All carboxylic acids are weak, but some are far weaker than others. Electron-withdrawing chlorine atoms near the COOH group stabilise the carboxylate ion and strengthen the acid; electron-donating alkyl groups do the opposite. Methanoic and ethanedioic acids can also be oxidised.
Carboxylic acids are the strongest of the O–H acids: carboxylic acid > phenol > water > alcohol, because the charge of the carboxylate ion is shared equally over two O atoms. Anything that stabilises RCOO− further makes the acid stronger. Electron-withdrawing Cl atoms pull electron density away from the COO− group and spread its charge: the more Cl atoms, and the closer they are to the COOH group, the stronger the acid (pKa ethanoic 4.76, chloroethanoic 2.86, dichloroethanoic 1.29, trichloroethanoic 0.65). Electron-donating alkyl groups destabilise the anion, so ethanoic acid is weaker than methanoic acid. Methanoic acid, HCOOH, still has an H on its C=O carbon, so it is oxidised to CO2 by Tollens', Fehling's and acidified KMnO4; ethanedioic acid decolourises warm acidified KMnO4, giving CO2.
Change the substituents in the simulator and watch the pKa and the charge on the carboxylate ion. Then check the patterns in the tables.
| Acid | Formula | pKa | Ka / mol dm−3 |
|---|---|---|---|
| ethanoic acid | CH3COOH | 4.76 | 1.74 × 10−5 |
| chloroethanoic acid | ClCH2COOH | 2.86 | 1.38 × 10−3 |
| dichloroethanoic acid | Cl2CHCOOH | 1.29 | 5.13 × 10−2 |
| trichloroethanoic acid | Cl3CCOOH | 0.65 | 0.224 |
| Effect tested | Acids compared (pKa) | Conclusion |
|---|---|---|
| position of Cl | 2-chlorobutanoic 2.86, 3-chlorobutanoic 4.05, 4-chlorobutanoic 4.52, butanoic 4.82 | the closer the Cl to COOH, the stronger the acid |
| alkyl groups | methanoic 3.75, ethanoic 4.76, propanoic 4.87 | electron-donating alkyl groups weaken the acid |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which acid is the strongest?
Why is chloroethanoic acid stronger than ethanoic acid?
Which carboxylic acid gives a silver mirror with Tollens' reagent?
What is seen when ethanedioic acid is warmed with acidified potassium manganate(VII)?
Run Tollens' and Fehling's tests in the simulator. Methanoic acid behaves like an aldehyde because of its H–C=O group.
| Acid | Reagent | Observation | Equation |
|---|---|---|---|
| methanoic acid, HCOOH | Tollens', warm | silver mirror | HCOOH + [O] → CO2 + H2O |
| methanoic acid | Fehling's, warm | blue to brick-red Cu2O precipitate | HCOOH + [O] → CO2 + H2O |
| methanoic acid | acidified KMnO4 | purple to colourless | HCOOH + [O] → CO2 + H2O |
| ethanedioic acid, HOOCCOOH | warm acidified KMnO4 (about 60 °C) | purple to colourless, CO2 given off | 5H2C2O4 + 2MnO4− + 6H+ → 2Mn2+ + 10CO2 + 8H2O |
| ethanoic acid | any of these | no change | not oxidised |
Electron-withdrawing groups make an acid stronger; electron-donating groups make it weaker. The explanation must be about the stability of the carboxylate anion, not about the O–H bond alone. More Cl is stronger, and Cl closer to the COOH group is stronger. A smaller pKa means a stronger acid. Of the simple carboxylic acids, only methanoic gives positive Tollens' and Fehling's tests; ethanedioic acid needs warm acidified KMnO4.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Which list has the acids in order of INCREASING acid strength?
The pKa of ethanoic acid is 4.76 and of chloroethanoic acid is 2.86. How many times larger is Ka for chloroethanoic acid?
25.0 cm3 of 0.0500 mol dm−3 ethanedioic acid is titrated with 0.0200 mol dm−3 KMnO4 in warm acid. What is the titre? (5H2C2O4 + 2MnO4− + 6H+ → 2Mn2+ + 10CO2 + 8H2O)
Why is propanoic acid (pKa 4.87) a weaker acid than methanoic acid (pKa 3.75)?
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