Methods of Preparation of Alcohols

Alcohols are prepared industrially and in the laboratory from three quite different classes of

starting material, and the choice of route usually depends on which alcohol is wanted and what

starting material is conveniently available.

From alkenes (acid-catalysed hydration). An alkene reacts with water in the presence of a

strong acid catalyst, usually dilute $\text{H}_2\text{SO}_4$, by an electrophilic addition

mechanism exactly analogous to $\text{HX}$ addition: the alkene's pi electrons protonate to form

the more stable carbocation (Markovnikov's rule), and water then attacks that carbocation, followed

by loss of a proton to give the alcohol. For an unsymmetrical alkene such as propene, this places

the new $-\text{OH}$ on the more substituted carbon, so propene gives propan-2-ol as the major

product, not propan-1-ol. An alternative laboratory route, hydroboration-oxidation

($\text{B}_2\text{H}_6$ then alkaline $\text{H}_2\text{O}_2$), instead adds $-\text{OH}$ with the

opposite, anti-Markovnikov regiochemistry, giving the less-substituted alcohol when the two routes

are compared on the same alkene.

From haloalkanes (nucleophilic substitution). Boiling a haloalkane with aqueous NaOH or KOH

substitutes the halide by hydroxide: for a primary haloalkane this proceeds by a clean, single-step

$S_N2$ mechanism (hydroxide attacks the carbon from the side opposite the leaving halide, inverting

its configuration and displacing $\text{X}^-$ directly), giving the corresponding primary alcohol

in good yield, e.g. $\text{CH}_3\text{CH}_2\text{Br} + \text{NaOH} \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{NaBr}$. Secondary and, especially, tertiary haloalkanes can

instead react by an $S_N1$ pathway (ionising first to a carbocation), which competes with

elimination to an alkene as a side reaction and can lower the alcohol yield.

From carbonyl compounds (reduction). Aldehydes and ketones are reduced cleanly to alcohols:

a hydride-transfer reducing agent such as sodium borohydride ($\text{NaBH}_4$, milder, chemoselective)

or lithium aluminium hydride ($\text{LiAlH}_4$, stronger, also reduces esters and acids) delivers a

hydride ion to the electrophilic carbonyl carbon, and aqueous work-up then protonates the resulting

alkoxide. An aldehyde ($\text{RCHO}$) is reduced to a primary alcohol ($\text{RCH}_2\text{OH}$),

while a ketone ($\text{RCOR}'$) is reduced to a secondary alcohol ($\text{RCH(OH)R}'$) --

so the class of alcohol obtained is fixed entirely by whether the starting carbonyl compound was an …