What is the chemical test for alkanes and alkenes?

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ashley227 eNotes educator| Certified Educator

Alkanes and alkenes are both hydrocarbons. Alkanes, however, contain only single bonds between the carbons while alkenes contain at least one double bond between carbons. Tests can be conducted to differentiate these two groups of molecules based on whether or not the double bonds can be broken by a halogen.

If one takes two test tubes with bromine water and adds an alkene to tube A and an alkane to tube B and then shakes the two tubes, a color change will occur in tube A (the color of the solution will change from brown to colorless) while no color change will occur in tube B. This is because in tube A, the double bond is broken and the carbons become halogenated while no reaction occurs in tube B.

The reaction that occurs in tube A is as follows:

Bromine is a diatomic element. It regularly occurs as Br2. In a molecule of Br2 one bromine has a partial positive charge and the other has a partial negative charge. It only becomes polarized as it approaches the electron-dense double bond of the alkene.

The electrophilic bromine attacks the double bond and a bond is formed between one of the carbons involved in the double bond and the electrophilic bromine. The other bromine is a good leaving group and leaves as a bromine ion.  

This leaves the former alkene as a carbo-cation; there is a positive charge on the carbon that now only has three bonds (due to the breaking of the double bond). This is an unstable intermediate, but the positive carbon is then attacked by the negative bromine ion and a bond is formed. The resulting product is 1,2-dibromoethane which is colorless while diatomic bromine is dark red/brown.