Chemistry · Ch 12 — Carbonyl Compounds and Carboxylic Acids
Acidity of Carboxylic Acids
Acidity of Carboxylic Acids
Carboxylic acids ionise in aqueous solution to produce H+ and a carboxylate anion; the resulting carboxylate ion is stabilised by resonance (its negative charge delocalised equally over both oxygen atoms), which is exactly what makes a carboxylic acid donate its proton so much more readily than a simple alcohol does (whose corresponding alkoxide anion has no comparable resonance stabilisation). The strength of a carboxylic acid is expressed quantitatively by its dissociation (acidity) constant, Ka = [RCOO-][H3O+] / [RCOOH]; the STRONGER the acid, the HIGHER its Ka value. The same strength can equivalently be expressed as pKa = -log(Ka) -- on this inv …
| Carboxylic acid | Molecular formula | pKa |
|---|---|---|
| Trichloroacetic acid | Cl3CCOOH | 0.64 |
| Dichloroacetic acid | Cl2CHCOOH | 1.26 |
| Fluoroacetic acid | FCH2COOH | 2.59 |
| o-Nitrobenzoic acid | o-NO2C6H4COOH | 2.17 |
| Chloroacetic acid | ClCH2COOH | 2.87 |
| Bromoacetic acid | BrCH2COOH | 2.90 |
| m-Nitrobenzoic acid | m-NO2C6H4COOH | 3.49 |
| p-Nitrobenzoic acid | p-NO2C6H4COOH | 3.44 |
| Iodoacetic acid | ICH2COOH | 3.17 |
| Formic acid | HCOOH | 3.75 |
Effect of Substituents on Acidity
(i) ELECTRON-RELEASING alkyl groups DECREASE acidity. A +I (electron-donating, inductive) substituent pushes extra electron density onto the already-negative carboxylate ion, further destabilising that extra negative charge and making the acid correspondingly more reluctant to lose its own proton in the first place -- so formic acid (HCOOH, no alkyl group at all) is a STRONGER acid than acetic acid (CH3COOH, one +I methyl group), which is in turn stronger than propionic acid (CH3CH2COOH, an even bigger +I ethyl group): HCOOH > CH3COOH > CH3CH2COOH. (ii) ELECTRON-WITHDRAWING substituents INCREASE acidity, by the exactly opposite logic: a -I (electron-withdrawing) group pulls electron density AWAY from the already-negative carboxylate ion, spreading out and thereby stabilising that negative charge, so the acid loses its proton more easily. Acidity increases with the substituent's electronegativity, e.g. among the haloacetic acids: FCH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH (fluorine being the most electronegative, hence most acid-strengthening, halogen). Acidity also increases with the NUMBER of electron-withdrawing substituents on the alpha-carbon (their -I effects add up): Cl3C-COOH > Cl2CH-COOH > ClCH2-COOH > CH3-COOH. The overall -I strength of common electron-withdrawing groups follows the order -NO2 > -CN > -F > -Cl > -Br > -I > -Ph. Putting several acid famili …