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 crystallize.
• 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