why a white precipitate is formed when aqeous bromine is added to aniline
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When aqueous bromine (Br2 dissolved in water) is added to aniline (a primary aromatic amine), a white precipitate is formed. This reaction is known as the bromination of aniline.
The white precipitate is actually an insoluble compound called 2,4,6-tribromoaniline. This compound forms when the bromine molecule (Br2) reacts with the amino group (-NH2) of aniline.
To understand why this reaction occurs, we need to look at the reactivity of aniline and the nature of the bromine molecule. Aniline has a lone pair of electrons on its nitrogen atom, which makes it a nucleophile. Bromine, on the other hand, is an electrophile because it has a partial positive charge due to its high electronegativity.
When bromine is added to aniline, a nucleophilic substitution reaction occurs. The lone pair of electrons on the nitrogen atom of aniline attacks the electrophilic bromine atom. This results in the replacement of one of the hydrogen atoms attached to the nitrogen with a bromine atom.
The reaction can be summarized as follows:
C6H5NH2 + Br2 -> C6H4BrNH2 + HBr
C6H5NH2 (aniline) + Br2 (bromine) -> C6H4BrNH2 (2,4,6-tribromoaniline) + HBr (hydrobromic acid)
Due to the presence of three bromine atoms, the product, 2,4,6-tribromoaniline, is less soluble in water compared to aniline. As a result, it precipitates out of the solution as a white solid.
Overall, the white precipitate observed when aqueous bromine is added to aniline is the result of a nucleophilic substitution reaction between the electron-rich amine group of aniline and the electrophilic bromine molecule.