Carboxylic Acids

Theory — Carboxylic Acids

A carboxylic acid contains the carboxyl group: a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon, written compactly as –COOH. The two functional groups influence each other strongly: the C=O makes the O–H much more acidic than an alcohol O–H (pKa ~5 vs ~16), and the lone pairs on the –OH oxygen donate into the C=O π* orbital, making the carbonyl C less electrophilic than that of an aldehyde or ketone. The result is a functional group with chemistry quite different from either C=O or –OH alone.

1. Naming carboxylic acids

The IUPAC suffix is -oic acid. The carboxyl carbon is always C1. No locant is needed for the COOH group itself (it can only be at the terminal position). Examples: methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid, butanoic acid, hexanoic acid.

Several common names are retained in IUPAC: formic acid (HCOOH; from formica, ant), acetic acid (CH₃COOH; from acetum, vinegar), benzoic acid (C₆H₅COOH), oxalic acid (HOOC-COOH), citric acid, lactic acid, malic acid, tartaric acid. For dicarboxylic acids, the suffix is -dioic acid: ethanedioic acid (oxalic), butanedioic acid (succinic), pentanedioic acid (glutaric), hexanedioic acid (adipic).

For aromatic carboxylic acids attached to a ring, the ring is named first, then -carboxylic acid: cyclohexanecarboxylic acid, 2-naphthalenecarboxylic acid. The COOH carbon is NOT counted in the ring carbon count for these.

2. Acidity — the defining property

The reason carboxylic acids are acidic (and alcohols are not) is resonance stabilisation of the conjugate base. When R–COOH loses H⁺, the resulting R–COO⁻ has its negative charge delocalised across the two oxygens equally:

Resonance in the carboxylate R-C(=O)-O⁻ ⇌ R-C(-O⁻)=O [two equivalent resonance structures]
The two C-O bonds become equal length (~127 pm) and equal in bond order (~1.5)
Result: pKa ~ 4-5 vs alcohol pKa ~ 16

Substituent effects on pKa. Anything that stabilises the carboxylate makes the acid more acidic (lower pKa). Anything that destabilises it makes the acid less acidic (higher pKa).

Acid	Structure	pKa	Why?
Trifluoroacetic acid	CF₃COOH	0.23	3× F (strong inductive –I) stabilises COO⁻ through bonds
Trichloroacetic acid	CCl₃COOH	0.66	3× Cl (strong –I) stabilises COO⁻
Dichloroacetic acid	CHCl₂COOH	1.29	2× Cl, weaker than CCl₃ but stronger than CH₂Cl
Chloroacetic acid	CH₂ClCOOH	2.86	1× Cl: small but significant –I
Formic acid	HCOOH	3.75	No alkyl group to donate (+I); slightly stronger than acetic
Benzoic acid	C₆H₅COOH	4.20	Aryl ring; resonance contribution + slight inductive withdrawal
Acetic acid	CH₃COOH	4.76	Methyl group donates electron density (+I), destabilises COO⁻ slightly
Propanoic acid	CH₃CH₂COOH	4.87	Slightly more alkyl donation than acetic
Phenol (for comparison)	C₆H₅OH	10.0	Resonance stabilises PhO⁻ but only onto C, not onto O — much weaker than COO⁻
Ethanol (for comparison)	CH₃CH₂OH	15.9	No resonance stabilisation of EtO⁻ at all

Note the dramatic effect of EWGs: trifluoroacetic acid (pKa 0.23) is ~30,000× more acidic than acetic acid (pKa 4.76). The fluorines pull electron density through the σ-bond network (inductive effect), stabilising the negative charge on the carboxylate.

3. Reactions — the carboxyl group as a precursor

(a) Acid-base reactions. Carboxylic acids react quickly with bases (NaOH, NaHCO₃, Na₂CO₃, amines) to give the carboxylate salt and water. With NaHCO₃ specifically, CO₂ is released — this gives the diagnostic effervescence test that distinguishes carboxylic acids from phenols (which are too weakly acidic to react with NaHCO₃).

