Electrophilic Substitution of Aniline

CHEReactions Of Amines

The $\text{-}NH_2$ group is a powerful activating, ortho/para-directing substituent, so aniline undergoes electrophilic substitution very readily — often too readily, giving over-substitution. ISC questions focus on bromination, controlled nitration, and sulphonation to sulphanilic acid.

Bromination (no catalyst)

C_6H_5NH_2 + 3\,Br_2 \rightarrow C_6H_2Br_3NH_2 + 3\,HBr

product is

2,4,6

-tribromoaniline, a white precipitate

Sulphonation to sulphanilic acid

C_6H_5NH_2 \xrightarrow{conc.\,H_2SO_4,\ 453\,K} p\text{-}H_2N\text{-}C_6H_4\text{-}SO_3H

the product exists as the zwitterion

p\text{-}{}^+H_3N\text{-}C_6H_4\text{-}SO_3^-

Aniline $+$ bromine water (no catalyst) $\rightarrow 2,4,6$ -tribromoaniline (white precipitate) $+ 3HBr$ ; shows how strongly $\text{-}NH_2$ activates the ring.
$\text{-}NH_2$ is a strong activator and an $o$ / $p$ -director because the N lone pair raises electron density at the ortho and para positions through resonance.
Clean monosubstitution requires protecting $\text{-}NH_2$ : acetylate to acetanilide first, then react, then hydrolyse off the acetyl group.
$p$ -nitroaniline: acetylate aniline $\rightarrow$ acetanilide, nitrate ( $conc.\,HNO_3/H_2SO_4$ , mainly $para$ ), then hydrolyse to release $p$ -nitroaniline.
Direct nitration of aniline fails to give a clean product because protonation to $\text{-}NH_3^+$ (meta-director) and oxidation by $HNO_3$ scramble the result.
The acetanilide route gives predominantly the $para$ product because the bulky $\text{-}NHCOCH_3$ group sterically blocks the ortho positions.
Sulphonation: aniline $+ conc.\,H_2SO_4$ forms anilinium hydrogen sulphate, then heating ('baking') at $\sim 453\,K$ gives $p$ -aminobenzenesulphonic acid (sulphanilic acid).
Sulphanilic acid exists as a zwitterion (inner salt): the basic $\text{-}NH_2$ is protonated to $\text{-}NH_3^+$ and the acidic $\text{-}SO_3H$ is deprotonated to $\text{-}SO_3^-$ in the same molecule.
The zwitterion explains sulphanilic acid's high melting point and low solubility in organic solvents, like an internal salt.
Sulphanilic acid is the standard partner for diazo coupling to make azo dyes (e.g. methyl orange).

Where the marks go

Writing monobromination of aniline with bromine water — without protection it gives the $2,4,6$ -tribromo product.
Trying to nitrate aniline directly for a clean $p$ -nitro product — protonation to $\text{-}NH_3^+$ (a meta-director) and oxidation spoil it; acetylation first is essential.
Forgetting that protonated aniline ( $\text{-}NH_3^+$ ) becomes meta-directing and deactivating, the opposite of free $\text{-}NH_2$ .
Drawing sulphanilic acid as a neutral molecule rather than the zwitterion $\,{}^+H_3N\text{-}C_6H_4\text{-}SO_3^-$ .
Calling $\text{-}NH_2$ a deactivating group — it is strongly activating and $o$ / $p$ -directing in the free amine.

How the board asks it

Give reasons $\text{-}NH_2$ as a strong activator and o/p-director
Account for the following: aniline reacts with bromine water in the absence of any catalyst to give a precipitate, whereas benzene does not react with bromine water at all.
Predict the productbromination of aniline with bromine water
Write the equation for the reaction of aniline with excess bromine water, naming the organic product formed and stating its appearance.
Conversionthe acetanilide protection route to $p$ -nitroaniline
How will you convert aniline into $p$ -nitroaniline? Give the equations and explain why aniline is first acetylated instead of being nitrated directly.
Conversionsulphonation of aniline to sulphanilic acid
How will you obtain sulphanilic acid from aniline? Give the reagents and conditions used at each step.
Structure / namingthe zwitterion structure of sulphanilic acid
Draw the structure of sulphanilic acid and explain why it exists as a zwitterion ( $\,{}^+H_3N\text{-}C_6H_4\text{-}SO_3^-$ ) with a high melting point and low solubility in organic solvents.
Give reasonsprotonation of $\text{-}NH_2$ to $\text{-}NH_3^+$ during direct nitration
Give reasons: direct nitration of aniline with $conc.\,HNO_3/H_2SO_4$ gives an appreciable amount of the meta product along with the ortho and para isomers.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.