Chemistry Case

Alcohols do not undergo nucleophilic substitution reactions because hydroxide ion is strongly

basic and a poor leaving group. Thus, alkyl bromides cannot be prepared by either an SN2 or an

SN1 reaction upon heating alcohols with bromide salts. However, alcohols readily undergo

nucleophilic substitutions (either SN2 or SN1) if the hydroxyl group is first activated to produce

a better leaving group. This is the basis for nearly all of the reactions of alcohols.

R OH X RBr + OH

R OH X R + OH Br RBr

SN2

SN1

Br

One of the easiest methods for activating an alcohol for a nucleophilic substitution reaction is to

protonate the hydroxyl oxygen with a strong acid in the presence of a nucleophile. The

R OH H R OH2

Nu

R Nu + H2

O

protonated hydroxyl group can now leave as a weakly basic water molecule instead of as a

strongly basic hydroxide ion. If the alcohol is primary, the protonated hydroxyl is readily

displaced by a nucleophile in an SN2 reaction, whereas, if the alcohol is tertiary, the protonated

hydroxyl-carbon bond fragments to form a carbocation by an SN1 process which then reacts with

a nucleophile to form a substitution product. The protonated hydroxyl of a secondary alcohol

may undergo replacement by a nucleophile either an SN2 or an SN1 mechanism, depending upon

the exact structure of the alkyl group.

X RCH2 OH2 RCH2 X + H2O

R3C OH2 R3C

+

H2O

X R3C X

Primary and tertiary alcohols are readily converted into the corresponding alkyl bromides or

iodides upon treatment with concentrated hydrobromic or hydriodic acids.

Tertiary alcohols also react with concentrated hydrochloric acid to give alkyl chlorides, whereas

primary alcohols are inert to concentrated hydrochloric acid.

R OH R X + H2

+ HX O

R = primary, X = Br or