NaHCO₃ effervescence test R-COOH + NaHCO₃ → R-COO⁻Na⁺ + H₂O + CO₂↑ (visible bubbles)
Phenol + NaHCO₃ → no reaction (phenol pKa ~10 vs HCO₃⁻ pKa ~6.4)
Effervescence with NaHCO₃ = carboxylic acid (NOT phenol)

(b) Fischer esterification. Carboxylic acid + alcohol with acid catalyst (typically conc. H₂SO₄ or p-TsOH) gives an ester. Reaction is reversible; equilibrium is driven to the ester by removing water (Dean–Stark trap, molecular sieves) or using excess alcohol. Mechanism: protonation of C=O → nucleophilic addition of alcohol → proton shuffles → loss of water → deprotonation. Six steps, all reversible.

Fischer esterification R-COOH + R'-OH ⇌ R-COO-R' + H₂O [H⁺ cat., reversible]
Drive forward: remove H₂O OR use excess R'-OH
Reverse (saponification with base): R-COO-R' + NaOH → R-COO⁻Na⁺ + R'-OH
Acid catalysis: equilibrium. Base catalysis (saponification): irreversible (carboxylate cannot react back)

(c) Conversion to acyl chloride. Treatment with thionyl chloride (SOCl₂) or oxalyl chloride ((COCl)₂) converts R–COOH → R–COCl, with byproducts SO₂ + HCl (gases, easy removal) or CO + CO₂ + HCl. Acyl chlorides are far more reactive than the parent carboxylic acid and are key intermediates in synthesis.

Activation as acyl chloride R-COOH + SOCl₂ → R-COCl + SO₂↑ + HCl↑
R-COOH + (COCl)₂ + cat. DMF → R-COCl + CO↑ + CO₂↑ + HCl↑
Acyl chloride is then attacked by ANY nucleophile — alcohols, amines, organometallics

(d) Amide formation. Direct reaction of carboxylic acid + amine gives an ammonium carboxylate salt at room temperature. To get the amide, the salt must be heated (lose water) or activated by a coupling reagent (DCC, EDC, HATU). DCC (dicyclohexylcarbodiimide) is the classic peptide-coupling reagent: it activates the COOH for nucleophilic attack and produces dicyclohexylurea (DCU) as a separable byproduct.

(e) Reduction. LiAlH₄ in dry THF reduces R–COOH all the way to R–CH₂OH (primary alcohol). NaBH₄ does NOT reduce carboxylic acids — it is too mild. For partial reduction (R–COOH → R–CHO, the aldehyde), use DIBAL-H at low temperature (one equivalent, –78 °C, careful stoichiometry).

(f) Decarboxylation. Most carboxylic acids do not lose CO₂ on heating, but β-keto acids and 1,3-dicarboxylic acids decarboxylate readily because the resulting carbanion is stabilised by the adjacent C=O group. Important in malonic ester synthesis and acetoacetic ester synthesis.

Decarboxylation of a β-keto acid R-CO-CH₂-COOH ^Δ→ R-CO-CH₃ + CO₂↑
Six-membered transition state involving H-transfer to carbonyl O
Only β-keto and β-dicarboxylic acids decarboxylate easily; simple R-COOH does NOT

4. Diagnostic tests (see Section IV)

Test	Positive result	What it detects	Distinguishes from
NaHCO₃ effervescence	Visible CO₂ bubbles	Carboxylic acid (only)	Phenol (negative); alcohol (negative)
Litmus / pH paper	Red colour (pH 2-4)	Acidic compounds	Neutral or basic compounds
Solubility — 5% NaOH	Dissolves	Carboxylic acid OR phenol	Alcohol, ester, amine (mostly insoluble)
Solubility — 5% NaHCO₃	Dissolves with effervescence	Carboxylic acid (only)	Phenol (does NOT dissolve)
FeCl₃	Purple/blue colour	Phenol (some)	Carboxylic acid (negative or yellow)
Fischer esterification	Pleasant fruity odour	Carboxylic acid + alcohol	Useful confirmation, not specific

