Carboxylic Acids

Carboxylic acids are one of the most widely encountered functional groups in organic chemistry and in everyday life. They contain the carboxyl group (-COOH), which combines a carbonyl (C=O) and a hydroxyl (O-H) in a single functional group. From ethanoic acid in vinegar to citric acid in lemons, carboxylic acids play central roles in biology, industry, and synthesis.

Structure and Naming

The carboxyl group is written as -COOH or -CO2H. The carbon of the carboxyl group is always carbon 1 in the naming system, and the suffix -oic acid is used:

Methanoic acid (HCOOH) -- one carbon
Ethanoic acid (CH3COOH) -- two carbons
Propanoic acid (CH3CH2COOH) -- three carbons
Butanoic acid (CH3CH2CH2COOH) -- four carbons

The carboxyl group is always at the end of the chain, so no positional number is needed for it.

Dicarboxylic Acids

Some carboxylic acids have two -COOH groups. These are called dicarboxylic acids and are named with the suffix -dioic acid:

Ethanedioic acid (oxalic acid) -- HOOCCOOH
Hexanedioic acid (adipic acid) -- HOOC(CH2)4COOH

Dicarboxylic acids are important as monomers for condensation polymers (polyesters and polyamides).

Physical Properties and Hydrogen Bonding

Carboxylic acids have notably high boiling points for their molecular mass. This is because the -COOH group can form two hydrogen bonds per molecule -- one through the O-H and one through the C=O lone pairs. In fact, in the liquid state and in non-polar solvents, carboxylic acids often exist as dimers, where two molecules are linked by a pair of hydrogen bonds forming an eight-membered ring.

Short-chain carboxylic acids (up to about four carbons) are miscible with water because they form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, the non-polar portion dominates and solubility decreases.

Acid	Formula	Mr	Boiling Point (degrees C)	Solubility
Methanoic acid	HCOOH	46	101	Miscible
Ethanoic acid	CH3COOH	60	118	Miscible
Propanoic acid	C2H5COOH	74	141	Miscible
Butanoic acid	C3H7COOH	88	164	Miscible
Hexanoic acid	C5H11COOH	116	205	Slightly soluble
Butan-1-ol (comparison)	C4H9OH	74	117	Soluble

Notice that ethanoic acid (Mr = 60) has a higher boiling point than butan-1-ol (Mr = 74), even though it has a lower molecular mass. This demonstrates the effectiveness of the double hydrogen bonding in carboxylic acid dimers.

Acidity of Carboxylic Acids

Carboxylic acids are weak acids. They dissociate partially in water to release a proton (H+):

CH3COOH <=> CH3COO- + H+

The equilibrium lies to the left, meaning only a small proportion of molecules are dissociated at any given time. This is in contrast to strong acids like HCl, which dissociate completely.

Why Are Carboxylic Acids Acidic?

The acidity arises because the carboxylate ion (RCOO-) formed after proton loss is stabilised by delocalisation. The negative charge is spread equally over both oxygen atoms through a delocalised pi system. This stabilisation of the conjugate base drives the equilibrium towards dissociation more than would occur for a simple alcohol (where the alkoxide ion R-O- has a localised charge and is much less stable).

Delocalisation in the Carboxylate Ion -- A Closer Look

In the carboxylate ion, the two C-O bonds are identical in length (intermediate between a C=O double bond and a C-O single bond). Each oxygen carries half of the negative charge. This can be represented by two resonance structures:

Structure 1: negative charge on the left oxygen, double bond to the right oxygen
Structure 2: double bond to the left oxygen, negative charge on the right oxygen

The true structure is a hybrid of both, with 1.5 bond order on each C-O bond.

Comparison of Acidity

Species	pKa	Classification	Reason
Ethanoic acid (CH3COOH)	4.76	Weak acid	Carboxylate ion stabilised by delocalisation
Ethanol (CH3CH2OH)	~16	Very weak acid	Ethoxide ion (CH3CH2O-) has localised charge -- unstable
Phenol (C6H5OH)	10.0	Very weak acid	Phenoxide ion partially stabilised by delocalisation into the ring
Hydrochloric acid (HCl)	-7	Strong acid	Full dissociation; Cl- is very stable

The carboxylate ion is far more stable than an alkoxide, explaining why carboxylic acids are approximately 10^11 times more acidic than ethanol.

Effect of Substituents on Acid Strength

Electron-withdrawing groups near the -COOH group increase acid strength by stabilising the carboxylate ion further:

Acid	pKa	Explanation
CH3COOH	4.76	Baseline: methyl group has small electron-releasing effect
ClCH2COOH	2.86	Cl withdraws electron density (inductive effect), stabilises COO-
Cl2CHCOOH	1.29	Two Cl atoms -- stronger withdrawal
Cl3CCOOH	0.65	Three Cl atoms -- very strong withdrawal
FCH2COOH	2.66	F is more electronegative than Cl -- stronger effect per atom

This trend is important for understanding why some carboxylic acids are stronger than others.

