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convert- a)ethyne to bromo ethane b)1-bromopropane to 2-bromopropane
If you need bromoethene (BrCH=CH2), then the above bromoethane is converted to ethene (CH2=CH2) by the action of alcoholic KOH, which is then converted to dibromoethane (BrCH2-CH2Br), this on reaction with alc.KOH first followed by NaNH2 in liq. NH3 gives acetylene, further addition of HBr to acetylene gives bromoethene.



2-bromopropane is heated under reflux with a concentrated solution of sodium or potassium hydroxide in ethanol. Heating under reflux involves heating with a condenser placed vertically in the flask to avoid loss of volatile liquids. Propene is formed and, because this is a gas, it passes through the condenser and can be collected.

Everything else present (including anything formed in the alternative substitution reaction) will be trapped in the flask.

The mechanism

In elimination reactions, the hydroxide ion acts as a base - removing a hydrogen as a hydrogen ion from the carbon atom next door to the one holding the bromine.

The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces propene.


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