5. Hydrogen bonding and physical properties

Carboxylic acids form unusually strong intermolecular H-bonds. In the gas phase and in non-polar solvents, they exist as cyclic dimers with two reciprocal H-bonds. This raises the boiling point dramatically: acetic acid b.p. 118 °C versus ethanol b.p. 78 °C and acetone b.p. 56 °C. The dimer is so stable that the apparent molecular weight in benzene solution is approximately twice that of the monomer.

Lower carboxylic acids (C1–C4) are fully water-miscible. Solubility decreases with increasing chain length: pentanoic acid is somewhat soluble; longer-chain acids (palmitic, stearic) are essentially insoluble in water and behave as "fatty acids."

Instructions

This lab's Simulation section has four parts. Complete them in order.

Section I — Naming & Acidity. Eight carboxylic acids are shown as structural drawings. For each, identify the IUPAC name, predict the approximate pKa range, and identify the structural feature most influencing acidity.

Section II — Reaction Bench. Six reactions of carboxylic acids, presented as an interactive flask with reagent dispensers. Click a reagent bottle to add it to the flask, watch the reaction, then identify the product from four options.

Section III — Reactivity & Mechanism. Eight problems testing acidity ordering, EWG vs EDG effects, mechanism of Fischer esterification, why amides hydrolyse slower than esters, decarboxylation conditions, and dimer formation.

Section IV — SDS & Microscale Tests. Read SDS extracts for four reagents (16 questions on glacial acetic acid, formic acid, thionyl chloride, DCC). Then run six microscale diagnostic tests to identify unknown carboxylic acids and distinguish them from phenols.

Prepare your lab notebook. Use the Example Report as your template.

Prerequisite: Complete (or be familiar with) Lab Skills & Safety, Functional Group Tests, Acids and Bases, and Aldehydes & Ketones before starting this lab. The carboxylic acid is closely connected to those topics.

Simulation

Four interactive parts. Use the ↺ Reset Simulation button at any time to clear all answers and start over.

Eight carboxylic acids. For each: (a) IUPAC name, (b) pKa range, (c) structural feature most affecting acidity.

Score: 0 / 24 (3 questions × 8 acids)

Six reactions of carboxylic acids. For each: read the prompt, click the reagent in the dispenser shelf to add it to the flask, then click the predicted product from the four options.

Score: 0 / 6

Eight conceptual problems on acidity, mechanism, and reactivity. Choose the best answer for each.

Score: 0 / 8

Round 1 — SDS interpretation

Four key reagents used in carboxylic acid chemistry. Each has 4 questions.

SDS score: 0 / 16

Round 2 — Microscale diagnostic tests

Six unknown samples. For each, run the indicated test and identify the functional group present.

Microscale score: 0 / 6

Team Questions

Discuss with your team before answering.

Question 1 — Naming. What is the IUPAC name of HOOC-CH₂-CH₂-COOH (the di-acid found in unripe apples)?

Question 2 — Acidity. Which is more acidic: 2-chloroacetic acid (ClCH₂COOH, pKa 2.86) or 3-chloropropanoic acid (ClCH₂CH₂COOH, pKa 4.10)? Why?

Question 3 — Diagnostic test. A student has two compounds, both giving a yellow precipitate with 2,4-DNP. Compound A produces effervescence with NaHCO₃; compound B does not. Compound A also has a higher pKa value than compound B. Identify what classes A and B might be.

Question 4 — Fischer esterification. You want to make ethyl acetate from acetic acid and ethanol. The reaction is reversible. Suggest TWO ways to drive the equilibrium to the right (favour ester formation).

Question 5 — Decarboxylation. Why does 3-oxobutanoic acid (CH₃-CO-CH₂-COOH) easily lose CO₂ on warming to give acetone, but propanoic acid (CH₃-CH₂-COOH) does not?

