6.2.5- Organic Synthesis

Question 1

When purifying by recrystallisation, hy is the minimum volume of hot solvent used?

Answer 1

So that a saturated solution is created, so that as many crystals will fall out of solution as possible when it is cooled

Question 2

Why is the solution filtered hot when purifying by recrytallisation?

Answer 2

To remove insoluble impurities and ensure that the crystals do not form in the filter paper

Question 3

Why is the solution cooled in an ice bath when purifying by recrystallisation?

Answer 3

To ensure that as many crystals as possible fall out of solution- yield is higher

Question 4

Why are crystals washed with cold water when purifying by recrystallisation?

Answer 4

To remove soluble impourities

Question 5

How would you separate the crystals from the reaction mixture when purifying by recrystallisation?

Answer 5

Filter under reduced pressure using a Buchner funnel

Question 6

Why might % yield be below 100% (practical reasons)?

Answer 6

• Product is lost during filtration, drying and weighing- spills, not all transferred from one piece of apparatus to another
• Product is left dissolved in the solution- some does not crystallise. Some left on filter paper. Sample still wet.

Question 7

Describe how Quickfit apparatus is connected

Answer 7

Grease the joints using some petroleum jelly on the inside of the joints before connecting the pieces together.

Question 8

In a distillation setup, why is it necessary to have a continuous water flow around the condenser?

Answer 8

So that the water remains cool in order for the mixture to be distilled

Question 9

Describe a method that can be used to separate immiscible liquids

Answer 9

• Pour mixture into a separating funnel and some distilled water
• Add stopper and invert flask to mix the mixture
• Equalise pressure by opening the stopper as required
• Continue shaking until there is no ‘whistle’ sound
• To collect water in the lower layer, open stopper and place a beaker under the spout
• Use another beaker to collect the desired organic layer
• Shake the liquid with some drying agent

Question 10

Name two drying agents

Answer 10

1- Magnesium sulphate
2- Calcium chloride

Question 11

How is a drying agent used?

Answer 11

• Add a selected drying agent to the organic product
• If the drying agent forms clumps add some more until they are moving freely
• Use gravity filtration to collect the dry product
• Filtrate is the product

Question 12

What does re-distillation mean?

Answer 12

When a liquid is purified by using multiple distillations

Question 13

What are the three key steps to purify an organic solid?

Answer 13

1. filtration under reduced pressure
2. recrystallisation
3. measurement of melting points