📖 In-Depth Theory
Structure of Carboxylic Acids
CARBOXYLIC ACIDS contain the functional group –COOH (carboxyl group).
First four members:
Methanoic acid: HCOOH
Ethanoic acid: CH₃COOH (the acid in vinegar)
Propanoic acid: C₃H₅COOH
Butanoic acid: C₄H₇COOH
NAMING:
Named from the corresponding alkane — replace '-e' with '-anoic acid'.
meth- → methanoic acid; eth- → ethanoic acid; prop- → propanoic acid.
PROPERTIES:
Acidic — donate H⁺ ions in solution.
Lower members dissolve in water — miscible.
Higher members less soluble.
Ethanoic acid (CH₃COOH) is the main component of vinegar.
Reactions of Carboxylic Acids
1. REACTION WITH CARBONATES:
Carboxylic acid + metal carbonate → salt + water + carbon dioxide
CH₃COOH + Na₂CO₃ → CH₃COONa + H₂O + CO₂
Ethanoic acid + sodium carbonate → sodium ethanoate + water + CO₂
Same pattern as mineral acids reacting with carbonates.
CO₂ gas produced — effervescence (bubbling).
2. REACTION WITH METALS:
Carboxylic acid + metal → salt + hydrogen gas
More dilute/weaker reaction than mineral acids.
3. DISSOLVING IN WATER:
Forms weakly acidic solution — pH 3–5 (not as low as strong acids at the same concentration).
Due to weak dissociation.
4. REACTION WITH ALCOHOLS — ESTERIFICATION:
Carboxylic acid + alcohol → ester + water
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Ethanoic acid + ethanol ⇌ ethyl ethanoate + water
Conditions: acid catalyst (H₂SO₄), warm, reversible reaction.
Esters have characteristic fruity smells — used in flavourings and perfumes.
Uses and Natural Sources
ETHANOIC ACID (CH₃COOH):
Main component of vinegar (typically 4–8% solution).
Preservative — inhibits bacterial growth.
Produced by oxidation of ethanol (e.g. wine left open).
METHANOIC ACID (HCOOH):
Found in ant stings (causes the burning sensation).
Used as a preservative in silage.
ESTERS:
Esters are formed from carboxylic acids + alcohols.
Fruity smells: ethyl ethanoate (nail polish remover), amyl acetate (banana), isoamyl acetate.
Used in flavourings, perfumes, solvents, plasticisers.
FATTY ACIDS:
Long-chain carboxylic acids (e.g. stearic acid, oleic acid).
Components of fats and oils — reacted with glycerol to form triglycerides.
CARBOXYLIC ACIDS AND pH:
Weaker than hydrochloric or sulfuric acid at the same concentration.
pH around 3–5 for typical dilute solutions.
Incompletely dissociate in water — equilibrium mixture of HA ⇌ H⁺ + A⁻.
⚠️ Common Mistake
Carboxylic acids are WEAK acids — they do not fully dissociate in water unlike HCl or H₂SO₄. At the same concentration, carboxylic acid has a HIGHER pH than a strong acid. Esterification is REVERSIBLE — both ester + water and acid + alcohol are present at equilibrium.
🎯 Test Yourself
Question 1 of 2
1. What type of compound forms when ethanoic acid reacts with ethanol, and what are the conditions?
An ester (ethyl ethanoate) — acid catalyst (H₂SO₄), warm conditions, reversible reaction
No reaction — carboxylic acids and alcohols don't react
An alkane — acid + alcohol always produces an alkane and water
An aldehyde — oxidation of the alcohol by the acid produces an aldehyde
2. Why does ethanoic acid have a higher pH than hydrochloric acid at the same concentration?
Ethanoic acid is a weak acid — it only partially dissociates in water, producing fewer H⁺ ions than HCl at the same concentration
Ethanoic acid has a larger molecule — bigger molecules are always less acidic
Hydrochloric acid contains chlorine — chlorine makes it more acidic than carbon compounds
Ethanoic acid reacts with water to form a base — neutralising some of its own acidity