Organic Chemistry

438 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

convert an OH group into a weaker base is to protonate it. Protonation changes the

leaving group from (a strong base) to (a weak base), which is weak enough

to be displaced by a nucleophile. The substitution reaction is slow and requires heat to

take place in a reasonable period of time.

Because the OH group of the alcohol has to be protonated before it can be displaced by

a nucleophile, only weakly basic nucleophiles can be used in the substi-

tution reaction. Moderately and strongly basic nucleophiles

would be protonated in the acidic solution and, once protonated, would no longer be

nucleophiles or would be poor nucleophiles

Primary, secondary, and tertiary alcohols all undergo nucleophilic substitution

reactions with HI, HBr, and HCl to form alkyl halides.

The mechanism of the substitution reaction depends on the structure of the alcohol.

Secondary and tertiary alcohols undergo reactions. The carbocation intermediate

formed in the reaction has two possible fates: It can combine with a nucleophile

and form a substitution product, or it can lose a proton and form an elimination

product. However, only the substitution product is actually obtained, because any

alkene formed in an elimination reaction will undergo a subsequent addition reaction

with HX to form more of the substitution product.

SN 1

SN 1

1-propanol

a primary alcohol

cyclohexanol

a secondary alcohol

bromocyclohexane

1-iodopropane

CH 3 CH 2 CH 2 OH HI CH 3 CH 2 CH 2 I

OH

+ + H 2 O

∆

+ HBr + H 2 O

∆

Br

2-methyl-2-butanol

a tertiary alcohol

2-chloro-2-methylbutane

CH 3 CH 2 COH + HCl CH 3 CH 2 CCl + H 2 O

CH 3

CH 3

CH 3

CH 3

(+NH 4 ,RNH 3 +) (CH 3 OH).

(NH 3 ,RNH 2 ,CH 3 O-)

(I-,Br-,Cl-)

CH 3 OH + HBr CH 3 OH CH 3 Br + H 2 O

Br−

weak base

H ∆

+

• OH H 2 O

CH 3 COH H

H

Br

CH 3

CH 3

+

+ H 2 O

+

+

−

CH 3 COH

CH 3

CH 3

CH 3 CBr

CH 3

CH 3

CH 3 C+

CH 3

CH 3

CH 3 C

CH 3

HBr

CH 2

Br

elimination product

mechanism of the SN1 reaction

substitution

product

H+

tert-butyl alcohol

a tertiary alcohol

reaction of the carbocation

with a nucleophile

protonation of the

most basic atom formation of

a carbocation

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