Bromobutane
Halogenoalkanes
Halogenoalkanes are hydrocarbon
chains that have one or more hydrogen
atom(s) exchanged for halogen atom(s).
Module 2
Describe the laboratory preparation
of a halogenoalkane from the
corresponding alcohol, with specific
reference to refluxing, using a
separating funnel, removal of acidic
impurities, drying, simple distillation.
Preparation of
1-Bromobutane from the
corresponding alcohol
Bromobutane is prepared by reacting
butan-1-ol with hydrogen bromide.
C4H9OH + HBr  C4H9Br + H2O
Preparation of Reaction
Mixture
Butan-1- ol, solid sodium bromide and
water are put into a pear shaped flask.
Concentrated sulphuric acid is added
from a tap funnel, slowly, in order to
dissipate heat as this is a very
exothermic reaction.
Preparation of Reaction
Mixture
Hydrogen bromide is made in situ (this
means in the reaction mixture) by the
reaction between concentrated
sulphuric acid and sodium bromide.
H2SO4 + NaBr  NaHSO4 + HBr
The HBr then reacts with butan-1-ol.
C4H9OH + HBr  C4H9Br + H2O
During the preparation
brown fumes are observed.
The concentrated H2SO4 oxidises HBr
producing bromine. This is a side
reaction.
2HBr + H2SO4  Br2 + SO2 +2H2O
So excess NaBr and concentrated
sulphuric acid must be used.
Reflux
The reaction mixture is then refluxed for
30 – 45 mins.
Reflux
Continuous evaporation and
condensation used to prevent loss of a
volatile liquid. This allows an organic
reaction mixture to be heated without
losing any reactants or products.
Condenser in a vertical position.
First Distillation
The mixture is allowed to cool and is
then distilled until no more oily drops of
1-bromobutane carry over.
This separates the 1-bromobutane from
the reaction mixture.
Water Out
Water In
Use of A Separating
Funnel
Separating funnels are used to remove
impurities from crude organic liquids.
The organic liquids are shaken with an
aqueous solution and then the organic
layer is separated from the aqueous
layer.
Use of A Separating
Funnel
Washing with water removes water
soluble impurities.
Washing with sodium carbonate or
sodium hydrogencarbonate solution
removes acidic impurities.
Separating Funnel
Removal of Acidic
Impurities
A separating funnel is used to shake the
crude bromobutane with sodium
hydrogencarbonate solution to remove
the acidic impurities.
2NaHCO3 + H2SO4  Na2SO4 + 2H2O + 2CO2
NaHCO3 + HBr  NaBr + H2O + CO2
Because carbon dioxide is produced
pressure builds up inside the funnel and it
is necessary to release the pressure
periodically by inverting the funnel and
carefully opening the tap.
Aqueous or Organic
Layer?
The separating funnel is left to settle.
Two layers are formed – an aqueous
layer and an organic layer.
To find out which is the aqueous layer
add a few drops of water – the layer that
increases in size is the aqueous layer,
the other the organic layer.
Drying the Product –
Removal of Water
In this preparation the organic layer is
the bottom layer.
1-bromobutane in the organic layer is
run off into a conical flask
A drying agent such as anhydrous
sodium sulphate is added and the
conical flask is swirled.
Other Drying Agents
Anhydrous Magnesium Sulphate
Anhydrous Calcium Chloride
The bromobutane is then filtered and
redistilled into a pre-weighed clean, dry
conical flask.
The flask is then weighed and the actual
yield is calculated.
The percentage yield can be calculated.
Percentage Yield
% YIELD =
Actual Mass
x 100
Theoretical Mass
Theoretical Yield
8.1g butan-1-ol 0.109 moles
C4H9OH + HBr  C4H9Br + H2O
Therefore 0.109 moles of bromobutane
is theoretical yield.
0.109 x 137 =14.93g
Percentage Yield
Actual Yield = 7.77g
% YIELD = Actual Mass
x 100
Theoretical Mass
% YIELD = 7.77
14.93
= 52.04%
Referring to practical and theoretical
considerations why is the % yield is not
100%?
Practical:
Loss during transfer
Loss during distillation
Theoretical : Side reactions
Reaction does not go
to completion.