Physical properties and acidity

Aliphatic acids with up to nine carbons are colourless liquids with unpleasant smells; the higher acids are wax-like, nearly odourless solids because they hardly evaporate.

Acids boil higher than aldehydes, ketones and even alcohols of similar mass because their molecules are held together more extensively by hydrogen bonds. These bonds survive even in the vapour, where most acids, like those in aprotic solvents, exist as dimers.

Acids with up to four carbons mix with water through hydrogen bonding; solubility falls as the chain grows, and higher acids are practically insoluble. Benzoic acid is nearly insoluble in cold water. All dissolve in benzene, ether, alcohol and chloroform.

Acids give hydrogen with electropositive metals and salts with alkalis. Unlike phenols they also react with carbonates and hydrogencarbonates to release CO₂, the test for a carboxyl group.

In water an acid gives a resonance-stabilised carboxylate ion and H₃O⁺. Strength is quoted as pKa = −log Ka; a smaller pKa means a stronger acid. pKa < 1 is strong, 1–5 moderately strong, 5–15 weak and above 15 extremely weak.

pKa values: hydrochloric acid −7.0; trifluoroacetic acid (the strongest carboxylic acid) 0.23; benzoic acid 4.19; acetic acid 4.76; phenol 10; ethanol about 16.

Acids are stronger than phenols: the carboxylate ion has two equivalent resonance forms with the charge on the two electronegative oxygens, while phenoxide's forms are unequal and put charge on less electronegative ring carbons.

Electron-withdrawing groups stabilise the carboxylate and raise acidity; electron-donating groups lower it. Group effect, increasing: Ph < I < Br < Cl < F < CN < NO₂ < CF₃. Strongest first: CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NC–CH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH.

A phenyl or vinyl group attached directly to –COOH raises acidity, because the sp² carbon it bonds to is more electronegative. On the ring: 4-nitrobenzoic acid pKa 3.41 < benzoic acid 4.19 < 4-methoxybenzoic acid 4.46.