Identify alcohols and describe their key reactions
Organic chemistry • Reactions of alkenes and alcohols
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Key concepts
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Definition and functional group
An alcohol contains an –OH (hydroxyl) group attached to a saturated carbon atom. The hydroxyl group determines many properties by enabling hydrogen bonding and by providing a reactive site for substitution or oxidation reactions. Naming uses the suffix "-ol" (for example, methanol CH3OH, ethanol C2H5OH). The general molecular formula for the homologous series of alcohols is CnH2n+1OH where n is the number of carbon atoms.
Recognising alcohols from names and formulae
Names that end in "-ol" indicate an alcohol; position numbers appear for isomeric alcohols (for example, propan-1-ol versus propan-2-ol) to show which carbon carries the –OH. Structural formulae show the –OH group explicitly; displayed or skeletal formulas that include an O bonded to an H identify the alcohol functional group. The presence of an –OH in the molecular formula can be inferred by subtracting the hydroxyl hydrogen and oxygen (OH) from the saturated hydrocarbon formula; for example, ethanol C2H5OH matches C2H6O and contains the hydroxyl group.
Main uses of methanol, ethanol, propanol and butanol
Methanol serves as an industrial solvent, a feedstock for formaldehyde production and a fuel or fuel additive in some applications. Ethanol functions as a solvent, a recreational beverage component (denatured when industrial), and a biofuel additive. Propanol and butanol act primarily as solvents, intermediates in organic synthesis and as potential biofuels. Increasing carbon chain length increases suitability as nonpolar solvents and decreases volatility.
Combustion in air
Complete combustion of an alcohol in excess oxygen produces carbon dioxide and water and releases energy. Example: ethanol burns to give CO2 and H2O (C2H5OH + 3O2 → 2CO2 + 3H2O). Energy release explains alcohols use as fuels. Incomplete combustion (limited oxygen) produces carbon monoxide or carbon (soot) and reduces energy yield. Bright, sooty flames indicate incomplete combustion for longer-chain alcohols or poor oxygen supply.
Reaction with sodium metal
Alcohols react with sodium metal to produce a metal alkoxide and hydrogen gas: 2R–OH + 2Na → 2R–O–Na+ + H2. Bubbling or fizzing of hydrogen gas occurs on contact with sodium. Reactivity is similar for methanol, ethanol, propanol and butanol under the same conditions; the reaction demonstrates the –OH hydrogen's weak acidity relative to water and forms ionic alkoxide salts.
Solubility in water
Hydrogen bonding between the alcohol's hydroxyl group and water molecules causes solubility. As carbon chain length increases, the nonpolar hydrocarbon part reduces solubility. Methanol and ethanol are fully miscible with water. Propanols show reduced miscibility and butanols show limited solubility and can form a separate organic layer when excess alcohol is present.
Oxidation by oxidising agents
Primary alcohols oxidise to an aldehyde and further to a carboxylic acid with strong or prolonged oxidation. Example (ethanol): ethanol + [O] → ethanal; ethanal + [O] → ethanoic acid. Acidified potassium dichromate (VI) acts as a common oxidising agent; its orange Cr2O7(2-) solution turns green on reduction to Cr3+. Secondary alcohols oxidise to ketones (no further oxidation to carboxylic acids under mild conditions). Tertiary alcohols resist oxidation unless harsh conditions break C–C bonds, producing a mixture of products.
Key notes
Important points to keep in mind