9.6.7. Electrophilic substitution

You have read earlier that aniline is a resonance hybrid of five structures. Where do you find the maximum electron density in these structures? Ortho- and para-positions to the –NH2 group become centres of high electron density. Thus –NH2 group is ortho and para directing and a powerful activating group.

Aniline, which is an amine with an amino group (-NH2) directly attached to a benzene ring, exhibits resonance. This means that the structure can be represented in multiple ways, allowing for the delocalization of electrons. The resonance forms show that the ortho (next to) and para (across from) positions relative to the amino group have higher electron density. Because of this, these positions become particularly reactive in electrophilic substitution reactions, allowing new groups to attach to the benzene ring at these locations.

Think of the electron density like a crowded party. The ortho and para positions are the hottest spots at the party where everyone wants to be—there’s more energy and excitement there. Just like how guests are likely to hang out where the action is, chemical reactions are more likely to happen at these electron-rich areas on the benzene ring.

Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.

When aniline is treated with bromine water, it takes place readily due to the high electron density at the ortho and para positions. The amino group enhances the reactivity of the benzene ring, making it easier for bromine to add to those sites. The result is the formation of 2,4,6-tribromoaniline, which appears as a white precipitate. Bromination is a classic example of electrophilic substitution where the benzene part of aniline is substituted by bromine atoms.

Imagine trying to add decorations to a cake (the cake being the benzene ring). The more colorful and exciting the cake is (thanks to the amino group), the easier it is to add decorations (bromine atoms) on the most attractive spots (ortho and para positions).

The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para-positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH2 group be controlled? This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.

The high reactivity of aniline during electrophilic substitution can be a challenge, especially when you only want to add one group to the ring. To manage this, chemists can 'protect' the amino group. Protecting the amino group involves converting it into an acetamido group (-NHCOCH3) using acetylation, which decreases its electron-donating ability and thus reduces its reactivity. This allows a controlled substitution at the benzene ring. Once the desired substitution is accomplished at a lower reactivity, the protecting group can be removed through hydrolysis to yield the final product.

Imagine a celebrity trying to go out without attracting too much attention. They might wear a disguise (the protection) so that they can go to a quiet restaurant (perform the substitution reaction) without being swarmed by fans. Once they’re safely in, they can take off the disguise and enjoy their meal in peace (perform hydrolysis to yield the aromatic amine).

Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amounts of meta derivative are also formed.

When attempting to nitrate aniline, the presence of both the amino group and acidic conditions creates complex reactions. Nitration typically aims to introduce a nitro group (-NO2) into the aniline. However, the strongly acidic medium can protonate the amine group, forming the anilinium ion. This protonation changes the directing influence of the amino group to a meta position rather than ortho or para, leading to a mix of products, including unwanted tarry by-products.

It’s like trying to organize a community cleanup event. If you have excited volunteers (the amino group) but suddenly they are given confusing instructions (the acidic conditions), some might end up going in the wrong direction (producing meta nitration products), leading to a mixed and potentially messy result.

Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.

In the presence of concentrated sulfuric acid, aniline forms a product called anilinium hydrogensulphate. This is a soluble salt that can be treated further. When heated, this salt converts into sulphanilic acid by replacing the hydrogen in the amino group with a sulfonic acid group (-SO3H). This reaction is significant because sulphanilic acid is an important compound in various applications, including dye production.

Think of aniline like a sponge soaking up a new liquid (sulfonic acid). Just as a sponge can hold onto liquid until it is full, aniline adds sulfonic acid to become sulphanilic acid—an important compound that can ‘dye’ fabrics just like how a sponge can be used to clean or color things.

Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.

Friedel-Crafts reactions involve electrophilic aromatic substitution, where an alkyl or acyl group is introduced to the benzene ring. However, since aniline can form a salt with aluminum chloride (a Lewis acid), it leads to the protonation of the amino nitrogen. This protonated form acts as a deactivating group due to its positive charge, making it less reactive towards further electrophilic attack and thus unable to participate effectively in Friedel-Crafts reactions.

Think of aniline as a club bouncer who has to keep other rowdy guests in line (the electrophiles). But if they become overwhelmed (protonated), they either can’t do their job or end up pushing everyone away (becoming a deactivating presence), letting fewer new guests enter (alkyl or acyl groups added).

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