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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

Required Practicals

1 Moles determination 2 Acid–base titration 3 Enthalpy determination 4 Qualitative analysis of ions 5 Synthesis of an organic liquid 6 Synthesis of an organic solid 7 Qualitative analysis of organic functional groups 8 Electrochemical cells 9 Rates of reaction – continuous monitoring method 10 Rates of reaction – initial rates method 11 pH measurement 12 Research skills

Core Practical 6: Synthesis of an Organic Solid

Aim: Prepare and purify aspirin via the reaction of salicylic acid with ethanoic anhydride. Key techniques: reflux, crystallisation, filtration under reduced pressure, drying, and melting point analysis.

Safety Notes

Apparatus and Chemicals

Apparatus

Chemicals

Method Overview

Reaction (Esterification)

  1. Mix salicylic acid and ethanoic anhydride in a conical flask.
  2. Add a few drops of phosphoric acid (catalyst).
  3. Heat gently in a water bath (~70 °C) for 15–20 minutes.

Crystallisation

  1. Pour the hot reaction mixture into cold water in a beaker to precipitate aspirin.
  2. Cool further in an ice bath to maximise crystal formation.

Filtration

  1. Collect the crude aspirin by filtration under reduced pressure using a Buchner funnel.

Recrystallisation

Recrystallisation setup for aspirin purification.
  1. Dissolve crude aspirin in a minimum of warm ethanol.
  2. Immediately filter the hot solution through fluted filter paper.
  3. Allow the filtrate to cool and crystallise.
  4. Filter the crystals under reduced pressure using a Buchner funnel.

Purity Check

  1. Measure the melting point of aspirin using a melting point apparatus and compare to literature (~135 °C).
  2. Run TLC with pure aspirin and salicylic acid as references and compare Rf values.

Thin Layer Chromatography (TLC) of Aspirin

Objective: Assess the purity of prepared aspirin by TLC, comparing it with pure aspirin and salicylic acid.

Apparatus and Chemicals

Procedure

  1. Prepare plate: Draw baseline 1 cm from bottom.
    Mark spots:
    • A (pure aspirin)
    • S (salicylic acid)
    • P (product)
  2. Spot samples with capillaries and dry and re-spot if needed.
TLC plate baseline and spotted samples: aspirin, salicylic acid, and product.
  1. Develop: Place plate in chamber with ~0.5 cm solvent. Ensure solvent level is below baseline. Cover and allow solvent to rise.
TLC plate in solvent chamber with solvent front rising.
  1. Visualise: Remove plate when solvent front is ~1 cm from top. Mark solvent front with pencil. View under UV or spray with locating agent.
Visualisation of TLC plate spots with UV lamp or locating agent.
  1. Calculate Rf: Rf = distance moved by spot ÷ distance moved by solvent front.
Rf value calculation diagram for TLC plate.

Interpreting Results

Example Calculation: % Yield

Note: Yield > 100% indicates impurities or incomplete drying.

Sources of Error