🧪

Carboxylic Acids and Derivatives

Sep 9, 2025

View transcript

Overview

This lecture covers carboxylic acids and their derivatives, focusing on their structures, naming, reactions (including esterification and hydrolysis), uses, and essential purification techniques for AQA A-level Chemistry.

Carboxylic Acids: Structure and Naming

  • Carboxylic acids contain the carboxyl group (-COOH) with both a carbonyl (C=O) and hydroxyl (OH) group.
  • The carboxyl carbon is always carbon 1 and located at the end of the molecule.
  • Name by finding the longest carbon chain and ending with ‘-oic acid’; e.g., ethanoic acid.
  • Aromatic carboxylic acids such as benzoic acid exist.

Reactions and Properties of Carboxylic Acids

  • Carboxylic acids are weak acids; they partially dissociate in water.
  • React with carbonates or hydrogen carbonates to form salt, water, and carbon dioxide.

Esters: Formation, Naming, and Uses

  • Esters are formed by reacting carboxylic acids (or acid anhydrides) with alcohols using a sulfuric acid catalyst (esterification).
  • Name esters: first part from alcohol (alkyl), second part from acid (carboxylate); e.g., ethyl propanoate.
  • Used in perfumes, food flavorings, solvents, glues, and plasticizers.

Ester Hydrolysis

  • Acid hydrolysis: esters react with water and acid catalyst to form carboxylic acid and alcohol.
  • Base hydrolysis (saponification): esters react with NaOH to form carboxylate salt and alcohol.

Fats, Oils, Soaps, and Biodiesel

  • Fats and oils are esters of glycerol and fatty acids.
  • Saturated fatty acids (no C=C bonds): found in animal fats, solid at room temp, higher melting points.
  • Unsaturated fatty acids (C=C bonds): found in oils, liquid at room temp, lower melting points.
  • Saponification: hydrolysis of fats/oils with NaOH produces soap (a carboxylate salt) and glycerol.
  • Biodiesel: made by transesterification of vegetable oils with methanol using KOH catalyst, producing methyl esters.

Carboxylic Acid Derivatives: Acid Chlorides and Anhydrides

  • Acid chlorides (acyl chlorides): -COCl group, named by replacing ‘-oic acid’ with ‘-oyl chloride’.
  • Acid anhydrides: formed from two carboxylic acids, named by replacing ‘acid’ with ‘anhydride’.

Reactions of Acid Chlorides and Anhydrides

  • Both react with water → carboxylic acid.
  • With ammonia → amide.
  • With alcohol → ester.
  • With primary amine → N-substituted amide.
  • Acid chlorides release HCl gas (very vigorous); anhydrides produce carboxylic acids (safer).

Nucleophilic Addition-Elimination (Mechanism)

  • Nucleophiles (e.g., alcohol, water, ammonia, amines) attack the delta+ carbon of acyl chlorides/anhydrides.
  • Mechanism involves nucleophilic attack, addition intermediate, elimination of chloride (or carboxylate).

Industrial Use: Synthesis of Aspirin

  • Aspirin is made by reacting salicylic acid with ethanoic anhydride (preferable to acid chloride for safety and cost).

Purification and Practical Techniques

  • Reflux: Heats volatile or flammable liquids safely without loss.
  • Distillation: Separates compounds by boiling point; used to isolate products.
  • Redistillation/Separation: Further purifies products and removes water or impurities by separating layers.
  • Drying agents: Remove residual water (e.g., CaCl₂).
  • Vacuum Filtration: Separates solids from liquids using reduced pressure.
  • Recrystallization: Purifies solid products using minimal hot solvent, followed by cooling and filtration.
  • Testing Purity:
    • Liquids: Measure boiling point; pure substances boil sharply at known values.
    • Solids: Use melting point apparatus; purity indicated by sharp melting range.

Key Terms & Definitions

  • Carboxyl Group — Functional group: -COOH.
  • Esterification — Formation of an ester from a carboxylic acid and alcohol.
  • Saponification — Base hydrolysis of fats to produce soap and glycerol.
  • Acid Chloride (Acyl Chloride) — Derivative of carboxylic acid with -COCl group.
  • Acid Anhydride — Compound from two carboxylic acids losing water (-CO-O-CO-).
  • Amide — Compound with -CONH₂ group.
  • Nucleophile — Electron pair donor in chemical reactions.

Action Items / Next Steps

  • Review mechanisms for esterification, hydrolysis, and nucleophilic addition–elimination.
  • Practice naming and drawing structures of carboxylic acids, esters, and derivatives.
  • Complete assigned homework or past paper questions on ester and derivative reactions.
  • Prepare for practical lab on purification techniques.

Full transcript