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ChemistryCarboxylic Acids
Arrange the following carboxylic acids in decreasing order of acidity:
I. Formic acid (HCOOH)
II. Acetic acid (CH₃COOH)
III. Isobutyric acid ((CH₃)₂CHCOOH)
IV. Trimethylacetic acid / Pivalic acid ((CH₃)₃CCOOH)
Options
1
I > II > III > IV
2
IV > III > II > I
3
II > I > IV > III
4
III > IV > I > II
Correct Answer
I > II > III > IV
Solution
1

+I effect of alkyl groups destabilises RCOO⁻ → decreases acidity (increases pKa).

Number of alkyl groups: HCOOH(0) < CH₃COOH(1) < (CH₃)₂CHCOOH(2) < (CH₃)₃CCOOH(3)

2

pKa: HCOOH=3.74 < CH₃COOH=4.74 < (CH₃)₂CHCOOH=4.84 < (CH₃)₃CCOOH=5.05

Lower pKa = stronger acid. Order of decreasing acidity:

$$\boxed{I > II > III > IV}$$
More alkyl groups (+I effect) → weaker acid
HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH
Theory: Carboxylic Acids
1. Acidity of Carboxylic Acids

Carboxylic acids ionise in water: $RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+$. $K_a = [RCOO^-][H_3O^+]/[RCOOH]$. pKa = $-\log K_a$. Lower pKa = stronger acid (larger Ka). Factors affecting acidity: (1) Electron-withdrawing groups (EWG) on R: -I effect stabilises RCOO⁻ → stronger acid → lower pKa. (2) Electron-donating groups (EDG/alkyl groups): +I effect destabilises RCOO⁻ → weaker acid → higher pKa. (3) Resonance: carboxylate anion RCOO⁻ is resonance-stabilised (negative charge delocalised over both O atoms) → carboxylic acids are much more acidic than alcohols (ROH, pKa ≈ 16-18 vs RCOOH pKa ≈ 3-5). (4) Distance: inductive effect decreases rapidly with distance from COOH. (5) Aromaticity: aromatic carboxylic acids (benzoic acid, pKa=4.20) are more acidic than aliphatic because aromatic ring has -I effect + partial resonance withdrawal of electrons.

2. Inductive Effect on Carboxylic Acid Acidity

Inductive effect: transmission of electronic effect through sigma bonds, decreases rapidly with distance. +I (electron-donating) groups: alkyl groups (increasing order: CH₃ < C₂H₅ < (CH₃)₂CH < (CH₃)₃C). Alkyl groups push electrons toward COOH → increased electron density on O of COO⁻ → anion less stable → weaker acid. -I (electron-withdrawing) groups: F, Cl, Br, I, NO₂, CN, CHO. Withdraw electrons from COOH → decreased electron density → anion more stable → stronger acid. Effect decreases with chain length and distance. Comparison: ClCH₂COOH (pKa 2.85) > Cl₂CHCOOH (pKa 1.25) > Cl₃CCOOH (pKa 0.65). Each Cl adds -I effect. ClCH₂COOH (pKa 2.85) > ClCH₂CH₂COOH (pKa 4.07) — Cl further away has less effect. FCH₂COOH (pKa 2.57) > ClCH₂COOH (pKa 2.85) > BrCH₂COOH (pKa 2.86) > ICH₂COOH (pKa 3.12) — F is most electronegative, strongest -I effect.

3. Resonance Stabilisation of Carboxylate Anion

The carboxylate anion RCOO⁻ is stabilised by resonance: negative charge delocalised equally between two identical C-O bonds. Both C-O bonds are equivalent in RCOO⁻ (bond length intermediate between C-O and C=O ≈ 1.26 Å vs C-O 1.43 Å and C=O 1.20 Å). This resonance stabilisation makes carboxylic acids (pKa ≈ 4-5) much more acidic than: alcohols (ROH, pKa ≈ 16-18, no resonance stabilisation of RO⁻), phenols (pKa ≈ 10, some resonance in phenoxide), carbonic acid (pKa₁ ≈ 6.4). The resonance energy of acetate ion: ~59 kJ/mol. This is why acetic acid is 10¹² times more acidic than ethanol! Carboxylate is the most stable common anion → carboxylic acids are the strongest common organic acids.