Reactions of Carboxylic Acids

With Bases (Neutralisation)

Carboxylic acids react with bases to form a salt and water:

CH3COOH + NaOH --> CH3COONa + H2O

The salt formed is called a carboxylate -- in this case sodium ethanoate.

With Carbonates and Hydrogen Carbonates

Carboxylic acids react with carbonates and hydrogen carbonates to produce a salt, water, and carbon dioxide:

2CH3COOH + Na2CO3 --> 2CH3COONa + H2O + CO2

CH3COOH + NaHCO3 --> CH3COONa + H2O + CO2

The effervescence (bubbling of CO2) provides a simple test for the presence of a carboxylic acid. Alcohols and other organic compounds do not produce CO2 with sodium carbonate. This is the key diagnostic test that distinguishes carboxylic acids from alcohols (both of which react with PCl5 to give HCl fumes).

With Reactive Metals

Carboxylic acids react with reactive metals (e.g., magnesium, zinc) to produce a salt and hydrogen gas:

2CH3COOH + Mg --> (CH3COO)2Mg + H2

Ester Formation (Fischer Esterification)

Carboxylic acids react with alcohols in the presence of a concentrated sulfuric acid catalyst to form esters and water. This is a condensation reaction:

CH3COOH + C2H5OH <=> CH3COOC2H5 + H2O

This is known as Fischer esterification. Key points:

The reaction is reversible (reaches equilibrium)
The concentrated H2SO4 acts as a catalyst (protonates the carbonyl oxygen, making the carbon more electrophilic) and also removes water to shift the equilibrium towards products
The reaction is slow and requires heating under reflux
The ester product often has a pleasant fruity smell

Mechanism of Fischer Esterification (Outline)

H2SO4 protonates the carbonyl oxygen of the carboxylic acid, increasing the delta+ on the carbonyl carbon.
The lone pair on the alcohol oxygen attacks the now highly electrophilic carbonyl carbon (nucleophilic addition).
Proton transfers occur within the tetrahedral intermediate.
Water is eliminated (lost as a leaving group) and the C=O reforms.
The proton catalyst is regenerated.

The overall mechanism involves six steps and is not usually required in full at A-Level, but understanding why the acid catalyst is needed (step 1) is important.

Reduction of Carboxylic Acids

Carboxylic acids can be reduced to primary alcohols, but NaBH4 is not strong enough to do this. Instead, a more powerful reducing agent is needed -- lithium aluminium hydride (LiAlH4) in dry ether:

CH3COOH --> CH3CH2OH (with LiAlH4)

Why is NaBH4 too weak? The -OH group of the carboxylic acid delocalises its lone pairs into the C=O bond, which reduces the delta+ on the carbonyl carbon and makes it harder for the mild H- from NaBH4 to attack. LiAlH4 is a much more powerful source of H-, overcoming this resistance.

LiAlH4 reacts violently with water, which is why dry ether is used as the solvent. After the reaction, dilute acid is added carefully to hydrolyse the aluminium alkoxide intermediate and release the alcohol product.

Acyl Chlorides -- More Reactive Derivatives

Carboxylic acids themselves are relatively unreactive in nucleophilic acyl substitution because the -OH group is a poor leaving group. To overcome this, carboxylic acids can be converted to acyl chlorides (RCOCl), which are far more reactive.

Acyl chlorides are formed by reacting a carboxylic acid with SOCl2 (thionyl chloride) or PCl5 (phosphorus pentachloride):

CH3COOH + SOCl2 --> CH3COCl + SO2 + HCl

The Cl in the acyl chloride is a much better leaving group than -OH, so acyl chlorides react rapidly with nucleophiles (water, alcohols, amines) without needing a catalyst or prolonged heating. This makes them invaluable in synthesis, as covered in a later lesson.

Synthesis Route Map from Carboxylic Acids

flowchart LR
    A[Carboxylic Acid RCOOH] -->|SOCl2| B[Acyl Chloride RCOCl]
    A -->|LiAlH4, dry ether| C[Primary Alcohol RCH2OH]
    A -->|ROH, conc. H2SO4, reflux| D[Ester RCOOR']
    A -->|NaOH| E[Sodium Carboxylate RCOONa]
    B -->|ROH| D
    B -->|RNH2| F[Amide RCONHR']
    B -->|NH3| G[Primary Amide RCONH2]
    B -->|H2O| A
    C -->|K2Cr2O7 / H2SO4, reflux| A

Common Exam Mistakes

Using NaBH4 to reduce a carboxylic acid. NaBH4 is too mild. You must use LiAlH4 in dry ether.
Confusing the carbonate test. Alcohols do NOT react with Na2CO3 to give CO2. Only carboxylic acids do. However, both alcohols and carboxylic acids give HCl fumes with PCl5.
Forgetting that esterification is reversible. The yield from Fischer esterification is limited by equilibrium. To improve yield, remove water or use excess of one reactant.
Writing the wrong products for the reaction with Na2CO3. Remember: salt + water + CO2. Students often forget the CO2 or write it as H2 by mistake.

