WM - Practical techniques Flashcards
What does refluxing make sure?
You don’t lose any volatile organic substances.
Why do some organic reactions involve refluxing?
Because they can be slow and the substances are usually flammable and volatile (they’ve got low boiling points). If you stick them in a beaker and heat them with a Bunsen burner they’ll evaporate or catch fire before they have time to react.
How do you reflux something?
Heat the solution in a flask fitted with a vertical Liebig condenser - this causes the solution to continuously boil/evaporate and then condense back into the flask, giving it time to react.
Heat the solution electronically - hot plates, heating mantles, or electrically controlled water baths are normally used. This avoids naked flames that might ignite the compounds.
What does distillation do?
Separates substances with different boiling points.
How do you distill something?
- Gently heat it in distillation apparatus. The substances will evaporate out of the mixture in order of increasing boiling point.
- When an evaporated substance enters the condenser, it is cooled and condenses back to a liquid, which can be collected by placing a flask at the open end of the condenser.
How can you collect a pure product in using distillation?
If you know the boiling point of the pure product you can use the thermometer to find out when it is evaporating and therefore when to collect it.
How can volatile liquids be purified?
By redistillation.
How can volatile liquids be purified by redistillation?
- After distillation, the product will probably still contain impurities.
- If a product and its impurities have different boiling points, then redistillation can be used to separate them. You just use the same distillation apparatus but this time you heat the impure product, instead of the reaction mixture.
- When the liquid you want boils (this is when the thermometer is at the boiling point of the liquid), place a flask at the open end of the condenser ready to collect your pure product.
- When the thermometer shows the temperature is changing, put another flask at the end of the condenser because a different liquid is about to be delivered.
What does separation do?
Removes any water soluble impurities from the product.
When can separation be used?
If a product is insoluble in water.
Explain how separation works
Once the reaction to form the product is completed, pour the mixture into a separating funnel and add water.
Shake the funnel and then allow it to settle. The organic layer is less dense than the aqueous layer so should float on top. Any water soluble impurities will have dissolved in the lower aqueous layer.
You can then open the stopper on the separating funnel and run off the aqueous layer. This leaves you with the product in the funnel.
How do you remove water from a purified product?
By drying it.
Explain how to remove water from a purified product by drying it
If you use separation to purify a product the organic layer will end up containing trace amounts of water, so it has to be dried.
To do this, add an anhydrous salt such as magnesium sulfate or calcium chloride. The salt is used as a drying agent - it binds to any water present to become hydrated.
When you first add the salt to the organic layer it will be lumpy. This means you need to add more. You know that all the water has been removed when you can swirl the mixture and it looks like a snow globe.
You can then filter the mixture to remove the solid drying agent.
What is filtration used to do?
Isolates solid organic products.
Explain how filtration can be used to isolate solid organic products
If the reactants in an organic reaction are liquids and the product you are after is a solid, you can isolate the product from the reaction mixture by filtration.
Pour the reaction mixture, containing the liquid and solid, into a Buchner funnel with filter paper on the bottom.
Below the funnel there should be a sealed side-arm flask attached to a vacuum line. The reduced pressure inside the flask, caused by the vacuum line removing air, will force the liquid through the funnel, leaving the solid product on the filter paper to be collected.
