6.2.5: Organic synthesis

Question 1

When purifying by recrystallisation, why 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

recrystallisation?

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 the crystals washed with cold water when

purifying by recrystallisation?

Answer 4

To remove soluble impurities

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 percentage 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 the other
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 the mixture into a separating funnel and some distilled water
● Add the stopper and invert the flask to mix the mixture
● Equalise the pressure by opening the stopper as required
● Continue shaking until there is no ‘whistle’ sound
● To collect the water in the lower layer, open the 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

Magnesium sulphate and 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