Testing for Alkenes & Haloalkanes
Testing for an Alkene
- Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)
- Br2(aq) is an orange-yellow solution, called bromine water
- The unknown compound is shaken with the bromine water
- If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise
The bromine water test is the standard test for unsaturation in alkenes
Testing for a haloalkane
- The haloalkane must first be warmed with sodium hydroxide in a mixture of ethanol and water
- The haloalkane will undergo nucleophilic substitution releasing the halide ion into solution, which can then be tested
- The released halide ion is tested for using the silver nitrate test:
-
- Acidify the sample with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution, AgNO3
- The acidification is done to remove carbonate ions that might give a false positive result
- If a halide is present it forms a silver halide precipitate:
- Acidify the sample with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution, AgNO3
Ag+ (aq) + X– (aq) → AgX (s)
-
- Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
- Silver chloride is white, silver bromide is cream and silver iodide is yellow
- Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
Each silver halide produces a precipitate of a different colour
Examiner Tip
The acidification step in the halide ion test must be done with nitric acid rather than hydrochloric acid, as HCl contains chloride ions which would interfere with the results.