4. Effect of Substituents on Aromatic Carboxylic Acids

Benzoic acid: pKa = 4.20. Electron-withdrawing substituents on ring increase acidity (lower pKa): p-NO₂-C₆H₄-COOH: pKa = 3.44. m-NO₂: pKa = 3.49. o-NO₂: pKa = 2.17 (o-effect: steric + inductive). p-Cl: pKa = 3.98. m-Cl: pKa = 3.83. p-F: pKa = 4.14. Electron-donating substituents decrease acidity (raise pKa): p-CH₃: pKa = 4.37. p-OCH₃: pKa = 4.47. p-OH: pKa = 4.54. p-NH₂: pKa = 4.92 (strong +M effect). Ortho-effect: always increases acidity (regardless of nature of substituent!) due to steric inhibition of resonance (COOH and ortho substituent geometrically interact) and field/inductive effects. o-CH₃-C₆H₄-COOH: pKa = 3.91 (more acidic than benzoic acid despite +I of CH₃, due to ortho steric effect).

5. Reactions of Carboxylic Acids

Acidity: react with NaOH, Na₂CO₃, NaHCO₃ to give carboxylate salts (distinguishes carboxylic acids from phenols: phenol reacts with NaOH but NOT with NaHCO₃). RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂ (fizzing). Esterification (Fischer): RCOOH + R'OH ⇌ RCOOR' + H₂O (H⁺ catalyst, equilibrium). Mechanism: protonation of carbonyl O → nucleophilic attack by alcohol → tetrahedral intermediate → dehydration → ester. Acyl chloride formation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl. Acid anhydride: 2RCOOH → (RCO)₂O + H₂O (P₂O₅ or heat). Amide: RCOOH + NH₂R' → RCONHR' + H₂O (heat). Decarboxylation: RCOOH → RH + CO₂ (mild for β-keto acids; requires strong heat for simple acids; RCOOAg + Br₂ → RBr + CO₂ = Hunsdiecker reaction).

6. Dicarboxylic Acids

HOOCCOOH (oxalic acid, C2): pKa₁ = 1.25, pKa₂ = 4.27. Strongest dicarboxylic acid (two COOH groups in proximity, mutual -I effect). HOOC-CH₂-COOH (malonic acid, C3): pKa₁ = 2.83, pKa₂ = 5.70. Active methylene between two COOHs (used in malonic ester synthesis). HOOC(CH₂)₂COOH (succinic acid, C4): pKa₁ = 4.21, pKa₂ = 5.64. Fumaric acid (trans, C4): pKa₁ = 3.03, pKa₂ = 4.44. Maleic acid (cis, C4): pKa₁ = 1.83, pKa₂ = 6.59. Maleic acid more acidic (strong intramolecular H-bond after first ionisation stabilises maleate monoanion; steric destabilisation of COOH). Phthalic (ortho): similar to maleic. Glutaric (C5): pKa₁ = 4.34. Adipic (C6): pKa₁ = 4.44 (used in nylon synthesis). As chain length increases: effect of one COOH on other decreases → both pKa₁ values approach that of corresponding monocarboxylic acid.

7. Unsaturated Carboxylic Acids

Acrylic acid (CH₂=CHCOOH): pKa = 4.25. More acidic than propionic acid (pKa 4.87) due to -I effect of vinyl group (sp² C is more electronegative than sp³ C). Crotonic acid (CH₃CH=CHCOOH): pKa = 4.69 (closer to saturated analog). Benzoic acid: pKa = 4.20. Phenylacetic acid (C₆H₅CH₂COOH): pKa = 4.31 (aryl group less directly connected). α,β-Unsaturated acids: vinyl group withdraws electrons from COOH. In general: sp C > sp² C > sp³ C in electronegativity. More s-character → more electronegative → better -I effect → more acidic. Propiolic acid (HC≡CCOOH): pKa = 1.84 (very strong for aliphatic acid — sp carbon adjacent to COOH).

8. Industrial Applications of Carboxylic Acids

Acetic acid: 75% used to make vinyl acetate (VAM → polyvinyl acetate → adhesives, paints). Also: acetic anhydride, cellulose acetate (photographic film, textiles), aspirin synthesis. Production: carbonylation of methanol (Monsanto/Cativa process using Rh or Ir catalysts). Formic acid: leather tanning, preservation, ant venom. Citric acid: food additive (sour taste in soft drinks, E330), chelating agent. Tartaric acid: wine, baking powder. Lactic acid: fermented foods, biodegradable polylactic acid (PLA) plastics. Oxalic acid: bleaching, cleaning (removes rust). Adipic acid + hexamethylenediamine → Nylon-6,6. Terephthalic acid + ethylene glycol → PET (polyethylene terephthalate, plastic bottles, polyester clothing). Propionic acid: food preservative (bread). Butyric acid: rancid butter smell.