Question 6 — Reactivity ordering. Rank from highest to lowest acid strength: ethanol, phenol, acetic acid, hydrochloric acid (HCl).

Example Lab Notebook Entry

Use the format below as a template.

Carboxylic Acids — Lab Notebook Entry

Submitted by: [Student Name]

Course: Organic Chemistry I · Section: 201-A · Date: April 26, 2026

Objective

To recognise carboxylic acids by structure and IUPAC name; to predict the pKa of a carboxylic acid based on structural features (alkyl chain length, EWG/EDG substituents, aromatic vs aliphatic); to predict the products of common reactions (Fischer esterification, conversion to acyl chloride, reduction with LiAlH₄, salt formation with bases, amide formation, decarboxylation); to interpret SDS information for common reagents; and to identify carboxylic acids by diagnostic microscale tests (NaHCO₃ effervescence, FeCl₃, solubility behaviour, esterification fragrance).

Naming & pKa summary (Section I results)

Structure	IUPAC name	pKa	Key feature
HCOOH	Methanoic acid (formic)	3.75	No alkyl group; H instead of R
CH₃COOH	Ethanoic acid (acetic)	4.76	Methyl donates +I
CH₃CH₂COOH	Propanoic acid	4.87	Slightly more +I from longer chain
C₆H₅COOH	Benzoic acid	4.20	Aryl ring withdraws inductively, slightly +M
HOOC-COOH	Ethanedioic acid (oxalic)	1.27 (1st), 4.27 (2nd)	Two COOH groups; first is acidified by neighbouring –I
CCl₃COOH	Trichloroethanoic acid	0.66	Three Cl exert strong –I
2-OH-C₆H₄-COOH	2-Hydroxybenzoic acid (salicylic)	2.97	Intramolecular H-bond stabilises COO⁻
CH₃CHOH-COOH	2-Hydroxypropanoic acid (lactic)	3.86	α-OH withdraws inductively

Reaction bench observations (Section II)

Acid	Reagent	Product (predicted)	Class of product
Acetic acid	EtOH + cat. H₂SO₄, Δ	Ethyl ethanoate (ethyl acetate)	Ester
Benzoic acid	SOCl₂	Benzoyl chloride + SO₂↑ + HCl↑	Acyl chloride
Propanoic acid	LiAlH₄ in dry THF, then H₂O	Propan-1-ol	1° alcohol
Acetic acid	NaOH (aq)	Sodium acetate (CH₃COO⁻Na⁺)	Carboxylate salt
Benzoic acid + cyclohexylamine	DCC, CH₂Cl₂	N-cyclohexylbenzamide + DCU	Secondary amide
3-Oxobutanoic acid	Heat, no catalyst	Acetone + CO₂↑	Methyl ketone (decarboxylation)

Microscale test results (Section IV, Round 2)

Unknown	Test applied	Observation	Identified as
Sample 1	NaHCO₃ (aq)	Visible effervescence, CO₂ gas evolution	Carboxylic acid (NOT a phenol)
Sample 2	FeCl₃ (aq)	Deep purple-violet colour	Phenol (NOT a carboxylic acid)
Sample 3	Solubility — 5% NaOH and 5% NaHCO₃	Dissolves in NaOH and in NaHCO₃	Carboxylic acid
Sample 4	Solubility — 5% NaOH and 5% NaHCO₃	Dissolves in NaOH only; insoluble in NaHCO₃	Phenol
Sample 5	Esterification (cat. H₂SO₄ + EtOH, Δ)	Pleasant fruity ester odour	Carboxylic acid (Fischer esterification confirms)
Sample 6	Universal pH paper	pH ≈ 3 (red colour)	Carboxylic acid (or other strong-ish acid)

Discussion

The defining feature of carboxylic acids is their unusually high acidity (pKa 4-5) compared to alcohols (pKa ~16): the conjugate base, the carboxylate, is dramatically stabilised by resonance delocalisation across two equivalent C–O bonds. The two C–O bonds of the carboxylate are equivalent (~127 pm), each having a bond order of approximately 1.5 — a clear signature of resonance.

