Alcohols

THEORY OF ALCOHOLS

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Alcohols are organic compounds that contain the hydroxyl group (-OH). Methanol is the simplest alcohol, it is obtained by reducing carbon monoxide with hydrogen.

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Rule 1. The longest chain that contains the -OH group is chosen as the main chain.

Rule 2. The main chain is numbered so that the -OH group takes the lowest locant. The hydroxyl group has preference over carbon chains, halogens, double and triple bonds.

Read more: Alcohol Nomenclature

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Alcohols are amphoteric (amphiprotic) species, they can act as acids or bases. In aqueous solution an equilibrium is established between alcohol, water and their conjugate bases.

Writing the equilibrium constant (Ka)

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Alcohols can be obtained from haloalkanes by S_N2 and S_N1 reactions

Synthesis of alcohols using S_N2

Primary haloalkanes react with sodium hydroxide to form alcohols. Secondary and tertiary haloalkanes eliminate to form alkenes.

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Both sodium borohydride $(NaBH_4)$ and lithium aluminum hydride $(LiAlH_4)$ reduce aldehydes and ketones to alcohols.

Ethanal is transformed by reduction with sodium borohydride to ethanol.

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Another method of preparing alcohols is the reduction of aldehydes or ketones to alcohols. The simplest method is the hydrogenation of the carbon-oxygen double bond, using hydrogen in the presence of a platinum, palladium, nickel, or ruthenium catalyst.

Ethanal [1] is transformed by hydrogenation of the double bond into ethanol [2].

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Alcohols can be obtained by opening epoxides (oxacyclopropanes). This opening can be done using organometallic reagents or the lithium aluminum reductant.

Oxacyclopropane [1] is transformed by reduction with lithium aluminum hydride in ethanol [2].

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A synthesis method for alcohols, already studied in the section on alkenes, consists in hydrating the alkene. The addition of -OH can be on the more substituted carbon of the alkene (Markovnikov), or on the less substituted carbon (anti-Markovnikov).

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Carboxylic acids and esters are reduced to alcohols with lithium aluminum hydride. Milder reducers such as sodium borohydride are unable to reduce these compounds.

Ethanoic acid [1] is transformed by reduction with lithium aluminum hydride in ethanol [2].

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Primary and secondary alcohols can be converted to haloalkanes with reagents such as: phosphorus tribromide, phosphorus trichloride, thionyl chloride, and phosphorus pentachloride.

preparacion-haloalcanos-a-partir-alcoholes

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The oxidation of alcohols forms carbonyl compounds. Aldehydes are obtained by oxidizing primary alcohols, while oxidizing secondary alcohols forms ketones.

1. Oxidation of primary alcohols to aldehydes

Chromium trioxide with pyridine in dichloromethane allows the isolation of aldehydes with good yield from primary alcohols.

Chromium trioxide with pyridine and hydrochloric acid in dichloromethane is known as PCC (pyridinium chlorochromate). This reagent also converts primary alcohols to aldehydes.

2. Oxidation of primary alcohols to carboxylic acids

Chromium trioxide in an aqueous acidic medium (Jones' reagent), potassium permanganate, and potassium dichromate oxidize primary alcohols to carboxylic acids.