33.1 Carboxylic acids
- Syllabus
- 9701–2028–2029
- Topic
- 33.1
- Level
- A2
Learning objectives
33.1.1Preparation of benzoic acid• Recall preparation of benzoic acid: (a) reaction of an alkylbenzene with hot alkaline KMnO₄ and then dil. acid, exemplified by methylbenzene33.1.2Carboxylic acids to acyl chlorides• Describe the reaction of carboxylic acids with PC l ₃ and heat, PCl ₅ or SOCl ₂ to form acyl chlorides33.1.3Oxidation of carboxylic acids• Recognise that some carboxylic acids can be further oxidised:- (a) the oxidation of methanoic acid, HCOOH, with Fehling’s reagent or Tollens’ reagent or acidified KMnO₄ or acidified K₂Cr₂O₇ to form carbon dioxide and water- (b) the oxidation of ethanedioic acid, HOOCCOOH, with warm acidified KMnO₄ to form carbon dioxide33.1.4Acidities of carboxylic acids, phenols• Describe/explain: the relative acidities of carboxylic acids, phenols and alcohols33.1.5Acidities of chlorine-substituted carboxylic• Describe/explain: the relative acidities of chlorine-substituted carboxylic acids
Oxidise an alkylbenzene side chain to benzoic acid
An alkylbenzene with at least one benzylic hydrogen can be oxidised by hot alkaline KMnO4. After acidification, the side chain is converted to a carboxylic acid attached directly to the ring.
The carbon skeleton beyond the benzylic carbon is removed during vigorous oxidation, so different alkylbenzenes can give the same benzoic-acid ring product.
Methylbenzene + hot alkaline KMnO4, then dilute acid -> benzoic acid. The methyl carbon becomes the carboxyl carbon.
Do not oxidise the aromatic ring itself or stop at an alcohol; this is a strong side-chain oxidation followed by acidification.
Three chlorinating reagents convert RCOOH into RCOCl
A carboxylic acid is converted to the corresponding acyl chloride by replacing the -OH of its -COOH group with Cl. The carbon skeleton and carbonyl group remain unchanged: RCOOH becomes RCOCl.
| Chlorinating reagent | Required condition | General equation and by-products |
|---|---|---|
| PCl3 | heat | 3RCOOH + PCl3 -> 3RCOCl + H3PO3 |
| PCl5 | no additional condition specified | RCOOH + PCl5 -> RCOCl + POCl3 + HCl |
| SOCl2 | no additional condition specified | RCOOH + SOCl2 -> RCOCl + SO2 + HCl |
With any of the three reagent choices, ethanoic acid, CH3COOH, gives ethanoyl chloride, CH3COCl. Keep the acid's root name and change the ending from -oic acid to -oyl chloride.
Only PCl3 is paired with heat in the official outcome. Do not write the same by-products for all three reagents, and do not replace a hydrogen on the hydrocarbon chain: the functional-group change is -COOH -> -COCl.
Methanoic and ethanedioic acids are exceptional further-oxidation cases
Most carboxylic acids resist further oxidation under ordinary syllabus conditions, but methanoic acid and ethanedioic acid are named exceptions. Recognise the acid first, then choose only its permitted oxidant and products.
HCOOH+[O]COX2+HX2O
HOOCCOOH+[O]2COX2+HX2O
| Acid | Oxidising reagent | Products | Distinguishing observation |
|---|---|---|---|
| methanoic acid | Fehling's reagent, warmed | CO2 + H2O | brick-red Cu2O precipitate |
| methanoic acid | Tollens' reagent, warmed | CO2 + H2O | silver mirror or grey silver deposit |
| methanoic acid | acidified KMnO4 | CO2 + H2O | purple solution decolourises |
| methanoic acid | acidified K2Cr2O7 | CO2 + H2O | orange solution turns green |
| ethanedioic acid | warm acidified KMnO4 | CO2 | purple solution decolourises and CO2 is evolved |
Do not generalise the Fehling's or Tollens' result to all carboxylic acids. Ethanedioic acid requires warm acidified KMnO4 in this objective, while methanoic acid is the one accepted with all four listed oxidising systems.
Conjugate-base stability gives carboxylic acid > phenol > alcohol
The more stable and less basic the conjugate base after H+ loss, the stronger the parent O-H acid. The decisive difference is where the negative charge can be placed and how attached groups affect it.
| Parent acid | Conjugate base | Charge distribution and stability | Relative acidity |
|---|---|---|---|
| carboxylic acid, RCOOH | carboxylate, RCOO- | charge is equally delocalised over two electronegative O atoms; most stabilised | strongest of the three |
| phenol, C6H5OH | phenoxide, C6H5O- | charge is delocalised from O into the aromatic ring, but some is placed on less electronegative C atoms | intermediate |
| alcohol, ROH | alkoxide, RO- | charge remains localised on one O and an alkyl group donates electron density toward it; least stabilised | weakest of the three |
carboxylic acid>phenol>alcohol
A carboxylic acid is strong enough to react with aqueous carbonate and release CO2. Phenol does not do so under ordinary conditions, and an alcohol is weaker still. This behaviour is consistent with, rather than a substitute for, the conjugate-base explanation.
Delocalisation alone is not enough to rank the ions: carboxylate is especially stabilised because both main charge-bearing atoms are oxygen. Do not claim phenoxide is more stable merely because its charge can spread across more atoms.
More and nearer chlorine substituents make a carboxylic acid stronger
Chlorine withdraws electron density through sigma bonds by its negative inductive effect. This stabilises the carboxylate conjugate base, makes recombination with H+ less favourable, and therefore increases acidity.
| Structural comparison | Acidity trend | Reason |
|---|---|---|
| add Cl atoms beside -COOH | ethanoic acid < chloroethanoic acid < dichloroethanoic acid < trichloroethanoic acid | more electron-withdrawing Cl atoms reinforce conjugate-base stabilisation |
| move one Cl nearer -COOH | 3-chloropropanoic acid < 2-chloropropanoic acid | inductive withdrawal is stronger over fewer sigma bonds and weakens with distance |
When comparing chlorine-substituted acids, first hold the carbon skeleton constant, then count chlorine atoms and compare their distance from the carboxyl group. More chlorine and shorter distance both predict the stronger acid.
This is an inductive effect transmitted through sigma bonds, not chlorine's directing effect in an aromatic substitution. A chlorine atom does not need to be attached to an aromatic ring to change carboxylic-acid strength.