Substituent effects on pKa were dramatic: trichloroacetic acid (pKa 0.66) is more than 12,000× more acidic than acetic acid (pKa 4.76). Three chlorines pull electron density through the σ-bond network (inductive withdrawal, –I), stabilising the negative charge on the carboxylate. Conversely, the methyl group of acetic acid donates electron density (+I), slightly destabilising COO⁻ and making acetic acid less acidic than formic acid (pKa 3.75). In the aromatic series, benzoic acid is slightly more acidic than acetic because the sp² aryl carbon withdraws electron density inductively; salicylic acid is much more acidic still because the ortho-OH forms an intramolecular H-bond with the carboxylate, stabilising it specifically.

The reactions explored in Section II show why the carboxylic acid is a synthetic hub: from a single COOH, six different functional groups were accessible — ester (Fischer), acyl chloride (SOCl₂), amide (DCC + amine), alcohol (LiAlH₄), salt (NaOH), and even another carbonyl (decarboxylation of β-keto acids). Importantly, these reactions are not interchangeable. NaBH₄ does NOT reduce carboxylic acids — only LiAlH₄ does. Direct heating of acid + amine gives only the salt (ammonium carboxylate); the free amide requires either dehydration (200 °C, prolonged heating, low yield) or activation by a coupling reagent like DCC.

Section IV's microscale tests highlighted the most useful diagnostic distinction: NaHCO₃ effervescence separates carboxylic acids from phenols. Both are "acidic" enough to react with NaOH, but only carboxylic acids (pKa 4-5) are stronger than carbonic acid (pKa 6.4), so only they can protonate HCO₃⁻ to release CO₂. Phenol (pKa 10) is too weak. The FeCl₃ test is the complementary check: phenols give a strong purple colour from the iron–phenoxide complex; carboxylic acids do not (apart from a slight pale-yellow shift sometimes observed).

Conclusion

Carboxylic acids combine the chemistry of a carbonyl (electrophilic at C) with that of an alcohol (acidic at O–H), but the combination produces unique reactivity: very low pKa, strong intermolecular H-bonding (dimer formation), reduced electrophilicity at the carbonyl carbon (compared to an aldehyde or ketone), and accessibility to a wide range of carbonyl derivatives via straightforward reactions. Combined with their characteristic NaHCO₃ effervescence test, carboxylic acids are perhaps the easiest functional group to recognise in the laboratory.

References

1. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed., Oxford University Press, 2012, Chs 12, 13, 22.
2. Smith, M. B.; March, J. March's Advanced Organic Chemistry, 7th ed., Wiley, 2013, Ch 16.
3. IUPAC. Recommendations on Organic Nomenclature, 2013.
4. Sigma-Aldrich SDS for acetic acid (CAS 64-19-7), formic acid (CAS 64-18-6), thionyl chloride (CAS 7719-09-7), DCC (CAS 538-75-0), accessed online March 2026.

Practice Questions

Work through each before peeking at the hint.

Practice 1 — Naming

Name (CH₃)₂CHCOOH using IUPAC rules.

Hint: 2-methylpropanoic acid (common name: isobutyric acid). Three-carbon parent chain (propan-) with COOH (C1) and a methyl branch on C2.

Practice 2 — pKa ordering

Rank by acid strength (most → least): chloroacetic acid (pKa 2.86), acetic acid (pKa 4.76), benzoic acid (pKa 4.20), formic acid (pKa 3.75).

Hint: ClCH₂COOH > HCOOH > C₆H₅COOH > CH₃COOH (most acidic first). Lower pKa = stronger acid. Cl-CH₂-COOH gets a strong –I from chlorine; HCOOH lacks the alkyl +I that weakens acetic.

