Carboxylic Acids

Carboxylic acids bring together two familiar ideas: the C=O of a carbonyl and the O–H of an alcohol. The combination creates a functional group — –COOH — that has its own distinctive chemistry. Carboxylic acids are weak acids that react with bases, carbonates and metals just like dilute mineral acids, and they act as the starting point for almost every ester you will meet.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.1 (a)–(c): structure, nomenclature, acidity and reactions of carboxylic acids.

1. The Carboxyl Group

The carboxyl group, –COOH, can be thought of as a carbonyl (C=O) with a hydroxyl (O–H) attached to the same carbon. Its compact formula hides a great deal of chemistry.

Key Definition — Carboxyl group: The functional group –COOH, in which a carbon is double-bonded to one oxygen and single-bonded to a hydroxyl group.

The two oxygens of –COOH are not equivalent in any one molecule (one is doubly bonded and one singly bonded), but when the acid dissociates to give –COO⁻, the negative charge is delocalised evenly over both oxygens. This resonance stabilisation is the reason carboxylic acids are acidic at all — more on that in Section 3.

graph LR
  A[Carboxyl -COOH] --> B[C=O carbonyl]
  A --> C[O-H hydroxyl]
  A --> D[After loss of H+: -COO- with delocalised charge]

2. Nomenclature

Carboxylic acids take the suffix -oic acid. The –COOH carbon is always carbon 1, just like the –CHO carbon in aldehydes.

Rules:

Find the longest chain including –COOH.
Number from the –COOH carbon as C1.
Replace the final e of the parent alkane with oic acid.
Add locants for substituents.

Examples:

Formula	Name
HCOOH	Methanoic acid (formic acid)
CH₃COOH	Ethanoic acid (acetic acid)
CH₃CH₂COOH	Propanoic acid
CH₃CH₂CH₂COOH	Butanoic acid
CH₃CH(CH₃)COOH	2-methylpropanoic acid
HOOC–COOH	Ethanedioic acid (oxalic acid)
C₆H₅COOH	Benzoic acid

Dicarboxylic acids take the suffix -dioic acid — the final e of the alkane is kept. Note "ethanedioic", not "ethandioic".

3. Why Carboxylic Acids Are (Weakly) Acidic

Carboxylic acids ionise slightly in water:

$R-COOH + H_2O \rightleftharpoons R-COO^- + H_3O^+$

For ethanoic acid, Kₐ ≈ 1.8 × 10⁻⁵ at 25 °C, giving a pKₐ ≈ 4.76. Compared with a mineral acid (HCl pKₐ ≈ –7) this is very weak — only about 1 in 300 ethanoic acid molecules in a 0.1 mol dm⁻³ solution is ionised.

But it is much more acidic than a typical alcohol (ethanol pKₐ ≈ 16). Why?

3.1 Resonance stabilisation of the carboxylate

When an alcohol loses H⁺, the negative charge is localised on a single O atom. When a carboxylic acid loses H⁺, the negative charge is spread ("delocalised") over both oxygen atoms by resonance:

R-C(=O)-O(-) <--> R-C(-O-)=O

The real ion is a hybrid of these two structures, with two equivalent C–O bonds intermediate in length between single and double. Delocalisation lowers the energy of the anion, so the equilibrium lies further to the right than for an alcohol.

3.2 Effect of electron-donating or electron-withdrawing groups

Groups that withdraw electrons (–Cl, –NO₂, –COOH) stabilise the carboxylate further and make the acid stronger. Groups that donate electrons (–CH₃, –C₂H₅) destabilise the anion and make the acid weaker.

Acid	pKₐ	Why
Cl₃C–COOH (trichloroethanoic)	0.65	Three Cl pull electrons away, stabilising the anion
Cl–CH₂–COOH (chloroethanoic)	2.87	One Cl stabilises the anion
HCOOH (methanoic)	3.75	No alkyl group, baseline strength
CH₃–COOH (ethanoic)	4.76	Methyl group donates electrons, destabilising the anion
CH₃–CH₂–COOH (propanoic)	4.87	Ethyl donates slightly more

You should be able to compare two acids qualitatively and predict which is stronger. OCR will occasionally ask you to explain this in terms of inductive (±I) effects.

4. Reactions of Carboxylic Acids

Carboxylic acids undergo all the "typical acid" reactions you met at GCSE plus a set of organic transformations that you need for A-Level.

4.1 With metals (redox)

Carboxylic acids react with reactive metals (Mg, Zn, Fe) to give a salt and hydrogen gas:

$2CH_3COOH + Mg \longrightarrow (CH_3COO)_2Mg + H_2$

Observations: effervescence (bubbles of H₂), metal gradually disappears, warms slightly.

