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 In

Water Out

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 transferLoss during

distillationTheoretical : Side reactions

Reaction does not go to completion.