A variety of reaction modes are available to alcohols.
9-1
Reactions of Alcohols with Base: Preparation of
Alkoxides
Strong bases are needed to deprotonate alcohols completely.
Base strength must be stronger than that of the alkoxide.
Alkali metals also deprotonate alcohols, but by reduction of H+.
Vigorous:
Less Vigorous:
Relative reactivities:
Uses for alkoxides:
Hindered alkoxides
E2 reactions with haloalkanes to form alkenes.
Less hindered alkoxides
SN2 reactions with haloalkanes to form ethers.
9-2
Reactions of Alcohols with Strong Acids: Alkyloxonium
Ions in Substitution and Elimination Reactions of
Alcohols
Water has a high pKa (15.7) which means that its conjugate base, OH- is an
exceedingly poor leaving group.
The –OH group of an alcohol must be converted into a better leaving group for
alcohols to participate in substitution or elimination reactions.
9-2
Reactions of Alcohols with Strong Acids: Alkyloxonium
Ions in Substitution and Elimination Reactions of
Alcohols
Haloalkanes from primary alcohols and HX: Water can be a leaving
group.
Protonation of the hydroxy substituent of an alcohol to form an alkyloxonium ion
converts the –OH from the poor leaving group, OH-, to the good leaving group,
H2O.
Primary bromoalkanes and iodoalkanes can be prepared by the reaction with HBr
and HI. Chloroalkanes cannot be prepared by this method because Cl- is too poor
a nucleophile.
Secondary and tertiary alcohols undergo carbocation reactions with
acids: SN1 and E1.
Primary alkyloxonium ions undergo only SN2 reactions with acid. Their
carbocation transition state energies are too high to allow SN1 and E1 reactions
under ordinary laboratory conditions.
Secondary and tertiary alkyloxonium ions lose water when treated with acid to
form a carbocation.
When good nucleophiles are present, the SN1 mechanism predominates.
Here the tertiary carbocation is generated at a relatively low temperature,
which prevents the competing E1 reaction.
At higher temperatures, or in the absence of good nucleophiles, elimination
becomes dominant.
Secondary alcohols show complex behavior when treated with HX, following SN2,
SN1, and E1 pathways.
Relatively hindered (compared to primary alcohols):
Retarded SN2 reactivity
Slow to form carbocations (compared to tertiary alcohols):
Retarded SN1 reactivity.
E1 reactions of alcohols (dehydrations) result in the formation of alkenes. Nonnucleophilic acids, such as H3PO4 or H2SO4, are used in this case, rather than the
nucleophilic acids, HBr and HI.
Dehydrations of tertiary alcohols often occur just above room temperature.
Summary