Bromine water

Bromine water is a saturated aqueous solution of bromine .

properties

Since bromine is only sparingly soluble in water, a brown-colored, dilute bromine solution is created. Due to the disproportionation to bromide and hypobromite , bromine water reacts slightly acidic:

${\displaystyle \mathrm {Br_{2}\ +\ H_{2}O\longrightarrow \ Br^{-}\ +\ BrO^{-}\ +\ 2\ H^{+}} }$

The reaction is accelerated by light. Bromine water is therefore stored in brown, less translucent bottles.

use

Play media file

Differentiation between alkanes and alkenes. left : Cyclohexane does not react with bromine water right : Cyclohexene reacts with bromine water

Bromine water is used in the laboratory for the rapid detection of C = C multiple bonds in organic compounds such as alkenes or alkynes . If an organic compound is introduced as a gas (for example ethene ) into bromine water or shaken as a liquid with bromine water, whereby the solution becomes colorless, the sample substance contains at least one C = C double bond. This evidence is based on an electrophilic addition . A bromonium ion is formed with the π electrons of the double bond via a heterolytic cleavage of Br ₂ . This complex then reacts with a bromide ion (Br ^- ) to form 1,2-dibromoethane .

${\displaystyle \mathrm {H_{2}C{=}CH_{2}\ +\ Br_{2}\longrightarrow \ Br{-}CH_{2}{-}CH_{2}{-}Br} }$

Bromine adds to ethene. 1,2-Dibromoethane is formed.

In a side reaction of the bromonium ion with water, 2-bromoethanol is also formed :

${\displaystyle \mathrm {H_{2}C{=}CH_{2}\ +\ Br_{2}\ +\ H_{2}O\longrightarrow \ Br{-}CH_{2}{-}CH_{2}{-}OH+\ HBr} }$

If a larger excess of sodium bromide is dissolved in the bromine water beforehand, only 1,2-dibromoethane is formed.

The reaction of bromine water with an alkane takes place only with a catalyst and / or under high-energy light. The bromine water is also decolorized, but HBr is also formed , since the reaction mechanism is now a radical substitution . It is very easy to distinguish between alkanes and alkenes.

${\displaystyle \mathrm {C_{2}H_{6}\ +\ Br_{2}\longrightarrow \ C_{2}H_{5}Br\ +\ HBr} }$

Substitution reaction of ethane and bromine. This results bromoethane and hydrogen bromide.

safety instructions

The escaping bromine fumes are very poisonous. They must not be inhaled as the bromine fumes corrode the alveoli and the airways. For details see article Brom .

Individual evidence

1. ↑ ChidS: Electrophilic addition of bromine to ethene