RP10 - Preparation of organic solids and liquids

Question 1

What are the main steps in producing a pure organic

solid?

Answer 1

1. Synthesis of the compound (usually using reflux, distillation etc.)
2. Filtration (usually vacuum filtration)
3. Purification (recrystallisation)

Question 2

How do you use laboratory equipment to heat under

reflux?

Answer 2

Quickfit apparatus is used to heat a substance under reflux.

• The substance is boiled in a pear-shaped or round-bottomed flask.
• As it evaporates, it is cooled by the water in the liebig condenser and so condenses back into a liquid and drips back down into the flask to be heated again.

Question 3

Why is heating under reflux used?

Answer 3

● Allows heating for a long period of time
● Prevents the flask from boiling dry
● Prevents volatile reactants/products escaping
● Ensures even heating

Question 4

Why are anti-bumping granules used when heating

under reflux and distillation?

Answer 4

To allow smooth boiling- they prevent splashing up the sides of the flask due to
bubbles caused by vapours.

Question 5

How do you use laboratory equipment to filter under

reduced pressure?

Answer 5

Using a Buchner funnel and Buchner flask, connected by rubber tubing to the vacuum source.
● The funnel contains a layer of filter paper.
● Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask.
● The solid will remain on the paper.

Question 6

How do you purify a solid product?

Answer 6

By recrystallisation.
● Add minimum amount of warm solvent to the impure
sample until it has dissolved.
● Allow to cool, crystals should form.
● When no more form you can filter under reduced pressure to obtain a dry crystalline solid.

Question 7

How do you determine the melting point of a substance and why can this information be useful?

Answer 7

● Place a small sample of the solid in a capillary tube.
● Melt using the melting apparatus available, measuring the temperature with a thermometer.
● A pure substance will usually melt at a single temperature but an impure substance will melt over a range of temperatures
(usually lower than the pure substance).
● Record the starting and ending points of the melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively.
● You can then compare the melting point to known values to identify the
substance.

Question 8

How do you purify a liquid organic product?

Answer 8

● You can use a separating funnel to isolate the organic layer from the aqueous layer.
● Add anhydrous MgSO4
to the organic product to
get rid of all water.
● You can then purify the liquid by distillation, which
separates the substances by boiling point.

Question 9

How do you calculate the percentage yield of a product?

Answer 9

Percentage yield =

Actual yield/ theoretical yield x100