Organic Chemistry

160 CHAPTER 4 Reactions of Alkenes

How can we explain the regioselectivity of the preceding addition reaction? The

electrophile ends up on the carbon bonded to the greater number of hydro-

gens because, in the transition state for the second step of the reaction, the breaking of

the C Br bond has occurred to a greater extent than has the formation of the C O

bond. As a result, there is a partial positive charge on the carbon that is attacked by the

nucleophile.

Therefore, the more stable transition state is achieved by adding the nucleophile to the

most substituted carbon—the one bonded to the lesser numberof hydrogens—

because the partial positive charge will be on a secondary carbon rather than on a pri-

mary carbon. Thus, this reaction also follows the general rule for electrophilic addition

reactions: The electrophile adds to the carbon that is bonded to the greater number

of hydrogens. In this case, the electrophile is

When nucleophiles other than are added to the reaction mixture, they, too,

change the product of the reaction, just as water changed the product of addition

from a vicinal dibromide to a vicinal bromohydrin. Because the concentration of the

added nucleophile will be greater than the concentration of the halide ion generated

from or the added nucleophile will be the nucleophile most likely to partici-

pate in the second step of the reaction. (Ions such as and cannot form covalent

bonds, so they do not react with organic compounds. They serve only as counterions to

negatively charged species, so their presence generally is ignored in writing chemical

equations.)

PROBLEM 17

There are two nucleophiles in each of the following reactions:

a.

b.

c.

d.

For each reaction, explain why there is a greater concentration of one nucleophile than the

other. What will be the major product of each reaction?

CH 3 OH

CH 3 CH CHCH 3 + HBr

CHCH 3 HCl

H 2 O

CH 3 CH +

CH 2 CHCH 3 + 2 NaI + HBr

C

CH 3

Cl 2

CH 3 OH

CH 2 CH 3 +

CCH 3

CH 3 CH 3

ClOCH 3

CH 3 CH + Cl 2 + CH 3 OH CH 3 CHCCH 3

CH 2

Cl

CH 3 CH + Br 2 + NaCl CH 3 CHCH 2 Br

+

+ NaBr

HCl

Na+ K+

Br 2 Cl 2 ,

Br 2

H 2 O

Br+.

sp^2

sp^2

O

CH 3 CH CH 2

H

H

more stable transition state

Br

δ+

δ+

δ+

O

CH 3 CH CH 2

H

H

less stable transition state

Br

δ+

δ+

δ+

¬ ¬

(Br+) sp^2

Do not memorize the products of alkene
addition reactions. Instead, for each re-
action, ask yourself, “What is the elec-
trophile?” and “What nucleophile is
present in the greatest concentration?”