Organic Chemistry

230 CHAPTER 5 Stereochemistry

PROBLEM 49

a. What products would be obtained from the addition of to cyclohexene if the solvent

were instead of

b. Propose a mechanism for the reaction.

PROBLEM 50

What stereoisomers would you expect to obtain from each of the following reactions?

a. d.

b. e.

c. f.

PROBLEM 51

a. What is the major product obtained from the reaction of propene and plus

excess

b. Indicate the relative amounts of the stereoisomers obtained.

5.20 Stereochemistry of Enzyme-Catalyzed Reactions

The chemistry associated with living organisms is called biochemistry. When you

study biochemistry, you study the structures and functions of the molecules found in

the biological world and the reactions involved in the synthesis and degradation of

these molecules. Because the compounds in living organisms are organic compounds,

it is not surprising that many of the reactions encountered in organic chemistry are also

seen when you study the chemistry of biological systems. Living cells do not contain

molecules such as HBr, or so you would not expect to find the addition of

such reagents to alkenes in biological systems. However, living cells do contain water

and acid catalysts, so some alkenes found in biological systems undergo the acid-

catalyzed addition of water (Section 4.5).

The organic reactions that occur in biological systems are catalyzed by enzymes.

Enzyme-catalyzed reactions are almost always completely stereoselective. In other

words, enzymes catalyze reactions that form only a single stereoisomer. For example,

the enzyme fumarase, which catalyzes the addition of water to fumarate, forms only

(S)-malate—the Renantiomer is not formed.

An enzyme-catalyzed reaction forms only one stereoisomer because the binding

site of an enzyme is chiral. The chiral binding site restricts delivery of reagents to only

one side of the functional group of the reactant. Consequently, only one stereoisomer

is formed.

+ H 2 O

H COO−

−OOC H

fumarase

COO−

C

−OOCCH

2

H

OH

fumarate (S)-malate

CC

Cl 2 , BH 3 ,

Cl-?

Br 2

CH 2 CH 3

H 2

Pt/C

CH 3 CH 3 CH 3

Br 2

CH 2 Cl 2

CH 3 CH 3

H 2

Pt/C

H 2

Pt/C

CH 3 CH 2 CH 3

H 3 C CH 2 CH 3

CC

Br 2

CH 2 Cl 2

CH 3 CH 2 CH 3

CC

CH 3 CH 2 CH 3

CH 3 CH 2 CH 3

Br 2

CH 2 Cl 2

H 2 O CH 2 Cl 2?

Br 2