Organic Synthesis Techniques

Spec Mapping — OCR H432 Module 4.2.3 — Organic synthesis techniques, covering the laboratory preparation and purification of organic compounds; reflux apparatus (vertical Liebig condenser, anti-bumping granules, open top); simple and fractional distillation (horizontal condenser, still head, thermometer); solvent extraction with a separating funnel; drying with anhydrous inorganic salts (Na₂SO₄, MgSO₄, CaCl₂); recrystallisation as a purification method for solids; filtration under reduced pressure (Buchner funnel + Buchner flask + water pump); melting-point determination as a purity check; the full synthesis-purification workflow exemplified by the preparation of 1-bromobutane from butan-1-ol (PAG 5 anchor); yield, atom-economy and safety considerations (refer to the official OCR H432 specification document for exact wording).

Knowing reaction equations is only half of organic chemistry — the other half is actually making the molecule in the lab. A practical organic chemist must (i) drive the chemical transformation cleanly under conditions that maximise the desired product, (ii) separate the product from the reaction mixture (which contains unreacted starting material, solvent, by-products and inorganic salts), (iii) purify the product to a defined standard (typically >95 % for a teaching lab, >99 % for pharmaceutical work), and (iv) characterise it to confirm structure and purity. OCR A-Level Chemistry asks you to understand, describe and select between the main laboratory techniques used in this workflow: reflux (controlled heating of a volatile reaction mixture), distillation (separation of liquids by boiling point, simple or fractional), solvent extraction with a separating funnel, drying with an anhydrous inorganic salt, recrystallisation of a solid product, and filtration under reduced pressure (Buchner filtration) to collect the crystals. These techniques are central to the PAG 3 (organic synthesis and purification) and PAG 5 (synthesis of an organic liquid) practical activities, and frequently appear in 6-mark exam questions that ask you to draw or describe a complete procedure. This lesson develops each technique in detail — what it does, when it is used, the apparatus, the procedure, the safety considerations — and then puts them together in two worked examples (the preparation of 1-bromobutane from butan-1-ol, and a solid recrystallisation), with yield calculations and a discussion of common practical errors.

Key Distinction: Reflux has a vertical Liebig condenser that returns vapour to the reaction flask, keeping the volatile mixture confined while you heat it for long times. Distillation has a horizontal (downward-tilted) Liebig condenser that returns vapour to a separate collection flask, letting you remove a volatile component from the reaction mixture. The angle of the condenser is the single most-examined distinguishing feature in OCR practical chemistry diagrams.

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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

• The organic liquid becomes clear (water layer gone).
• The drying agent no longer clumps on further addition — it moves freely at the bottom of the flask.

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5. Recrystallisation

Recrystallisation is a technique for purifying a solid organic product. It relies on the different solubilities of the main product and its impurities in a carefully chosen solvent at different temperatures.

5.1 The Ideal Solvent

The ideal recrystallisation solvent:

• Dissolves the product well when hot.
• Dissolves the product poorly when cold.
• Dissolves the impurities equally well hot and cold (so impurities stay in solution on cooling).
• Is not too volatile (won't boil away during cooling).
• Does not react with the product.

5.2 Procedure

1. Dissolve the crude solid in the minimum volume of hot solvent. Use a hot plate; heat the flask and the solvent together.
2. If there are insoluble impurities ("gunk"), filter the hot solution through a hot glass funnel to remove them.
3. Cool slowly — to room temperature first, then in an ice bath. Crystals of the pure product form as the solution becomes supersaturated.
4. Filter under reduced pressure (next section) to collect the crystals.
5. Wash the crystals with a small amount of cold solvent.
6. Dry the crystals (air-dry or gentle oven).
7. Measure the mass, calculate yield, and determine melting point to check purity.

5.3 Why It Works

Because we used the minimum volume of hot solvent, the solution is saturated in the product when hot. As it cools, solubility of the product drops rapidly, and the excess crystallises out. The impurities, which were present in much smaller amounts, do not reach saturation at the lower temperature and stay dissolved. When we filter, the crystals are pure and the impurities are washed away in the filtrate.

graph TD
  A[Crude impure solid] --> B[Add minimum hot solvent]
  B --> C[Hot solution - all dissolved]
  C --> D[Filter hot to remove insoluble impurities]
  D --> E[Cool slowly]
  E --> F[Pure crystals form, impurities stay in solution]
  F --> G[Filter under reduced pressure]
  G --> H[Wash with cold solvent, dry]
  H --> I[Pure product]

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6. Filtration Under Reduced Pressure (Buchner Filtration)

This is the standard way of collecting a solid from a liquid mixture — faster and more efficient than gravity filtration.

6.1 Apparatus

graph TD
  A[Buchner funnel with filter paper] --> B[Side-arm conical flask Buchner flask]
  B --> C[Rubber tubing to water vacuum pump / filter pump]
  C --> D[Reduced pressure in flask pulls liquid through filter paper]

• Buchner funnel — porcelain or plastic funnel with a flat perforated plate.
• Filter paper — cut to the exact diameter of the funnel plate.
• Buchner flask (a thick-walled conical flask with a side arm) — withstands low pressure.
• Water pump or aspirator connected to the side arm.

6.2 Procedure

1. Place the filter paper on the Buchner funnel and wet it with a little solvent to seal.
2. Turn on the water pump.
3. Pour the cold mixture containing crystals into the funnel.
4. The liquid is pulled quickly through the filter paper by the reduced pressure.
5. Wash the crystals with a little cold solvent while the pump is still running.
6. Let air draw through the crystals for a minute or two to aid drying.
7. Switch off the pump and transfer the crystals to a watch glass or oven for final drying.

6.3 Advantages over Gravity Filtration

• Much faster — solvent is sucked through.
• Gives drier crystals, because air is drawn through them.
• Easier to wash without disturbing the crystal cake.

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7. Determining Purity: Melting Point

After recrystallisation, you check whether your product is pure by measuring its melting point and comparing it to a literature value.

• A pure compound melts over a sharp range (≤ 1 °C).
• An impure compound melts over a wide range (several °C), and at a lower temperature than the pure compound.

This is called melting point depression and happens because impurities disrupt the crystal lattice, lowering its stability.

Example: Benzoic acid has a literature mp of 122.4 °C. If your recrystallised sample melts at 121–122 °C, you can be confident it is pure. If it melts at 110–118 °C, you know there is still ~10% impurity.

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8. Worked Example — Full Synthesis Question

Describe how you would prepare and purify 1-bromobutane from butan-1-ol, starting from 10 g of butan-1-ol.

Reaction:

$C_4H_9OH + NaBr + H_2SO_4 \rightarrow C_4H_9Br + NaHSO_4 + H_2O$C4​H9​OH+NaBr+H2​SO4​→C4​H9​Br+NaHSO4​+H2​O

Procedure: