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

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

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Answer 1 · 2015-11-23T14:53:38

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

Explanation:

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

$H_{2} C = C H_{2} + B r_{2} \to H_{2}^{+} C - C H_{2} B r + B r^{-}$ $H_2C=CH_2 + Br_2 rarr H_2^+C-CH_2Br + Br^-$

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 $H O C H_{2} - C H_{2} B r$ $HOCH_2-CH_2Br$ (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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How does bromine water show if something is an alkane or alkene?

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