Organic Chemistry

220 CHAPTER 5 Stereochemistry

In discussing the stereochemistry of electrophilic addition reactions, we will look

first at reactions that form a product with one asymmetric carbon. Then we will look at

reactions that form a product with two asymmetric carbons.

Addition Reactions that Form One Asymmetric Carbon

When a reactant that does not have an asymmetric carbon undergoes a reaction that

forms a product with oneasymmetric carbon, the product will be a racemic mixture.

For example, the reaction of 1-butene with HBr forms identical amounts of

(R)-2-bromobutane and (S)-2-bromobutane. Thus, an electrophilic addition reaction

that forms a compound with one asymmetric carbon from a reactant without any

asymmetric carbons is not stereoselective because it does not select for a particular

stereoisomer. Why is this so?

You will see why a racemic mixture is obtained if you examine the structure of

the carbocation formed in the first step of the reaction. The positively charged

carbon is hybridized, so the three atoms to which it is bonded lie in a plane

(Section 1.10). When the bromide ion approaches the positively charged carbon

from above the plane, one enantiomer is formed, but when it approaches from below

the plane, the other enantiomer is formed. Because the bromide ion has equal access

to both sides of the plane, identical amounts of the Rand Senantiomers are obtained

from the reaction.

PROBLEM 41

a. Is the reaction of 2-butene with HBr regioselective?

b. Is it stereoselective?

c. Is it stereospecific?

d. Is the reaction of 1-butene with HBr regioselective?

e. Is it stereoselective?

f. Is it stereospecific?

When HBr adds to 2-methyl-1-butene in the presence of a peroxide, the product has

one asymmetric carbon. Therefore, we can predict that identical amounts of the Rand

Senantiomers are formed.

CH 3 CH 2 CCH 2

CH 3

2-methyl-1-butene

+ HBr

peroxide

CH 3 CH 2 CHCH 2 Br

CH 3

1-bromo-2-methylbutane*

asymmetric

carbon

CH 3

CH 3

C

C

CH 3 CH 2 Br−

(S)-2-bromobutane

(R)-2-bromobutane

H

+

CH 3

CH 3 CH 2 H

Br

C

Br

CH 3 CH 2

H

sp^2

Tutorial:

Review addition of HBr

tutorial in Chapter 3