Organic Synthesis - Practical Preparation Techniques (A-Level Chemistry)

Practical Preparation Techniques

Refluxing

The rate of reaction of organic reactions tends to be slow unless the reaction mixture gets heated up.

However, organic compounds also tend to be highly flammable and have low boiling points, which means that if heated with an open flame, they are likely to evaporate or burn before a reaction can take place.

Instead, the reacting mixture in organic reaction is heated under reflux.

The flask containing the reaction mixture is connected to a vertical Liebig condenser. As a result, any reactants that boil off will immediately condense and fall back into the flask so that the reaction can continue.

Distillation

Simple Distillation

The products of a reaction heated under reflux will also evaporate, condense, and fall back into the flask so that they react further, making it impossible for them to be collected.

You can prevent this by using simple distillion.

A distillation apparatus works as follows:

1. The flask containing the reaction mixture is connected to a condenser. The condenser leads into an empty flask to collect any substances that get distilled off.

2. The reaction mixture gets heated gently. The temperature inside the flask increases so that it is the highest at the bottom and the lowest at the top.

3. Substances in the reaction mixture evaporate in order of volatility. As the reaction mixture gets heated and vapors rise, the substances with the highest boiling points will condense and fall back into the flask as they reach the top of the flask. This is because the temperature at the top of the flask is below its boiling point. Substances with lower boiling points, will not condense at the top of the flask, entering the condenser instead.

4. Products evaporate as they form. If products have lower boiling points than any of the reactants, the reaction mixture can be heated up to the boiling point of the products. The products will evaporate and enter the condenser so that they can be collected in the flask at the end.