Carboxylate Salts and Acid-Base Reactions

Introduction

Carboxylic acids are weak acids, but they are strong enough to react with every common laboratory base, including the mild hydrogencarbonate ion that phenols and alcohols leave untouched. The product of each of these reactions is a carboxylate salt , an ionic compound that behaves very differently from the parent acid. Sodium ethanoate, sodium benzoate and soap are all carboxylate salts. Knowing how to write these reactions, name the salts and predict which form dominates in water is basic equipment for the rest of carbonyl chemistry.

Core explanation

Reaction with metal hydroxides. A carboxylic acid transfers its O–H proton to hydroxide, giving a carboxylate salt and water:

CH₃COOH + NaOH → CH₃COONa + H₂O

The equilibrium lies almost completely to the right because water (pKa about 15.7) is a far weaker acid than ethanoic acid (pKa 4.76). The ionic equation is simply RCOOH + OH⁻ → RCOO⁻ + H₂O.

Reaction with carbonates. Carbonates accept two protons in turn, releasing carbon dioxide:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Reaction with hydrogencarbonates. Carbonic acid has a first pKa of about 6.4, so a carboxylic acid (pKa about 4–5) is strong enough to protonate hydrogencarbonate:

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

The carbonic acid formed decomposes to water and carbon dioxide, which escapes as bubbles. This removal of a product drives the reaction to completion. Phenol (pKa about 10) is too weak to do this, and alcohols (pKa about 16) are weaker still, so fizzing with sodium hydrogencarbonate is a classic test for the –COOH group.

Reaction with reactive metals. Like other acids, carboxylic acids react with metals such as magnesium to give a salt and hydrogen: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. The reaction is slower than with hydrochloric acid of the same concentration because the concentration of H⁺ is much lower.

Naming salts. Replace the "-oic acid" ending with "-oate" and put the cation first: sodium ethanoate, potassium propanoate, calcium ethanoate, ammonium benzoate.

Properties of the salts. Carboxylate salts are ionic solids with high melting points. Salts of sodium, potassium and ammonium are usually very soluble in water because the charged carboxylate group is strongly hydrated by ion–dipole attractions. A salt of a long-chain acid, such as sodium stearate, has an ionic head and a long nonpolar tail, which is why it acts as a soap.

Which form exists in water? The ratio of carboxylate to acid depends on pH and pKa, as described by the Henderson–Hasselbalch relationship. When pH = pKa the two forms are present in equal amounts. Each unit of pH above the pKa increases the ratio [RCOO⁻]/[RCOOH] tenfold. At the pH of blood (about 7.4), ethanoic acid is more than 99% ionised, which is why biochemists speak of "acetate", "lactate" and "pyruvate" rather than the acids.

Formulae

pH = pKa + log₁₀([RCOO⁻]/[RCOOH]). Fraction ionised = 1 ÷ (1 + 10^(pKa − pH)).

Step-by-step reasoning

To predict and write an acid–base reaction of a carboxylic acid:

1. Identify the acidic proton: the H of the –COOH group, not a C–H. 2. Identify the base and its conjugate acid (OH⁻ → H₂O, HCO₃⁻ → H₂CO₃ → H₂O + CO₂). 3. Compare pKa values: the reaction favours formation of the weaker acid. 4. Write the salt by pairing the carboxylate with the cation, then balance charges and atoms. 5. Name the salt using the "-oate" ending.

Visual explanation

Picture a titration flask of ethanoic acid with a pH meter. As sodium hydroxide is added, the pH climbs slowly through a buffer region centred on pH 4.76, where acid and ethanoate are balanced, then rises steeply near the equivalence point, which lies above 7 because ethanoate is a weak base.

Real-world analogy

A carboxylic acid is like a person holding a ball they are happy to pass on. Offer a strong grabber (hydroxide) or even a moderately keen one (hydrogencarbonate) and the ball changes hands. Phenol holds its ball more tightly, so the moderate grabber leaves empty-handed.

Real-world example

Sodium benzoate (E211) and potassium sorbate are carboxylate salts added to soft drinks and sauces as preservatives. They dissolve easily in water; in acidic drinks some of the benzoate is converted back to benzoic acid, the form that is most active against yeasts and moulds.

Why?

Why does ethanoic acid fizz with sodium hydrogencarbonate while phenol does not? Proton transfer favours the weaker acid. Ethanoic acid (pKa 4.76) is stronger than carbonic acid (pKa about 6.4), so the proton moves to hydrogencarbonate; phenol (pKa about 10) is weaker than carbonic acid, so the equilibrium lies on the side of phenol.

Common misconception

"Neutralising a carboxylic acid with sodium hydroxide gives a neutral solution." At the equivalence point the solution contains ethanoate ions, which are weak bases, so the pH is around 8–9, not 7.

Worked example

Question: Write the equation for propanoic acid reacting with calcium carbonate and name the salt.

Reasoning: Each carbonate removes two protons, so two acid molecules are needed. Calcium is 2+, so two propanoate ions balance one Ca²⁺.

Answer: 2CH₃CH₂COOH + CaCO₃ → (CH₃CH₂COO)₂Ca + H₂O + CO₂; the salt is calcium propanoate.

Quick check

1. What gas is released when benzoic acid is added to sodium hydrogencarbonate solution, and what salt forms? Answer: Carbon dioxide is released, and the salt formed is sodium benzoate.

Exam focus

Balance carbonate equations carefully (two acid molecules per CO₃²⁻) and use brackets for salts of 2+ ions. Be ready to use the hydrogencarbonate test to distinguish a carboxylic acid from a phenol or an alcohol, and to explain it with pKa values.

Advanced insight

The resonance-delocalised carboxylate is a poor nucleophile at carbon and a weak base, so it does not undergo the nucleophilic acyl substitutions that acids and their derivatives do; the negative charge actually repels nucleophiles. This is why a carboxylic acid must usually be converted into a more reactive derivative before forming amides efficiently.

Summary

Carboxylic acids react with hydroxides to give salts and water, with carbonates and hydrogencarbonates to give salts, water and carbon dioxide, and with reactive metals to give salts and hydrogen. Salts are named with the "-oate" ending and are ionic, high-melting and usually water-soluble. The form present in water depends on pH relative to pKa.

Practice questions

1. Write the ionic equation for methanoic acid reacting with hydroxide ions. Answer: HCOOH + OH⁻ → HCOO⁻ + H₂O. 2. Name the salt CH₃CH₂CH₂COOK. Answer: Potassium butanoate. 3. Suggest a simple test to distinguish ethanoic acid from phenol, giving the observation. Answer: Add sodium hydrogencarbonate solution; ethanoic acid gives effervescence of carbon dioxide, while phenol gives no bubbles. 4. What percentage of ethanoic acid (pKa 4.76) is ionised at pH 5.76? Answer: The ratio of ethanoate to acid is 10 : 1, so about 91% is ionised. 5. Why is the pH at the equivalence point of an ethanoic acid–sodium hydroxide titration above 7? Answer: The solution contains ethanoate ions, which are weak bases that react slightly with water to form hydroxide ions.