Organic Chemistry

Section 4.7 Addition of Halogens 159

dissolves both reactants, but does not participate in the reaction. The foregoing reac-
tions illustrate the way in which organic reactions are typically written. The reactants
are placed to the left of the reaction arrow, and the products are placed to the right of
the arrow. The reaction conditions, such as the solvent, the temperature, or any re-
quired catalyst, are written above or below the arrow. Sometimes reactions are written
by placing only the organic (carbon-containing) reagent on the left-hand side of the
arrow and writing the other reagent(s) above or below the arrow.

and are halogens, but they are not used as reagents in electrophilic addition re-
actions. Fluorine reacts explosively with alkenes, so the electrophilic addition of is
not a synthetically useful reaction. The addition of to an alkene is a thermodynami-
cally unfavorable reaction: The vicinal diiodides are unstable at room temperature, de-
composing back to the alkene and

If rather than is used as the solvent, the major product of the reaction
will be a vicinal halohydrin. A halohydrin(or more specifically, a bromohydrin or a
chlorohydrin) is an organic molecule that contains both a halogen and an OH group. In
a vicinal halohydrin, the halogen and the OH group are bonded to adjacent carbons.

The mechanism for halohydrin formation involves the formation of a cyclic bromo-
nium ion (or chloronium ion) in the first step of the reaction, because (or ) is
the only electrophile in the reaction mixture. In the second step, the bromonium ion
rapidly reacts with whatever nucleophile it bumps into. In other words, the elec-
trophile and nucleophile do not have to come from the same molecule. There are two
nucleophiles present in solution: and Because is the solvent, its con-
centration far exceeds that of Consequently, the bromonium ion is more likely to
collide with a molecule of water than with The protonated halohydrin that is
formed is a strong acid (Section 1.19), so it loses a proton.

Br-.

Br-.

H 2 O Br-. H 2 O

Br+ Cl+

propene

a bromohydrin

major product

minor product

OH Br

+

H 2 O

CH 3 CH CH 2 Br 2 CH 3 CHCH 2 Br + CH 3 CHCH 2 Br + HBr

2-methyl-2-butene

a chlorohydrin

major product

minor product

CH 3

ClOH Cl

+

H 2 O

CCH 3 Cl 2 + CH 3 CH + HCl

Cl

CH 3 CH CH 3 CHCCH 3 CCH 3

CH 3 CH 3

H 2 O CH 2 Cl 2

I I

+

CH 2 Cl 2

CH 3 CH CHCH 3 I 2 CH 3 CHCHCH 3

I 2.

I 2

F 2

F 2 I 2

ClCl

CH 2 Cl 2

Cl 2

CH 3 CH CHCH 3 CH 3 CHCHCH 3

CH 3 CH CH 2

Br

+ H 3 O

Br

−

Br

Br

Br

CH 2

OH

H

CH 3 CH

+

+

CH 3 CHCH 2 Br

+

OH

CH 3 CHCH 2

slow H 2 O

fast

H 2 O

fast

mechanism for halohydrin formation

Synthetic Tutorial:

Addition of halogens to

alkenes