Frequently Asked Questions
1. Why is HCOOH stronger acid than CH3COOH despite both being carboxylic acids? ⌄
Formic acid (HCOOH) has hydrogen directly attached to the carboxyl carbon. H has a slight -I effect compared to alkyl groups (H is less electron-donating than alkyl). So HCOO⁻ is slightly more stable than CH₃COO⁻. Additionally: the absence of any +I effect means there is no electron density increase on the carboxylate oxygen. Acetic acid (CH₃COOH) has a methyl group: +I effect of CH₃ pushes electrons toward the carboxylate → increases electron density on O in CH₃COO⁻ → anion less stable → higher pKa → weaker acid. pKa difference: HCOOH 3.74, CH₃COOH 4.74. Ratio of Ka values: $K_a(HCOOH)/K_a(CH_3COOH) = 10^{4.74-3.74} = 10^1 = 10$. HCOOH is exactly 10 times more acidic.
2. How do you predict acid strength from structure? ⌄
Step 1: Identify groups attached to COOH. Electron-withdrawing groups (EWG: halides -I, nitro -I-R, carbonyl -I-R, aryl -I): increase acidity (lower pKa). Electron-donating groups (EDG: alkyl +I): decrease acidity (higher pKa). Step 2: Count EWG/EDG: more EWG = stronger acid; more EDG = weaker acid. Step 3: Consider distance: substituents closer to COOH have stronger effect (1 bond away >> 2 bonds >> 3 bonds). Step 4: For aromatic acids, consider resonance: p-NO₂ (both -I and -R) > m-NO₂ (-I only, no -R at meta) in increasing acidity. Step 5: Hybridisation: sp³ C (alkyl) +I < sp² C (vinyl, aryl) -I < sp C (alkynyl) -I. More s-character = more electronegative.
3. Why do electron-withdrawing groups on alpha carbon increase acidity more than on beta? ⌄
Inductive effect (-I or +I) is transmitted through sigma bonds and diminishes with each bond. The effect on acidity $= \sigma_I \times f^n$ where $f \approx 0.3$-$0.5$ is the attenuation factor per bond and $n$ = number of bonds between substituent and COOH. For Cl on α-carbon (ClCH₂COOH, $n=1$): pKa = 2.85. For Cl on β-carbon (ClCH₂CH₂COOH, $n=2$): pKa = 4.07. For Cl on γ-carbon: pKa ≈ 4.50 (barely different from butyric acid 4.82). Each additional bond approximately halves the inductive effect. This exponential distance-dependence is why only α-substitution has strong effects in simple aliphatic carboxylic acids.
4. Compare acidity of carboxylic acid, phenol, alcohol, and water. ⌄
All are weak acids in water but with very different pKa values: Carboxylic acid (CH₃COOH): pKa ≈ 4.74. Most acidic — resonance stabilisation of carboxylate. Phenol (C₆H₅OH): pKa ≈ 10.0. Moderately acidic — resonance stabilisation of phenoxide (negative charge delocalised into ring). Water (H₂O): pKa = 15.7. Less acidic — OH⁻ has no resonance stabilisation. Alcohol (C₂H₅OH): pKa ≈ 16. Least acidic — ethoxide has no resonance stabilisation. Order: RCOOH >> ArOH >> H₂O > ROH. Practical test: all react with Na metal (H₂ evolved). Only RCOOH and ArOH react with NaOH. Only RCOOH reacts with NaHCO₃ (fizzing = CO₂). This test sequence distinguishes carboxylic acid (reacts with NaHCO₃) from phenol (does not).
5. What is the malonic ester synthesis and how does it use carboxylic acid chemistry? ⌄
Malonic ester (diethyl malonate, EtOOCCH₂COOEt) has an active methylene group (pKa ≈ 13) — quite acidic for a C-H bond because the carbanion is stabilised by resonance with TWO ester groups. Synthesis: diethyl malonate + NaOEt → sodium malonate anion (nucleophile) + RX (SN2 alkylation) → monoalkylated malonate. Hydrolyse with NaOH/H⁺ then heat (decarboxylation of β-keto acid intermediate) → monoalkyl acetic acid RCH₂COOH. Can do twice to get dialkyl: RR'CHCOOH. This synthesis extends carboxylic acid chains by 1 carbon (compared to starting alkyl halide) with excellent yield and few side products. Acetoacetic ester synthesis is similar, gives substituted acetic acid or methyl ketone. Both are classic named reactions used to demonstrate active methylene chemistry.
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