Organic Chemistry

Section 12.1 Substitution Reactions of Alcohols 439

Secondary and tertiary alcohols undergo

SN 1 reactions with hydrogen halides.

Primary alcohols undergo reactions

with hydrogen halides.

SN 2

Tertiary alcohols undergo substitution reactions with hydrogen halides faster than do

secondary alcohols because tertiary carbocations are easier to form than secondary car-

bocations (Section 4.2). Thus, the reaction of a tertiary alcohol with a hydrogen halide

proceeds readily at room temperature, whereas the reaction of a secondary alcohol with

a hydrogen halide would have to be heated to have the reaction occur at the same rate.

Primary alcohols cannot undergo reactions because primary carbocations are

too unstable to be formed (Section 11.1). Therefore, when a primary alcohol reacts

with a hydrogen halide, it must do so via an reaction.

Only the substitution product is obtained. No elimination product is formed because

the halide ion, although a good nucleophile, is a weak base, and strong base is needed

in an E2 reaction to remove a proton from a -carbon (Section 11.1). (Remember that

a -carbon is the carbon adjacent to the carbon that is attached to the leaving group.)

When HCl is used instead of HBr or HI, the reaction is slower because

is a poorer nucleophile than or (Section 10.3). The rate of the reaction can be

increased by using as a catalyst.

is a Lewis acid that complexes strongly with the lone-pair electrons on oxygen.

This weakens the bond and creates a better leaving group.

Cl

CH 3 CH 2 CH 2 OH Zn^2 + CH 3 CH 2 CH 2 OH CH 3 CH 2 CH 2 Cl

Zn+

+ + + +ZnOH

−

C¬O

Zn^2 +

ZnCl 2

CH 3 CH 2 CH 2 OH + HCl ∆ CH 3 CH 2 CH 2 Cl + H 2 O

ZnCl 2

Br- I-

SN 2 Cl-

b

b

ethyl alcohol

a primary alcohol

CH 3 CH 2 OH HBr CH 3 CH 2 OH CH 3 CH 2 Br

H

+ + + H 2 O

Br−

mechanism of the SN2 reaction

protonation of

the oxygen

back-side attack by

the nucleophile

SN 2

SN 1

CH 3 CCH 2 CH 3 HBr

CH 3

OH

+ CH 3 CCH 2 CH 3 H 2 O

CH 3

Br

+

CH 3 CHCH 2 CH 3 HBr

OH

+ CH 3 CHCH 2 CH 3 H 2 O

Br

+

∆

THE LUCAS TEST

Whether an alcohol is primary, secondary, or ter-

tiary can be determined by taking advantage of

the relative rates at which the three classes of alcohols react

with This is known as the Lucas test. The alcohol

is added to a mixture of HCl and —the Lucas reagent.

Low-molecular-weight alcohols are soluble in the Lucas

reagent, but the alkyl halide products are not, so the solution

ZnCl 2

HCl>ZnCl 2.

turns cloudy as the alkyl halide is formed. When the test is car-

ried out at room temperature, the solution turns cloudy immedi-

ately if the alcohol is tertiary, turns cloudy in about five minutes

if the alcohol is secondary, and remains clear if the alcohol is

primary. Because the test relies on the complete solubility of

the alcohol in the Lucas reagent, it is limited to alcohols with

fewer than six carbons.

BRUI12-437_480r3 27-03-2003 11:50 AM Page 439