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048 Lab Chemistry organic .pdf



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Chem Factsheet
January 2003

Number 48

Laboratory Chemistry - Organic Techniques
Before working through this Factsheet you should:
• Have some practical experience in organic chemistry;
• Undersatnd the Organic Chemistry covered so far at AS and A2 (covered
in Factsheets 15, 16, 17, 27, 31, 32, 33, 34, 35 and 39);
• Understand the laboratory techniques for separating and purifying
(covered in Factsheet 30).

2. Distillation
Distillation can be used to isolate a liquid product if the boiling point of the
product is significantly different to that of the rest of the reaction mixture.
Distillation is used to purify a liquid by boiling it and then condensing it
away from its impurities.

After working through this Factsheet you will:
• Have met and revised common practical techniques in organic chemistry;
• Be able to represent the relevant equipment with simple diagrams;
• Be able to assess simple organic practicals in terms of safety.

Thermometer

As several of these techniques were discussed in detail in Factsheet 30, this
Factsheet is designed to be more of a focused revision aid.

Thermometer pocket

It is necessary for candidates to display a familiarity with a variety of
practical techniques in organic chemistry when preparing for both written
exams and practical exams or assessment.

Liebig condenser

1. Heating under reflux
water
out

Reflux apparatus are necessary when a reactant has a low boiling point, or
the reaction is slow at room temperature. The condenser prevents the
escape of any of the volatile reagent or product.

water in
Heat

Receiver
Distillate

condenser
to sink

3. Fractional Distillation
Fractional distillation is most commonly used to separate two liquids if the
boiling points are quite close. Pure samples of each liquid can usually be
obtained, unless the boiling points are too close.
Thermometer
from tap
Thermometer pocket
Liebig condenser

reaction mixture
water
out

Fractionating
column

water in

to vacuum pump
for distillation
under reduced
pressure

heat
Distillate
Distillation flask

Heat

1

Chem Factsheet

Laboratory Chemistry - Organic Techniques
5. Recrystallisation

Candidates need to be familiar with the following types of graphs plotting
boiling points against composition.

To purify the solid product.
A suitable solvent for recrystallisation will often be evident (e.g. the solvent
from which the crude product was initially crystallised), and should be a
solvent which will not react with the solid. The solubility of the solid
should be high near the boiling point and low near room temperature.
Candidates should learn the method of recrystallisation.

Chemical A must be separated from chemical B. Chemical A has a higher
boiling point than chemical B, so chemical B is a more volatile liquid.
Boiling
point A




T1
T2

Boiling
point B

o

Temp. C

100% A
0% B
composition
X

composition
composition
Y






Dissolve the solid in the minimum volume of hot solvent.
Quickly filter the hot solution using preheated filter funnel and
fluted filter paper.
Collect the filtrate and allow to cool and the solid to recrystallise.
Use suction filtration (Buchner funnel) to collect the solid.
Wash the solid with a small amount of cold solvent.
Dry the solid product.

6. Melting Point Determination

0% A
100% B

The melting point of a solid is used to judge the purity of the product. A
solid should have a sharp melting point, and recrystallisation should be
repeated until this is obtained.

composition
Z

Melting point should be determined using melting point apparatus.

Consider a mixture of two chemicals A and B, of composition X. If it is
heated it will boil at T1 oC to give a vapour of composition Y.
Note that Y is richer in the more volatile component than X.

capillary tube

If Y is condensed and reboiled it will boil at a temperature T2oC, giving a
vapour of composition Z, where Z is richer in the more volatile component
B than Y was.

thermometer

electrically
heated block

Eventually pure B will be distilled off from the top of the fractionating
column and pure A will be left in the flask.
How the vapours are continuously condensed and reboiled will be discussed
in a later Factsheet, which will deal with the workings of fractional distillation
in even more detail.

illuminated
cavity

4. Filtration under reduced pressure
To isolate a solid product from a liquid, suction filtration using a Buchner
(or Hirsch) Funnel is an effective method.

sample
Buchner funnel
and flask
The sample is placed in a capillary tube, and the temperature increased
slowly until it melts.
suction

Melting points can also be used for identification purposes.

7. Boiling point determination
Boiling points can also be used for identification purposes, and to check
the purity of a product. Boiling points are most commonly measured in the
process of distillation (see section 2).

A small amount of cold solvent should be used to wet the filter paper prior
to filtration, and to remove any solid remains from the reaction flask.

2

Chem Factsheet

Laboratory Chemistry - Organic Techniques

8. Selecting an appropriate heat source
There are a variety of heat sources available in the laboratory, and heat is
often required for organic reactions to increase the rate of reaction or to
provide the activation energy to initiate the reaction.
Two things must be considered when selecting the type of heat source:
(a) The temperature or strength of heating required.
(b) Safety.
Consider the following options:
Bunsen burner
Direct, variable heat and accessible, but danger - do not use with flammable
reagents, solvents or products.
Electric heating mantle
No naked flame (so can be used with flammable chemicals), controllable
temperature, but a little cumbersome. Electricity, and necessary wiring,
obviously required.
Water bath
Useful when gentle heating required, as maximum temperature is that of
boiling water. Some water baths are electrically heated, whilst a simple
water bath is a beaker over a Bunsen flame, so candidates should be aware
if any flammable vapours may be produced.

organic chemial
water

heat

Oil bath
Same principle as water bath, but higher temperatures can be reached.
Take some time to reach required temperatures, and some time to cool
down again afterwards.
Exam Hint: - A note about safety during organic practicals.
Candidates should be able to make simple risk assessments
when it comes to practical work. A common type of exam question
will ask for safety precautions during a specified procedure. For
success in the exams and for your own personal safety, knowledge
of safe procedure is important.
It is assumed that students wear eye protection during practical
work, so the answer “wear goggles” will not suffice in response to
a question about safety.
Consider the following:
• Fume hoods should be used for all reactions involving toxic,
irritant or carcinogenic chemicals.
• No naked flames should be used with flammable chemicals.
• The use of gloves is required when dealing with corrosive or
irritant chemicals such as concentrated acids.

Acknowledgements:
This Factsheet was researched and written by Kieron Heath
Curriculum Press, Unit 305B, The Big Peg, 120 Vyse Street, Birmingham,
B18 6NF
ChemistryFactsheets may be copied free of charge by teaching staff or students,
provided that their school is a registered subscriber.
No part of these Factsheets may be reproduced, stored in a retrieval system, or
transmitted, in any other form or by any other means, without the prior permission
of the publisher. ISSN 1351-5136

Use these ideas and common sense when answering questions about safety.
When carrying out practical procedures, be careful to follow the prescribed
method and pay attention to the risk assessment and safety notes.

3


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