Sources of alcohols
Natural products
The common sources of methanol, ethanol, and isopropyl alcohol have been discussed above. Larger, more complicated alcohols are often isolated from volatile oils of plants by the process of steam distillation. The plant material is boiled in water, and the volatile oils are carried over by the steam, condensed, and separated from the water. Substances such as cholesterol, found in most animal tissues (and abundant in egg yolks), and retinol (vitamin A alcohol), extracted from fish liver oils, are examples of naturally occurring sources of alcohols.
Long-chain alcohols can be obtained from fats and waxes by hydrolysis in base, called saponification, followed by reduction.
Reduction of carbonyl compounds
The addition of two hydrogen atoms to a carbonyl group produces an alcohol. This is an example of a reduction. Ketones, aldehydes, and carboxylic acids are reduced by the catalytic addition of gaseous hydrogen (H2) or by a wide variety of specific reducing agents, such as lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).
Hydration of alkenes
The addition of water (hydration) across the double bond of an alkene yields an alcohol. Alkenes are available as products of coal tar and petroleum refining, and a variety of catalytic conditions can support the addition of water across the double bond. In most cases, water adds in the direction that places the new hydroxyl group on the more highly substituted end of the double bond according to the Markovnikov rule, as in acid-catalyzed hydrations. (The more highly substituted end of the double bond is the one that is bonded to more carbon atoms.)
Hydroboration-oxidation is also useful for adding water across the double bond of an alkene; however, hydroboration-oxidation gives an anti-Markovnikov orientation of the addition product, with the hydroxyl group adding to the less-substituted end of the double bond.
Alkenes may be converted to diols (dialcohols) by oxidizing agents such as potassium permanganate (KMnO4).
These oxidations must be carried out carefully to avoid overoxidation and breaking of the double bond.
Displacement of halides
A hydroxide ion can displace a halide ion from a primary alkyl halide (RCH2X, where X is a halogen) to give an alcohol. This displacement reaction is not frequently used to synthesize alcohols, however, because alkyl halides are more commonly synthesized from alcohols rather than vice versa.
Using Grignard and organolithium reagents
Grignard and organolithium reagents are powerful tools for organic synthesis, and the most common products of their reactions are alcohols. A Grignard reagent is formed by reaction of an alkyl halide (RX, where X is a halogen) with magnesium metal (Mg) in an ether solution. The product is written as R―Mg―X, although Grignard reagents are known to be more complicated than this simple structure suggests. Organolithium reagents (RLi) are formed in much the same way as Grignard reagents, except that an ether solvent is not required. Most reactions of organolithium reagents are similar to those of Grignard reagents; however, there are some important differences.
Grignard reagents add to carbonyl compounds (i.e., compounds containing the C=O functional group) to produce magnesium alkoxides (ROMgX) that are hydrolyzed to alcohols. A wide variety of alcohols can be synthesized by Grignard additions. A Grignard reagent adds to formaldehyde to give a primary alcohol with one additional carbon atom, to an aldehyde to give a secondary alcohol, and to a ketone to yield a tertiary alcohol.
Grignard reagents add twice to esters to give alcohols (upon hydrolysis). This technique is valuable for making secondary and tertiary alcohols with two identical alkyl groups. They also add to epoxides, yielding primary alcohols in which two additional carbon atoms have been added to the chain of the Grignard reagent.
