polar reagent is solvated, the for breaking the

dipole–dipole interactions between the solvent and the reagent has to be taken into ac-

count, and in a reaction in which a polar product is solvated, the for forming the

dipole–dipole interactions between the solvent and the product has to be taken into

account. In addition, solvation of a polar reagent or a polar product by a polar solvent

can greatly reduce the freedom of motion of the solvent molecules, and this will affect

the value of

PROBLEM 17

a. Using the bond dissociation energies in Table 3.1, calculate the for the addition of

HCl to ethene.

b. Calculate the for the addition of to ethene.

c. Are the reactions exothermic or endothermic?

d. Do you expect the reactions to be exergonic or endergonic?

Kinetics

Knowing whether a given reaction is exergonic or endergonic will not tell you how

fast the reaction occurs, because the of a reaction tells you only the difference be-

tween the stability of the reactants and the stability of the products; it does not tell you

¢G°

¢H° H 2

¢H°

¢S°.

¢H°

¢H°

¢H° ¢S°

d−

d−

d−

d− d−

d+

d+

d+

d+

d+

d+

d+

d+

d−

d−

d−

d+ d+

d+

d+

d+

d+

d+

d+

H

HO

H H

O

HH

O

H

H

O

H

H

O

H

+−

H

O

H

O

solvation of a negative charge

by water

solvation of a positive charge

by water

H

O H

H

¢G°;

T¢S° ¢H°

¢G°.

¢H°

¢H° ¢G°.

¢G° ¢H°

¢H° (-20 kcal>mol),

¢G°

¢H°

• 20 kcal>mol ¢H° ¢H°

63 kcal>mol p

p 63 kcal>mol.

DH°.

3-D Molecule:

Hydrated lithium cation

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