Alcohols are prepared via the reduction of carbonyls with LiAlH4 or NaBH4, the reaction of carbonyls with organometallic reagents, and the reduction of dicarbonyls.
A carbonyl is a functional group in which a carbon atom is double-bonded to an oxygen atom. Examples of carbonyls are carboxylic acids, ketones, aldehydes, amides, esters, acyl halides, and acid anhydrides. Reduction is a reaction that involves the addition of an electron to an atom. LiAlH4 can reduce ketones, aldehydes, carboxylic acids, and esters to alcohols, while NaBH4 can only reduce aldehydes and ketones to alcohols. LiAlH4 is often abbreviated LAH.
The reductions are nucleophilic addition reactions. LiAlH4 and NaBH4 are sources of hydrides (H–) and act as reducing agents. The hydrides react with the carbon of the carbonyl group to produce an alkoxide, which can be quenched with acid to yield an alcohol. The substituents on the carbonyl determine the type of alcohol product. The reduction of aldehydes yields primary alcohols, and the reduction of ketones produces secondary alcohols. Primary alcohols have only one other carbon attached to the carbon attached to the −OH. Secondary alcohols have two other carbons attached to the carbon attached to the −OH.
An organometallic is a compound that contains a metal bonded directly to a carbon. A Grignard reagent is a reagent that has the formula RMgX, where the halogen may be −Cl, −Br, or −I. The carbon-magnesium bond is highly polar, and because of its polarity the carbon in the C−Mg bond can act as a nucleophile.
Organometallic reactions are nucleophilic addition reactions with an organometallic reagent. The carbon in the C−Mg bond of the Grignard reagent acts as a nucleophile and adds to the carbon of the carbon-oxygen double bond, forming a new carbon-carbon bond and creating an alkoxide. Alkoxides are negatively charged oxygen atoms. Following the addition of a Grignard reagent, an acid is added to protonate the alkoxide ion that forms. Grignard reagents will protonate in the presence of acids, so an acid cannot be added at the same time as the Grignard reagent but must be added after the Grignard reaction is complete. Formaldehyde reacts with Grignard reagents to yield a primary alcohol. Aldehydes react with Grignard reagents to yield secondary alcohols. Ketones react with Grignard reagents to yield tertiary alcohols.
Reaction of Carbonyls with a Grignard Reagent
A diol, or glycol, is an alcohol with one additional hydroxyl group. Diols are formed via oxidation of alkenes into 1,2-diols. Oxidation is a reaction that adds oxygen to a compound or increases the amount of oxygen in a compound. Potassium permanganate (KMnO4) is a powerful oxidizing agent that creates a syn addition of two hydroxyl (−OH) groups to an alkene.
The Baeyer test (potassium permanganate and water) checks for the presence of unsaturation, double or triple bonds, in a compound. Potassium permanganate oxidizes the double bond and turns it into a diol. The permanganate solution starts as a purple color, and after reacting with an alkene it will form a brown precipitate of MnO2.
Oxidation of an Alkene and an Alkyne with Potassium Permanganate
Epoxidation with peroxy acids (RCO3H) followed by an acid creates an anti-addition of two hydroxyl (−OH) groups to an alkene. Epoxidation is a reaction that creates an epoxide, a three-membered oxygen-containing ring.