Question

Explain why Aniline with ${\text{B}}{{\text{r}}_{\text{2}}}{\text{/O}}{{\text{H}}^{\text{ - }}}$ gives ${\text{2,4,6 - tribromoaniline, while phNM}}{{\text{e}}_{\text{2}}}$ gives meta nitro derivative when mono nitrated with ${\text{HN}}{{\text{O}}_{\text{3}}}{\text{/}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$.

Answer 1

Aniline is an organic compound with the formula ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{H}}_{\text{2}}}$. With a phenyl group attached to an amino group, aniline is the simplest aromatic amine. Chemically, it has an electron-rich benzene derivative which reacts rapidly in electrophilic aromatic substitution reactions. Aniline is a weaker base and a poorer nucleophile than other structurally similar aliphatic amine.

Complete step by step answer:
Aniline is a benzene ring with an ${\text{N}}{{\text{H}}_{\text{2}}}$ group attached to it. And this ${\text{N}}{{\text{H}}_{\text{2}}}$ group has a lone pair of electrons and the hybridization is between ${\text{s}}{{\text{p}}^{\text{3}}}{\text{ and s}}{{\text{p}}^{\text{2}}}$. Because of which the lone pair is in an ${\text{s}}{{\text{p}}^{\text{x}}}$ hybrid orbital with high ${\text{p}}$ character. The amino group in aniline is flatter than that in an aliphatic amine, which helps in conjugation of lone pairs with the aryl substituent.
Here, a lone pair of electrons on nitrogen atom in ${\text{N}}{{\text{H}}_{\text{2}}}$ group spread into the benzene ring and delocalization of electron occur. The negative charge develops on the ortho and para (O AND P) positions and the change in density is higher during delocalization.

So, ${\text{ - N}}{{\text{H}}_{\text{2}}}$ group, activates the ortho and para positive.
When bromine water is added to aniline, the bromine water is decolourised and a white precipitate of 2,4,6, tribromo aniline is formed.
To generate the mono-substituted product i.e., ${\text{4}}$-bromo aniline, a protection with acetyl chloride is provided and then hydrolysed back to reform aniline.
${\text{PhNM}}{{\text{e}}_{\text{2}}}$ acts as a base in acidic condition and the formation of ${\text{NHM}}{{\text{e}}_{\text{2}}}$ which is meta directing group when mono nitrated with ${\text{HN}}{{\text{O}}_{\text{3}}}{\text{/}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$.

Additional Information:
Aniline is highly susceptible to electrophilic reactions which shows that it is an enamine which enhances the electron donating ability of the ring. For example, reaction of aniline with sulfuric acid (${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{) at 18}}{{\text{0}}^{\text{o}}}{\text{c}}$ produces sulfanilic acid (${{\text{H}}_{\text{2}}}{\text{N}}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{4}}}{\text{S}}{{\text{O}}_{\text{3}}}{\text{H}}$).

Note: The pyramidal geometry is because of two factor ${\text{1}}$) stabilization of the ${\text{N}}$ lone pair in an orbital with ${\text{s}}$character which favors pyramidalization ${\text{2}}$) delocalization of the ${\text{N}}$lone pair into the aryl ring favors planarity (giving the best overlap between the orbitals of the benzene ring and a lone pair in a pure ${\text{p}}$ orbital)