Alkenes
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Why does the practical work on alkenes in Nuffield Chemistry use bromine solution but all the theory is using bromine? Presumably bromine solution and ethene makes 2-bromoethanol, yet all the theory add bromine to an alkene to make a dibromo compound. I realise that using bromine liquid could be a safety issue, but would not an alkene change liquid bromine from orange to colourless. Why is the word solution or aqueous used in this context?
Igloo writes ...
As you surmise, the issue is one of safety.
Pure (liquid) bromine would, as you say, give the dibromo compound, as would a solution of bromine in an inert organic solvent. You are quite correct in claiming that the reaction of aqueous bromine (bromine-water) with ethene gives 2-bromoethanol as one of the products (actually the predominant one), but the accepted electrophilic addition mechanism also allows for the creation of 1,2-dibromoethane, which is certainly present in the mixture of products.
In order to simplify matters for Nuffield A-level students the creation of the bromoalkanol is not usually mentioned.
It is worth emphasising that the reaction of alkenes with bromine in organic solvents is very fast (with bromine-water it is very much slower because the mixture is a heterogeneous one and has to be shaken vigorously). The reaction with liquid bromine is probably instantaneous, almost certainly a very hazardous operation, and therefore unacceptable laboratory practice.
Always carry out a risk assessment before starting any practical work, and check with your teacher.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 27 January 2005
