Question

Why is Bromine used when dissolved in $CC{l_4}$ or water?

Answer 1

Bromine is reddish- brown in colour. When bromine in carbon tetrachloride is added to alkenes the reddish-brown colour of bromine gets discharged, due to the formation of vic- dibromide which is colourless. This is an important test for the detection of double bonds. Bromine dissolves in organic liquids like alcohol, ether etc. Also, bromine is slightly soluble in water.

Complete answer: Bromine dissolves in carbon tetrachloride, as both bromine and carbon tetrachloride are non-polar. Moreover, like dissolves in like, hence bromine dissolves in carbon tetrachloride.
Bromine- water cannot be stored for long- term because bromine will react with water to form hypobromous acid with molecular formula $HBr$ . But carbon tetrachloride solvent allows to store bromine for a longer term as carbon tetrachloride does not react with bromine.
Water helps in the formation of bromohydrin, which eventually helps to add bromine and $OH$ to the double bonds in alkenes. This leads to the breaking of the double bond of the alkene, while the bromine atom gets attached to each carbon atom, resulting in the formation of vicinal dibromides. The reddish brown colour of the bromine becomes colourless during this reaction.
$C{H_2} = C{H_2} + B{r_2}\xrightarrow{{CC{l_4}}}BrC{H_2}C{H_2}Br$

Note:
When alkenes are heated with bromine or chlorine at a high temperature of about $773\;K$ , the hydrogen atom of allylic carbon is substituted with halogen atoms forming alkyl halide. Hence, halogens do not add to the double bond at higher temperature but bring about substitution at allylic position. At higher temperatures, the addition reaction of halogens to alkenes is reversible so that the addition product formed during the reaction undergoes dehalogenation more rapidly than it is formed.