Organic Chemistry

152 CHAPTER 4 Reactions of Alkenes

*At a pH of 4, for example, the concentration of is whereas the concentration of

water in a dilute aqueous solution is 55.5 M.

HO- 1 * 10 -^10 M,

As we saw in Section 3.7, the addition of the electrophile to the alkene is relatively

slow, and the subsequent addition of the nucleophile to the carbocation occurs rapidly.

The reaction of the carbocation with a nucleophile is so fast that the carbocation com-

bines with whatever nucleophile it collides with first. In the previous hydration reac-

tion, there were two nucleophiles in solution: water and the counterion of the acid

that was used to start the reaction. (Notice that is not a nucleophile in

this reaction because there is no appreciable concentration of in an acidic solu-

tion.)* Because the concentration of water is much greater than the concentration of

the counterion, the carbocation is much more likely to collide with water. The product

of the collision is a protonated alcohol. Because the pH of the solution is greater than

the of the protonated alcohol (remember that protonated alcohols are very strong

acids; see Sections 1.17 and 1.19), the protonated alcohol loses a proton, and the final

product of the addition reaction is an alcohol. A reaction coordinate diagram for the

reaction is shown in Figure 4.5.

pKa

HO-

(e.g., Cl-) HO-

CH 3 CH CH 2

H 3 O+

H 3 O+

H

CH 3 CHCH 3

H 2 O

+

CH 3 CHCH 3

OH

CH 3 CHCH 3

OH

+

Free energy

Progress of the reaction

Figure 4.5N
A reaction coordinate diagram for
the acid-catalyzed addition of
water to an alkene.

Synthetic Tutorial:

Addition of water to an

alkene

A proton adds to the alkene in the first step, but a proton is returned to the reaction

mixture in the final step. Overall, a proton is not consumed. A species that increases

the rate of a reaction and is not consumed during the course of the reaction is called a

catalyst. Catalysts increase the reaction rate by decreasing the activation energy of the

reaction (Section 3.7). Catalysts do notaffect the equilibrium constant of the reaction.

In other words, a catalyst increases the rateat which a product is formed, but does not

affect the amountof product formed. The catalyst in the hydration of an alkene is an

acid, so the reaction is said to be an acid-catalyzed reaction.

PROBLEM 7

The of a protonated alcohol is about and the of an alcohol is about 15.

Therefore, as long as the pH of the solution is greater than _____ and less than _____ ,

more than 50% of 2-propanol (the product of the previous reaction) will be in its neutral,

nonprotonated form.

PROBLEM 8

Use Figure 4.5 to answer the following questions about the acid-catalyzed hydration of an

alkene:

pKa -2.5 pKa