Organic Chemistry

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes 227

PROBLEM 45

Reaction of 2-ethyl-1-pentene with with or with followed by

leads to a racemic mixture. Explain why a racemic mixture is obtained in

each case.

PROBLEM 46

How could you prove, using a sample of trans-2-butene, that addition of forms a cyclic

bromonium ion intermediate rather than a carbocation intermediate?

If the two asymmetric carbons in the product each have the same four substituents,
the erythro isomers are identical and constitute a meso compound. Therefore, addition
of to trans-2-butene forms a meso compound.

Because only anti addition occurs, addition of to a cyclohexene forms only the
enantiomers that have the added bromine atoms on opposite sides of the ring.

One way to determine what stereoisomers are obtained from many reactions that
create a product with two asymmetric carbons is the mnemonic CIS-SYN-ERYTHRO,
which is easy to remember because all three terms mean “on the same side.”
You can change any two of the terms but you can’t change just one. (For example,
TRANS-ANTI-ERYTHRO,TRANS-SYN-ERYTHREO, and CIS-ANTI-THREO
are allowed, but TRANS-SYN-ERYTHROis not allowed.) So if you have a trans reac-
tant that undergoes addition of (which is anti), the erythro products are obtained.
This mnemonic will work for all reactions that have a product with a structure that can be
described by erythro or threo.
A summary of the stereochemistry of the products obtained from addition reactions
to alkenes is given in Table 5.2.

PROBLEM-SOLVING STRATEGY

Give the configuration of the products obtained from the following reactions:

a. d.

b. e.

c.3-methyl-3-hexene+HBr+peroxide f. trans-3-hexene+Br 2

1-butene+HBr+peroxide trans-3-heptene+Br 2

1-butene+HCl cis-3-heptene+Br 2

Br 2

H H H

Br +

Br

Br 2

CH 2 Cl 2

H

H

Br

Br

H

Br 2

H H

Br

C C

H 3 C

H 3 C H

trans-2-butene a meso compound

perspective formula

Br (^2) =
CH 2 Cl 2
HH
Br
C C
H 3 C CH 3
Br
CH 3
CC
H
C CH 3
Br
plane of
symmetry
a meso compound
Fischer projection
CH 3
HBr
HBr
CH 3
Br 2
Br 2
HO-+H 2 O 2
Br 2 , H 2 >Pt, BH 3