Organic Synthesis Techniques

Knowing reaction equations is only half of organic chemistry — the other half is actually making the molecule in the lab. OCR A-Level asks you to understand, describe and select between the main practical techniques used in synthesis: reflux, distillation, separating funnel extraction, drying, recrystallisation, and filtration under reduced pressure. These are also central to the required practical activities (PAG 3 — organic synthesis and purification) and frequently appear in exam questions, sometimes worth 6 or more marks.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 4.2.3: organic synthesis techniques including reflux, distillation, solvent extraction, drying, recrystallisation and filtration under reduced pressure; safety considerations and yield.

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1. Heating Under Reflux

Reflux means heating a reaction mixture at its boiling point in an apparatus that condenses the vapour back into the flask, so that nothing is lost.

1.1 Why Reflux?

Many organic reactions are slow at room temperature (rates double with every 10 °C rise). Heating speeds them up, but at elevated temperatures the organic solvents and reagents are often volatile — they would evaporate and escape. Reflux solves this: you heat the mixture to boiling, but the condenser above the flask continuously condenses vapour back into the liquid. You can hold a reaction at its boiling point for hours with no loss of material.

1.2 Apparatus

graph TD
  A[Heat source<br/>water bath or electric mantle] --> B[Round-bottomed flask with reagents + anti-bumping granules]
  B --> C[Vertical Liebig condenser]
  C --> D[Water in at bottom]
  C --> E[Water out at top]
  C --> F[Open at top - NEVER sealed]

Key features:

• Round-bottomed flask — strong, distributes heat evenly.
• Anti-bumping granules — small porous chips that provide nucleation sites for bubbles, preventing violent bumping.
• Vertical Liebig condenser — water flows counter-current (in at the bottom, out at the top) for maximum efficiency.
• Open top — pressure cannot build up. A closed reflux apparatus would explode.
• Heat source — a water bath for flammable organics, an electric mantle for higher-boiling solvents. Never use a naked Bunsen flame with flammable volatile liquids.

1.3 Safety

• Flammable solvents require a water bath or electric mantle.
• The apparatus must be properly clamped and ventilated.
• Eye protection and gloves always.
• Do not seal the top — allow pressure release.

1.4 Examples of Reflux at A-Level

• Oxidation of a primary alcohol to a carboxylic acid (K₂Cr₂O₇ / H₂SO₄).
• Hydrolysis of an ester to give a carboxylic acid and alcohol.
• Nucleophilic substitution of haloalkanes (KCN in ethanol, NH₃ in ethanol).

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2. Distillation

Distillation separates liquids by boiling point. Vapour from the heated liquid travels through a condenser, condenses to a liquid, and is collected — but crucially, the condenser is tilted or horizontal, so the condensed liquid flows away from the reaction flask rather than back into it.

2.1 Why Distil?

Distillation is used:

• To stop an oxidation at the aldehyde stage (Lesson 2) — the aldehyde boils out of the flask before it can be oxidised further.
• To isolate a volatile product from a reaction mixture.
• To purify a liquid by separating it from non-volatile impurities or from higher-boiling co-products.

2.2 Apparatus

graph LR
  A[Pear-shaped flask with reagents + granules] --> B[Still head with thermometer]
  B --> C[Condenser tilted downwards]
  C --> D[Collection flask / receiver]
  E[Water in] --> C
  C --> F[Water out]

• Pear-shaped or round-bottomed flask with the reactants.
• Still head with a thermometer positioned at the top where the vapour exits — this gives the boiling point of the distillate.
• Condenser tilted downwards (Liebig, water counter-current).
• Collection flask / receiver at the lower end of the condenser.

2.3 Fractional Distillation

When two or more liquids with close boiling points need to be separated, you use a fractionating column packed with glass beads or rings between the flask and the condenser. This gives repeated cycles of vaporisation and condensation — each cycle enriches the vapour in the more volatile component. The result is a much cleaner separation.

Fractional distillation is essential when the boiling points of two components differ by less than ~25 °C, e.g. separating ethanol (78 °C) from water (100 °C) during fermentation.

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3. Solvent Extraction with a Separating Funnel

After a reaction, your product is often dissolved in an aqueous layer along with inorganic by-products (salts, acids, bases) you do not want. Solvent extraction uses an organic solvent immiscible with water to pull the product out of the aqueous layer and into the organic layer.

3.1 How It Works

Based on the partition of a solute between two immiscible liquids (think "like dissolves like"):

• Organic products are usually much more soluble in organic solvents (ethyl acetate, diethyl ether, dichloromethane) than in water.
• Ionic impurities (NaCl, K₂Cr₂O₇, HCl) stay in the water.
• Shaking the two layers in a separating funnel maximises contact.
• Letting them settle gives two clearly separated layers; the lower one is drained off first.

3.2 Procedure

1. Pour the reaction mixture into a separating funnel.
2. Add the organic solvent (~equal volume).
3. Stopper the funnel and invert, releasing pressure by opening the tap with the funnel upside-down (venting).
4. Shake vigorously and vent again.
5. Set the funnel in a retort stand and allow the two layers to separate.
6. Open the tap and drain the lower (denser) layer into one flask.
7. Drain the upper (less dense) layer into another flask (or let it out through the top).
8. Which layer is which? If you know the density of the solvent (ether is less dense than water, DCM is more dense) you know which is on top. If unsure, add a few drops of water to one layer — if the drops float, that layer is organic non-aqueous; if they mix, it is aqueous.

3.3 Safety

• Organic solvents are usually flammable and some are toxic — work in a fume cupboard.
• Pressure can build up when two reacting phases are shaken — vent the funnel by opening the tap with it upside-down.
• Ethyl acetate, dichloromethane, diethyl ether — all used regularly in school and university labs.

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4. Drying Agents

After solvent extraction, the organic layer still contains a small amount of dissolved water, which must be removed before distillation or other workup. Drying agents are anhydrous salts that absorb water without reacting with the organic compound.

Drying agent	Formula	Notes
Anhydrous sodium sulfate	Na₂SO₄	Neutral, mild, cheap, slow but reliable
Anhydrous magnesium sulfate	MgSO₄	Neutral, fast, efficient, a little warming
Anhydrous calcium chloride	CaCl₂	Acidic/basic reactive with some compounds — avoid with alcohols and amines
Anhydrous calcium sulfate	CaSO₄ (Drierite)	Fast, neutral, reusable by heating

4.1 Procedure

1. Pour the wet organic liquid into a clean conical flask.
2. Add a spatula-full of anhydrous drying agent.
3. Swirl.
4. If the drying agent clumps together, add more until some of it remains free-flowing at the bottom.
5. Leave for ~5 minutes.
6. Filter (gravity filter or decant) to remove the now-wet solid drying agent.
7. Proceed to distillation or evaporation.

4.2 How You Know It Is Dry