4.2 With metal oxides (neutralisation)

$2CH_3COOH + CaO \longrightarrow (CH_3COO)_2Ca + H_2O$

Observations: solid metal oxide dissolves.

4.3 With alkalis (neutralisation)

$CH_3COOH + NaOH \longrightarrow CH_3COONa + H_2O$

This is how "sodium ethanoate" is made. The neutralised salt is soluble in water and, for ethanoic acid, smells faintly vinegary.

4.4 With carbonates and hydrogencarbonates — a diagnostic test

$2CH_3COOH + Na_2CO_3 \longrightarrow 2CH_3COONa + H_2O + CO_2$

$CH_3COOH + NaHCO_3 \longrightarrow CH_3COONa + H_2O + CO_2$

This reaction gives rapid effervescence with aqueous sodium carbonate or sodium hydrogencarbonate, and the gas evolved turns limewater milky. This is the key test for a carboxylic acid in OCR exams — learn it.

Exam Tip: The phrase "effervescence with sodium hydrogencarbonate, gas turns limewater milky" is worth full marks for identifying a carboxylic acid. Phenols and alcohols do not give this reaction — phenol is too weak an acid to liberate CO₂ from HCO₃⁻.

4.5 Summary table

Reagent	Product(s)	Observation
Mg metal	Mg(RCOO)₂ + H₂	Effervescence, metal dissolves
NaOH	Na salt + H₂O	Neutralisation, warms
Na₂CO₃	Na salt + H₂O + CO₂	Effervescence, gas turns limewater milky
NaHCO₃	Na salt + H₂O + CO₂	Effervescence (gentler than carbonate)
Alcohol + H₂SO₄	Ester + H₂O	Sweet fruity smell (Lesson 4)

5. Physical Properties

Carboxylic acids have very high boiling points relative to alcohols, carbonyls and alkanes of similar size because they form two hydrogen bonds at once between two molecules — a so-called dimer.

Compound	Mᵣ	Boiling point (°C)
Propan-1-ol	60	97
Butanal	72	75
Propanoic acid	74	141
Butanoic acid	88	164

In the gas phase, two RCOOH molecules pair up via a pair of hydrogen bonds:

This effectively doubles the molecular mass for the purposes of boiling, giving very high boiling points.

Small carboxylic acids (C1–C4) are miscible with water because they can form multiple hydrogen bonds with water molecules. Higher members become progressively less soluble as the hydrocarbon tail grows. Long-chain ("fatty") acids with C ≥ 12 (e.g. lauric, palmitic, stearic) are essentially insoluble in water — the hydrocarbon tail dominates and the molecule behaves as a hydrophobic solid or wax. The sodium salts of these acids (sodium palmitate, sodium stearate) are the basis of all traditional soaps: the carboxylate head is hydrophilic and dissolves in water, while the hydrocarbon tail dissolves in oily dirt, allowing the dirt to be lifted off by surfactant action.

The dimer structure also explains some apparent anomalies in spectroscopy. In the infrared spectrum of a pure liquid carboxylic acid, the O–H stretch appears as a very broad band spanning 2500–3300 cm⁻¹ (compared with the sharp ~3300 cm⁻¹ of a free alcohol O–H) because the dimer's hydrogen bonds couple the two O–H stretches into a wide envelope of frequencies. Knowing the dimer structure makes this otherwise confusing observation fall straight out of first principles. The same dimer chemistry is responsible for the higher-than-expected vapour pressure of carboxylic acids at moderate temperatures: gas-phase ethanoic acid is dimer-rich up to ~150 °C and only fully dissociates into single monomeric molecules well above its boiling point. This was historically a source of confusion in early molecular-weight determinations by vapour density — ethanoic acid appeared to have M_r ≈ 120 (the dimer mass) rather than M_r 60 (the monomer mass), and was only resolved when X-ray crystallography and IR spectroscopy of the gas phase confirmed the dimer in the 1930s. Knowing this twenty-second piece of detail occasionally earns the final mark in a top-band exam answer when the question asks for an explanation of "unusually high boiling point" or "why vapour density gives a too-large M_r".