Summary

Carboxylic acids are versatile functional groups that participate in a wide range of reactions. Their weak acidity (explained by delocalisation in the carboxylate ion), hydrogen bonding (explaining high boiling points and dimer formation), and ability to form esters and acyl chloride derivatives make them central to organic chemistry at A-Level. Recognising the -COOH group and predicting its behaviour in reactions is an essential skill for the exam.

A-Level Deep Dive: Carboxylic Acids

Spec mapping

Edexcel 9CH0 specification, Topic 17 — Carboxylic acids and their derivatives, sub-strands 17.1–17.4 covers the structure of the carboxyl group (-COOH), the weak-acid behaviour of carboxylic acids in water (Ka of order 10⁻⁵, pKa ≈ 4–5), reactions of carboxylic acids with metals, bases, carbonates and hydrogencarbonates, esterification with alcohols under acid catalysis (concentrated H2SO4), and reduction by LiAlH4 in dry ether to primary alcohols (refer to the official specification document for exact wording). It links explicitly to Topic 12 (acid–base equilibria — Ka, pKa, buffer calculations), Topic 16 (oxidation of aldehydes gives carboxylic acids), Topic 18 (amino acids, condensation polymers, infra-red spectra of -COOH at ≈ 2500–3300 cm⁻¹ broad and C=O at 1700–1725 cm⁻¹) and CP14 (aspirin synthesis) and CP15 (preparation of an ester). Carboxylic acids are examined predominantly on Paper 2 with synoptic appearances on Paper 3.

Worked example with full mark scheme

Question (8 marks):

(a) Explain why ethanoic acid (pKa 4.76) is a stronger acid than ethanol (pKa 15.9). Refer to the structure and stability of the conjugate base in your answer. (4)

(b) A student has two unlabelled bottles, one containing aqueous ethanoic acid and one containing aqueous phenol. Describe a chemical test using sodium hydrogencarbonate (NaHCO3) that would distinguish the two solutions, including observations and equations where appropriate. (4)

Solution with mark scheme:

(a) Step 1 — identify the conjugate bases.

Ethanoic acid: CH3COOH ⇌ CH3COO⁻ + H+ (the conjugate base is the ethanoate ion). Ethanol: CH3CH2OH ⇌ CH3CH2O⁻ + H+ (the conjugate base is the ethoxide ion).

M1 — both equilibria written and conjugate bases identified.

Step 2 — explain stabilisation of the ethanoate ion.

In ethanoate (CH3COO⁻), the negative charge is delocalised over both oxygen atoms by resonance — the two C–O bonds are equivalent (C–O bond order ≈ 1.5). This delocalisation lowers the energy of the conjugate base relative to the protonated form, making the dissociation more favourable.

M1 — resonance/delocalisation in ethanoate stated explicitly. Drawing the two equivalent resonance structures with curly arrows scores this mark.

Step 3 — contrast with ethoxide.

In ethoxide (CH3CH2O⁻), the negative charge is localised on the single oxygen atom — there is no adjacent π-system to delocalise into. The conjugate base is therefore higher in energy and dissociation is correspondingly less favourable.

M1 — explicit contrast: localised vs delocalised charge.

A1 — concluding sentence linking lower-energy conjugate base to greater Ka and lower pKa, hence stronger acid.

(b) Step 1 — choose NaHCO3.

Add aqueous NaHCO3 to a portion of each solution. With ethanoic acid, effervescence is observed as CO2 gas is evolved; the gas turns limewater milky. With phenol, no effervescence occurs because phenol is too weak an acid to react with NaHCO3.

M1 — observation in ethanoic acid (CO2 effervescence). M1 — observation in phenol (no reaction).

Step 2 — write the equation.

CH3COOH(aq) + NaHCO3(aq) → CH3COONa(aq) + H2O(l) + CO2(g)

A1 — balanced equation.

Step 3 — explain why phenol does not react.

Phenol (pKa ≈ 10) is a weaker acid than carbonic acid (pKa1 ≈ 6.4); therefore phenol cannot donate H+ to HCO3⁻. Ethanoic acid (pKa 4.76) is stronger than carbonic acid and so does react.

A1 — pKa-based justification using the rule "an acid can protonate the conjugate base of any weaker acid".

Total: 8 marks (M5 A3).

Specimen question modelled on the Edexcel 9CH0 Paper 2 format

Question (7 marks): A student wishes to prepare ethyl ethanoate by refluxing ethanoic acid with ethanol in the presence of concentrated sulfuric acid catalyst.