Practice 3 — Fischer esterification

What ester forms from acetic acid + 1-pentanol with cat. H₂SO₄ and heat?

Hint: Pentyl ethanoate (n-pentyl acetate). Smells like banana — used in confectionery flavours. The acid contributes "ethanoate" (acyl part); the alcohol contributes "pentyl" (alkyl part).

Practice 4 — SOCl₂

What products result from butanoic acid + SOCl₂?

Hint: Butanoyl chloride (CH₃CH₂CH₂COCl) plus SO₂↑ and HCl↑ as byproducts. The two byproducts are gases that leave the reaction, simplifying purification.

Practice 5 — Reduction

Treat propanoic acid with (a) NaBH₄ in MeOH, (b) LiAlH₄ in dry THF then H₂O. What is the product in each case?

Hint: (a) NO REACTION — NaBH₄ is too mild to reduce a carboxylic acid. (b) Propan-1-ol — LiAlH₄ reduces all the way to the primary alcohol. To stop at the aldehyde would require DIBAL-H at –78 °C, one equivalent.

Practice 6 — Diagnostic tests

An unknown gives: (i) effervescence with NaHCO₃; (ii) red colour on universal indicator pH paper; (iii) when treated with EtOH and cat. H₂SO₄ at reflux, produces a fruity smell. Identify the functional class.

Hint: Carboxylic acid. (i) confirms acidity stronger than carbonic (pKa < 6.4); (ii) confirms low pH; (iii) confirms a Fischer esterification with EtOH, characteristic of carboxylic acids. Phenol would fail test (i); a strong mineral acid would fail test (iii).

Practice 7 — Decarboxylation

Which of these decarboxylates easily on warming, and why? (a) acetic acid, (b) malonic acid (HOOC-CH₂-COOH), (c) 3-oxobutanoic acid (CH₃-CO-CH₂-COOH).

Hint: Both (b) and (c). Malonic acid loses one CO₂ at 150 °C (the remaining CH₃COOH is not a β-keto acid and is stable). 3-oxobutanoic acid loses CO₂ readily at 100 °C via a six-membered cyclic transition state in which the carbonyl oxygen accepts the H. Acetic acid (a) does NOT decarboxylate — no β-stabilisation.

Practice 8 — Mechanism

Outline the six-step mechanism of Fischer esterification (acetic acid + ethanol + cat. H⁺ → ethyl acetate + water).

Hint: (1) Protonation of C=O by H⁺ activates carbonyl. (2) Nucleophilic addition of EtOH to C+. (3) Proton transfer from EtO⁺ to neighbouring –OH. (4) Loss of water (now a good leaving group as H₂O). (5) Resulting oxocarbenium-like ion. (6) Deprotonation of OEt⁺ to give the ester. ALL six steps are reversible — to drive forward, remove water (Le Chatelier).

Practice 9 — Salicylic acid acidity

Salicylic acid (2-hydroxybenzoic acid, pKa 2.97) is much more acidic than 4-hydroxybenzoic acid (pKa 4.54), even though both have an OH group on the ring. Why?

Hint: The ortho –OH in salicylic acid is positioned to form an INTRAMOLECULAR H-bond with the carboxylate after deprotonation. This stabilises the COO⁻ form by ~6 kJ/mol, lowering the pKa by ~1.5 units. The para isomer cannot form this H-bond geometrically. Same total resonance/inductive effect; only the conformational accessibility differs.

Practice 10 — Saponification

Why is base-catalysed ester hydrolysis (saponification) effectively irreversible, while acid-catalysed ester hydrolysis (the reverse of Fischer esterification) is reversible?

Hint: Saponification produces the carboxylATE (R-COO⁻Na⁺), which has a delocalised negative charge and is too unreactive to be re-attacked by an alcohol — it cannot reverse. Acid hydrolysis produces the neutral carboxylic acid, which IS still electrophilic enough to be re-attacked, so the reaction is reversible. The asymmetric reactivity makes saponification preferred for taking esters apart.