Organic Chemistry

Section 4.5 Addition of Water and Addition of Alcohols 151

Mechanistic Tutorial:

Addition of water to an

alkene

PROBLEM 6

The addition of HBr to which of the following alkenes is more highly regioselective?

a. b.

4.5 Addition of Water and Addition of Alcohols

Addition of Water
When water is added to an alkene, no reaction takes place, because there is no elec-
trophile present to start a reaction by adding to the nucleophilic alkene. The O H
bonds of water are too strong—water is too weakly acidic—to allow the hydrogen to
act as an electrophile for this reaction.

If, however, an acid (e.g., or HCl) is added to the solution, a reaction will occur
because the acid provides an electrophile. The product of the reaction is an alcohol.
The addition of water to a molecule is called hydration, so we can say that an alkene
will be hydratedin the presence of water and acid.

and HCl are strong acids, so they dissociate al-
most completely in an aqueous solution (Section 1.17). The acid that participates in
the reaction, therefore, is most apt to be a hydronium ion

The first two steps of the mechanism for the acid-catalyzed addition of water to an
alkene are essentially the same as the two steps of the mechanism for the addition of a hy-
drogen halide to an alkene: The electrophile adds to the carbon that is bonded to
the greater number of hydrogens, and the nucleophile (H 2 O)adds to the other sp^2 carbon.

(H+) sp^2

hydronium ion

H 2 SO 4 ++H 2 OH 3 O+ HSO 4 –

(H 3 O+).

H 2 SO 4 1 pKa=- 52 1 pKa=- 72

2-propanol

OH H

CH 3 CH CH 2 + H 2 O CH 3 CH CH 2

H+

H 2 SO 4

CH 3 CH CH 2 + H 2 O no reaction

¬

CH 2 CH 3

CH 3 CH 2 CCH 2 or or

CH 3

CHCH 3

CH 3

CH 3 C

CH 3 CH CH 2 + HOH CH 3 CHCH 3 + H 2 OCH 3 CHCH 3

OH

H

H

slow fast

fast

H 2 O

CH 3 CHCH 3 + H 3 O

OH

+

+

+ +

mechanism for the acid-catalyzed addition of water

addition of

the electrophile

addition of

the nucleophile

a protonated

alcohol

H 2 O removes a

proton, regenerating

the acid catalyst

an alcohol