7.4.4.3.1. Electrophilic aromatic substitution

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions. The –OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron-rich due to the resonance effect caused by –OH group.

Electrophilic aromatic substitution is a type of reaction that occurs when an electrophile replaces a hydrogen atom on an aromatic ring. In phenols, the hydroxyl (-OH) group makes the ring more reactive because it increases the density of electrons in the aromatic system. As a result, this reaction commonly occurs at the ortho (adjacent) or para (across) positions relative to the –OH group. This is due to the resonance structures that can be stabilized by the –OH group.

You can think of the aromatic ring as a party where the –OH group is the host making it more inviting for guests (electrophiles) to join and interact. Just like how a welcoming environment encourages guests to gather close, the resonance effect allows the electrophiles to easily approach and bond with the aromatic ring.

The common electrophilic aromatic substitution reactions taking place in phenol are as follows: (i) Nitration: With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols.

When phenol undergoes nitration, it reacts with dilute nitric acid (HNO3) under controlled conditions. This results in a mixture of products: ortho-nitrophenol and para-nitrophenol. The ortho product forms faster because it is in closer proximity to the –OH group, while the para product is also favored because it is more stable. The separation of these isomers can be done through steam distillation due to their differing boiling points.

Imagine you are making two types of fruit punch by adding lemon juice (phenol) to a mixture of water and sugar (nitration reaction). You can taste the punch immediately next to the lemon (ortho-nitrophenol) and the one further away (para-nitrophenol). Even though both recipes have lemon juice, the immediate one tastes different from the one a bit farther away since the flavors mix differently.

Halogenation: On treating phenol with bromine, different reaction products are formed under different experimental conditions. (a) When the reaction is carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed. (b) When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as a white precipitate.

Halogenation involves the introduction of halogen atoms (like bromine) into the phenol molecule. Under specific conditions, like using non-polar solvents, phenol can undergo a more controlled reaction resulting in monobromophenols. However, if bromine water is used, this encourages multiple bromination leading to 2,4,6-tribromophenol, which precipitates out of the solution due to its limited solubility.

Think of this like adding spices to a dish. If you sprinkle a little bit of salt (non-polar solvent), you'll get just the right amount of flavor. But if you pour a brine solution (bromine water), it overwhelms the dish and results in an entirely different taste, much like how adding too much leads to multiple bromine atoms being incorporated into the product.

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