The SN2 reaction of a haloalkane with hydroxide ion (HO-) is useful to prepare primary alcohols. Secondary and tertiary haloalkanes give significant amounts of elimination instead of substitution products. (Section 7.3A) See also: Glycols
Addition of water to an alkene provides an alcohol. This alkene hydration reaction requires acid to generate hydronium ion (H3O+), the actual electrophile that attacks the alkene. A carbocation intermediate is formed, so carbocation rearrangements are possible. The reaction follows Markovnikov's rule, so the more highly substituted alcohol is formed as the major product. Thus, the proton of hydronium ion becomes attached to the alkene end with the greatest number of hydrogens, and the HO end of the water becomes attached to the alkene end with the greatest number of carbons (Section 6.3B).
The addition of a Grignard reagent (RMgX) to formaldehyde (H2C=O) affords a primary alcohol. Similar reaction between a Grignard reagent and an aldehyde affords a secondary alcohol, while addition to an ester or ketone affords a tertiary alcohol. Grignard synthesis is a very broadly applicable protocol for preparing alcohols. (Section 11.5B)
An ester reacts with two molecules of Grignard reagent to afford a secondary alcohol from a formate ester, or a tertiary alcohol from other esters (Section 13.7). The reaction cannot be stopped at the addition of just one molecule of Grignard reagent to form a ketone, because a ketone is attacked much more rapidly than an ester.
Reduction of carbonyl group can afford an alcohol. Aldehydes and ketones can be reduced with molecular hydrogen and a transition metal catalyst such as platinum to provide primary or secondary alcohols, respectively (Section 11.10A). These reaction conditions also reduce an alkene to an alkane. Reduction of the alkene can be avoided by using sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4) for the reduction. Both of these reagents reduce aldehydes and well as ketones to the corresponding alcohols (Section 11.10B)
Reduction of a carboxylic acid (Section 12.5) or ester (Section 13.8A) by LiAlH4 also affords a primary alcohol. (This reduction also affords an alcohol from the OR end of the ester, but because an ester is often made from this alcohol, this reaction is rarely important.) Carboxylic acids and esters are inert to reduction by NaBH4.
Ester hydrolysis (saponification) affords a carboxylic acid and alcohol. The reaction can be conducted with acid (such as aqueous sulfuric acid) or base (such as aqueous KOH). As with LiAlH4 reduction of an ester, this is rarely an important route to alcohols. The reaction is another carboxylic acid derivative substitution reaction. In this case, the OR group of the ester is replaced with an OH group.
