Halogenoalkanes (3) Preparation of bromoethane (with problems to solve)

How can a haloalkane be prepared from a primary alcohol?

A typical experiment involves the halogenation of an alcohol like ethanol using hydrogen bromide (HBr).

The hydrogen bromide is prepared in situ.

The bromoalkane product is very volatile so it is collected under ice-cold water.

To purify the bromoalkane product it could be syphoned out and redistilled at room temperature.

Hydrogen bromide is prepared in the reaction flask from 50% sulphuric acid (H₂SO₄) and potassium bromide (KBr).

The flask contains the sulphuric acid and potassium bromide mixture and a few anti bumping granules to promote smooth boiling.

The ethanol is added slowly from the side-arm flask.

As the reaction is exothermic the addition of ethanol needs to be taken carefully to prevent over boiling and the expulsion of product with inorganic material into the condenser.

The receiving flask contains ice-cold water, the product distils over and, being denser than water, settles on the bottom of the flask.

The diagram does not show this but the delivery tube into the flask must be long enough to be placed under the ice-cold water. 

Here is the equation for the reaction:

CH₃CH₂OH    +      HBr              =      CH₃CH₂Br     +    H₂O

Ethanol                                              Bromoethane

You should be able to re-write equations for this preparation using structural formulae from different alcohols such as pentan-1-ol, propan-1-ol and methanol.

You should be able to re write these equations using skeletal formulae and displayed formulae.

You should be able to calculate the mass of bromoethane produced from 25g of ethanol

The formation/preparation of 2-chloro-2-methylpropane can be found here

You should also be able to calculate the volume of this mass of ethanol.