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How does bromine water show if something is an alkane or alkene?

Answer 1

Because an olefin, an alkene, will undergo facile bromination with bromine.

You know that the bromine molecule is polarizable, i.e. it can form #Br-Br^(delta+)# (the atom on the LHS is #delta^-#!). This electrophilic centre is capable of reaction with the electron-rich double bond to form a cationic intermediate, i.e. with ethylene:

#H_2C=CH_2 + Br_2 rarr H_2^+C-CH_2Br + Br^-#

The orange colour of bromine should therefore dissipate, and of course this is a positive test for olefinic bonds. Now that you have a carbocation, this will react with any nucleophile present: this could be water to give #HOCH_2-CH_2Br# (and of course water is a potent nucleophile). In the absence of water in an inert solvent it could be the bromide ion that was delivered in the first part of the bromination, to give 1,2-dibromoethane.

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Answer 2

Lucy Alessi

Bromine water is commonly used to distinguish between alkanes and alkenes. When bromine water is added to an alkane, there is no observable reaction because alkanes are inert to bromine water. However, when bromine water is added to an alkene, the orange-brown color of bromine water decolorizes rapidly due to the addition reaction between the alkene and bromine, forming a colorless dibromo compound. This color change serves as a positive test for the presence of an alkene.

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Answer 3

Sarah Angotti

Bromine water can be used to differentiate between alkanes and alkenes. When bromine water is added to an alkane, there will be no observable reaction because alkanes are saturated hydrocarbons and do not contain any double bonds. However, when bromine water is added to an alkene, it undergoes an addition reaction. The orange-brown color of bromine water will fade as the bromine molecules are added across the carbon-carbon double bond, resulting in a colorless solution. This is due to the formation of a colorless dibromo compound. Therefore, the decolorization of bromine water indicates the presence of an alkene.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.