What you'll learn

• How to plan organic syntheses as sequences of reactions, not isolated facts.
• How to choose practical techniques for separation, purification and purity testing.
• How addition polymers, polyesters and polyamides are formed.
• How to combine chemical tests, high-resolution ${}^{1}H$ NMR and chromatography to identify unknowns.

1. Organic synthesis: building a route

Organic synthesis is the preparation of an organic compound from simpler starting materials. At A level, the key skill is recognising which functional group you have and which one you need.

Useful reaction choices include:

• Haloalkane → alcohol: warm with aqueous OH⁻, usually under reflux.
• Haloalkane → nitrile: warm with ethanolic CN⁻; this extends the carbon chain by one carbon.
• Primary alcohol → aldehyde: acidified dichromate(VI), distil off the aldehyde.
• Primary alcohol → carboxylic acid: acidified dichromate(VI), reflux.
• Carboxylic acid + alcohol → ester: concentrated H₂SO₄ catalyst, reflux.
• Carboxylic acid → acyl chloride: SOCl₂.
• Acyl chloride + amine → amide: room temperature, often producing HCl.

Curly-arrow thinking in synthesis

For example, in the reaction of bromoethane with cyanide ions:

CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻

The mechanism is nucleophilic substitution. The cyanide ion attacks using the lone pair on the carbon atom of CN⁻. A curly arrow goes from that lone pair to the carbon bonded to bromine. At the same time, a curly arrow goes from the C–Br bond to Br, forming Br⁻.

2. Manipulation, separation and purification

Organic products are rarely obtained pure straight from the reaction mixture. You must be able to explain how to handle them.

Key practical techniques

• Reflux heats a reaction mixture without losing volatile reactants or products; vapour condenses and returns to the flask.
• Distillation separates liquids using different boiling temperatures.
• A separating funnel separates immiscible liquid layers, often an organic layer and an aqueous layer.
• A drying agent, such as anhydrous MgSO₄, removes traces of water from an organic liquid.
• Recrystallisation purifies a solid by dissolving it in minimum hot solvent, then allowing pure crystals to form on cooling.
• Melting temperature helps assess the purity of a solid.

For a liquid organic product, a typical method is: reflux the reaction mixture, transfer it to a separating funnel, wash with suitable aqueous solutions, run off the layers carefully, dry the organic layer, then distil the product.

For a solid organic product, a typical method is: collect the crude solid by filtration, recrystallise from a suitable solvent, filter the crystals under reduced pressure, wash with cold solvent, dry, then measure the melting temperature.

Melting temperature and purity

A pure solid has a sharp melting range, usually within about 1–2 °C, close to the data-book value. Impurities usually make the melting range lower and broader because they disrupt the regular crystal lattice.

3. Addition and condensation polymerisation

In addition polymerisation, monomers with C=C bonds join without losing any atoms. The double bond opens and forms a saturated polymer chain.

In condensation polymerisation, monomers join together while eliminating a small molecule, usually water or HCl.

Polyesters

A polyester contains ester links, –COO–. It can form from:

• a diol and a dicarboxylic acid, eliminating water
• a diol and a diacyl chloride, eliminating HCl

General idea:

HO–R–OH + HOOC–R′–COOH → polyester + water

Polyamides

A polyamide contains amide links, –CONH–. It can form from:

• a diamine and a dicarboxylic acid, eliminating water
• a diamine and a diacyl chloride, eliminating HCl

Nylon is an important example. Condensation polymers are industrially useful in fibres, packaging and engineering materials, but their persistence in the environment raises recycling and disposal issues.

4. Identifying unknown organic compounds

When identifying an unknown, do not rely on one test alone. Build evidence from chemical tests and spectra.

Planning chemical tests

A sensible test sequence uses small samples and separates possibilities logically:

• Bromine water decolourises with many alkenes.
• Sodium carbonate gives effervescence with carboxylic acids due to CO₂.
• 2,4-dinitrophenylhydrazine gives an orange precipitate with aldehydes and ketones.
• Tollens’ reagent or Fehling’s solution distinguishes aldehydes from ketones.
• Iodoform testing can identify CH₃CO– groups or CH₃CH(OH)– groups.
• Iron(III) chloride can give a coloured complex with phenols.

High-resolution ${}^{1}H$ NMR

High-resolution proton NMR gives four main types of information:

• Chemical shift, measured in ppm, suggests the proton environment.
• Integration gives the relative number of protons in each environment.
• Splitting shows neighbouring equivalent protons, using the $n+1$ rule.
• Number of peaks shows the number of different proton environments.

You should combine this with other data, such as IR absorptions, mass spectrometry molecular ion peaks, and carbon-13 NMR environments.

5. Chromatography and mixture composition

In paper chromatography and TLC, the stationary phase is the paper or coated plate, and the mobile phase is the solvent. The $R_f$ value is:

$$R_f=\frac{distance\; moved\; by\; spot}{distance\; moved\; by\; solvent\; front}$$

In two-way paper chromatography, the chromatogram is run in one solvent, dried, rotated by 90°, then run in a second solvent. This improves separation when spots overlap.

In GC and HPLC, each component has a retention time. Comparing retention times with standards helps identify compounds. Peak area is proportional to amount, so composition can be found using peak areas or calibration data.