6. Esterification — A Preview

Carboxylic acids react with alcohols, in the presence of a concentrated H₂SO₄ catalyst, to form esters. This is one of the most important reactions in carboxylic-acid chemistry and is the subject of the next lesson. In summary:

$R-COOH + R'-OH \xrightleftharpoons[\text{H}_2\text{SO}_4]{\text{warm, reflux}} R-COO-R' + H_2O$

The equilibrium typically lies at 65–70% ester formation. Esterification is also the link between carboxylic-acid chemistry and polymer chemistry — polyesters (e.g. PET, polyethylene terephthalate) are condensation polymers of a dicarboxylic acid and a diol (Lesson 9).

Key Insight: Whenever you see carboxylic acid + alcohol with concentrated H₂SO₄, think ester. Whenever you see acyl chloride + alcohol, also think ester — but faster, irreversibly, no acid needed. We will compare the two routes in Lessons 4 and 5.

7. Worked Example — Identifying an Unknown Acid

A liquid Z dissolves in water to give a solution with pH ~3. Adding aqueous NaHCO₃ produces vigorous effervescence, and the gas evolved turns limewater milky. Z has M_r = 60. Identify Z.

Approach:

The pH ~3 is consistent with a weak acid (mineral acids would give pH 1–2 at typical concentrations).
The NaHCO₃ + limewater test is diagnostic for carboxylic acid (other weak acids such as phenol do not effervesce with HCO₃⁻ at room temperature).
M_r = 60. Carboxylic acid formulae: HCOOH M_r = 46; CH₃COOH M_r = 60; CH₃CH₂COOH M_r = 74.

Z is ethanoic acid, CH₃COOH.

Confirmatory: ethanoic acid is the active component of vinegar (5% v/v solution); the M_r matches; pH 3 matches a ~0.1 mol dm⁻³ ethanoic acid solution (Kₐ ≈ 1.8 × 10⁻⁵ → [H⁺] ≈ √(Kₐ × c) ≈ 1.3 × 10⁻³, so pH ≈ 2.9).

8. Worked Example — Predicting Acid Strength

Arrange the following acids in order of increasing strength, with reasoning:

Ethanoic acid CH₃COOH (pKₐ 4.76)
Chloroethanoic acid ClCH₂COOH (pKₐ 2.87)
Dichloroethanoic acid Cl₂CHCOOH (pKₐ 1.35)
Trichloroethanoic acid Cl₃CCOOH (pKₐ 0.65)

Reasoning: Each Cl substituent on the α-carbon is electron-withdrawing (negative inductive effect, –I). Electron-withdrawing groups stabilise the negative charge of the carboxylate anion by pulling electron density along the σ framework towards them. More Cl substituents → more stabilisation of the anion → equilibrium for dissociation lies further to the right → stronger acid → lower pKₐ.

Order (weakest to strongest): $\text{CH}_3\text{COOH} < \text{ClCH}_2\text{COOH} < \text{Cl}_2\text{CHCOOH} < \text{Cl}_3\text{CCOOH}$

Trichloroethanoic acid is ~13,000 times stronger than ethanoic acid (Δ pKₐ ≈ 4.1). The pattern is universal — fluorine substituents would give an even stronger acid (Cl₃CCOOH vs F₃CCOOH: F is more electronegative).

Synoptic Links

Synoptic Links — Connects to:

ocr-alevel-chemistry-basic-organic / functional-groups-iupac (Lesson 1 — IUPAC nomenclature for -oic acid suffix; carboxylic acid is principal-group-priority above alcohol and carbonyl).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / carbonyl-compounds and carbonyls-reactions-and-tests (Lessons 1–2 — oxidation of aldehydes gives carboxylic acids; the synthetic on-ramp from previous lessons).

ocr-alevel-chemistry-alcohols-haloalkanes / combustion-and-oxidation-of-alcohols (oxidation of primary alcohols with hot concentrated acidified K₂Cr₂O₇ gives carboxylic acids directly).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / esters-esterification-hydrolysis (Lesson 4 — the immediate product of acid + alcohol).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / acyl-chlorides (Lesson 5 — carboxylic acid + SOCl₂ → acyl chloride, the fast onramp into ester/amide chemistry).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / condensation-polymers (Lesson 9 — polyesters built from dicarboxylic acids + diols).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / amino-acids-chirality (Lesson 7 — amino acids have both –COOH and –NH₂; the carboxylate at physiological pH is the basis of zwitterion structure).

ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / proton-nmr-combined-techniques (the –COOH proton appears at δ 11–12 ppm in ¹H NMR — diagnostic; the C=O carbon at δ 170–180 ppm in ¹³C NMR).

From AS Year 12: acid-base equilibria (Kₐ, pKₐ, weak-acid pH calculations); inductive effect from haloalkane chemistry.

Lesson 3: Carboxylic Acids