Carboxylic acids properties and reactions
Organic chemistry • Reactions of alkenes and alcohols
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Identification and general formula
Carboxylic acids contain the carboxyl group written as -COOH or shown structurally as R-COOH, where R is a hydrocarbon chain. The homologous series follows the general molecular formula CnH2nO2 for straight-chain acids. Recognition from names uses systematic nomenclature: methanoic (one carbon), ethanoic (two carbons), propanoic (three carbons) and butanoic (four carbons). Structural formulae show the -COOH group attached to the appropriate alkyl chain, for example CH3COOH for ethanoic acid. Identification from molecular formula or structural diagrams focuses on the -COOH group as the defining feature. When given a formula, locate the carbon double-bonded to oxygen and single-bonded to an OH; that arrangement confirms a carboxylic acid.
Dissolving in water and acidity
Carboxylic acids dissolve in water and set up an equilibrium in which some molecules donate a proton: R-COOH ⇌ R-COO− + H+. Proton donation increases the concentration of hydrogen ions in the solution and produces an acidic pH. The equilibrium lies mostly to the left so only a fraction of molecules ionise, producing a weakly acidic solution compared with strong mineral acids. Solubility in water decreases as the hydrocarbon chain length increases because the nonpolar chain reduces interaction with water. The first four carboxylic acids remain sufficiently soluble to form clear acidic solutions; longer chains show reduced solubility and weaker observable acidity in aqueous solution.
Reaction with carbonates
Carboxylic acids react with metal carbonates or hydrogen carbonates to form a salt, carbon dioxide and water. The acid supplies H+ ions that react with the carbonate ion CO3^2− to form CO2 and H2O, so effervescence (bubbling) of carbon dioxide is a diagnostic observation. General equation with a carbonate: 2 R-COOH + M2CO3 → 2 R-COOM + CO2 + H2O, where M represents a metal cation. The reaction occurs because acid protonation of carbonate converts it to carbonic acid which decomposes rapidly to carbon dioxide and water. The extent of reaction depends on the acid concentration and the carbonate used; soluble carbonates give faster observable effervescence.
Reaction with alcohols (esterification)
Carboxylic acids react with alcohols in the presence of an acid catalyst (commonly concentrated sulfuric acid) to form esters and water by a condensation (dehydration) reaction. The general equation is R-COOH + R'OH ⇌ R-COOR' + H2O. Naming of the ester uses the alkyl name from the alcohol followed by the acid-derived part as an -oate: for example, ethanoic acid + methanol → methyl ethanoate. The reaction is reversible and subject to equilibrium; removing water or using excess alcohol shifts the position of equilibrium to increase ester yield. Heating speeds up the reaction but does not change the position of equilibrium without other changes.
(HT) Weak-acid behaviour and pH
Carboxylic acids behave as weak acids because the ionisation equilibrium in water favours the un-ionised acid. The ionisation constant Ka for a carboxylic acid is relatively small, indicating limited dissociation into H+ and R-COO−. A small Ka produces a lower hydrogen-ion concentration [H+] than a strong acid at equal concentration, resulting in a higher pH (less acidic). The equilibrium expression is Ka = [H+][R-COO−] / [R-COOH]. For a given initial concentration, the degree of ionisation depends on Ka: smaller Ka → smaller fraction ionised. Factors that affect apparent acidity include concentration (dilution reduces [H+] but can increase the percentage ionised) and the electron-withdrawing or electron-donating character of substituents on the acid (electron-withdrawing groups stabilise R-COO− and increase acidity).
Key notes
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