Acidity

When a carboxylic acid dissolves in water, the O–H bond of the carboxyl group breaks heterolytically: the proton (H⁺) is donated to water, forming H₃O⁺, and the remaining carboxylate anion (RCOO⁻) retains both electrons. The equilibrium is described by Ka = [RCOO⁻][H₃O⁺] / [RCOOH]. Because Ka is small (typically 10⁻⁴ to 10⁻⁵ for simple acids), only a small fraction of molecules are ionized at any moment, yet this is still far greater than for alcohols or water.

The key to carboxylic acid acidity is what happens after the proton leaves. In the carboxylate anion (RCOO⁻), the negative charge is not localized on one oxygen. Instead, the lone pair of the negatively charged oxygen overlaps with the adjacent π* orbital of the C=O bond, creating a fully delocalized π system across O–C–O. This produces two equivalent resonance structures: [R–C(=O)–O⁻ ↔ R–C(–O⁻)=O]. The real anion is a hybrid — both C–O bonds are equivalent (bond length ~127 pm, intermediate between a C–O single bond at 143 pm and a C=O double bond at 122 pm), and the negative charge is spread equally over both oxygens. This delocalization lowers the free energy of the conjugate base, shifting the equilibrium toward ionization and increasing Ka.

When an alcohol (ROH) loses a proton, the resulting alkoxide anion (RO⁻) carries the full negative charge on a single oxygen with no resonance stabilization. The conjugate base is much higher in energy relative to the parent compound, so the equilibrium strongly favours the neutral molecule (pKa ~16–18). The dramatic difference in acidity between a carboxylic acid (pKa ~5) and an alcohol (pKa ~16) is almost entirely attributable to this resonance stabilization of the carboxylate.

An inductive effect operates through σ bonds: electronegative substituents polarize the electron density along the carbon chain toward themselves. If an electron-withdrawing group (EWG) such as –Cl, –F, –CN, or –NO₂ is attached near the carboxyl group, it draws electron density away from the carboxylate, further dispersing the negative charge and stabilizing the conjugate base — thereby increasing acidity (lowering pKa). Classic evidence: acetic acid has pKa = 4.76; chloroacetic acid (ClCH₂COOH) has pKa = 2.86; dichloroacetic acid (Cl₂CHCOOH) has pKa = 1.48; trichloroacetic acid (Cl₃CCOOH) has pKa = 0.66. Each successive chlorine substituent provides additional electron withdrawal and stabilizes the anion further.

Alkyl groups are mildly electron-donating (hyperconjugation and induction). They push electron density toward the carboxylate, slightly destabilizing the negative charge and making the acid slightly weaker. Formic acid (HCOOH, pKa = 3.75) is more acidic than acetic acid (CH₃COOH, pKa = 4.76) because the methyl group donates electron density to the carboxylate, raising the energy of the conjugate base relative to the neutral acid.

Inductive effects decay rapidly with distance because each additional σ bond attenuates the electronic polarization. For example: 4-chlorobutanoic acid (Cl at the γ carbon) has pKa ≈ 4.52, while 2-chlorobutanoic acid (Cl at the α carbon) has pKa ≈ 2.86. The same substituent causes a much smaller pKa change when it is three bonds away from the carboxyl group than when it is directly adjacent. By the time the substituent is four or more carbons away, its inductive effect is negligible.

• K_a — Acid dissociation constant (mol dm⁻³): Equilibrium constant for the ionization of the acid in water; larger Ka means stronger acid.
• [\text{RCOO}^-] — Carboxylate anion concentration (mol dm⁻³): Molar concentration of the conjugate base at equilibrium.
• [\text{H}_3\text{O}^+] — Hydronium ion concentration (mol dm⁻³): Molar concentration of hydronium ions produced by ionization.
• [\text{RCOOH}] — Undissociated acid concentration (mol dm⁻³): Molar concentration of the carboxylic acid that has not ionized.
• \text{p}K_a — pKa (dimensionless): Negative base-10 logarithm of Ka; lower values correspond to stronger acids.

Problem. Rank the following acids in order of increasing acidity (lowest Ka to highest Ka) and explain your reasoning: (A) propanoic acid CH₃CH₂COOH, (B) 2-fluoroproanoic acid FCH₂CH₂COOH wait — 2-fluoropropanoic acid FCH(CH₃)COOH, (C) 2,2-difluoropropanoic acid F₂C(CH₃)COOH, (D) 3-fluoropropanoic acid FCH₂CH₂COOH. Their pKa values are approximately: propanoic 4.87, 2-fluoro 2.66, 2,2-difluoro 1.48, 3-fluoro 4.11.

Answer. Order of increasing acidity: propanoic acid < 3-fluoropropanoic acid < 2-fluoropropanoic acid < 2,2-difluoropropanoic acid. Each step up in acidity is explained by greater electron withdrawal from the carboxylate anion — either by moving the substituent closer to the carboxyl group or by adding more electron-withdrawing fluorine atoms.

• Carboxylic Acids: Structure and Naming — Understanding the –COOH functional group geometry and IUPAC nomenclature provides the structural foundation for interpreting why resonance and induction affect acidity.
• Resonance and Electron Delocalization — The stabilization of the carboxylate anion is a direct application of resonance theory; mastering resonance structures is essential to understand why carboxylic acids are acidic.
• Acid Dissociation Constants: Ka and pKa — Ka and pKa are the quantitative tools used to express and compare the acidities discussed in this article.
• Reactions of Carboxylic Acids — The acidity of carboxylic acids directly controls their reactivity in esterification, amide formation, and decarboxylation reactions.
• Inductive Effect in Organic Chemistry — The inductive effect of substituents is one of the two main explanations for variation in carboxylic acid pKa values explored in this article.