Environmental and safety considerations are paramount when performing bromination and nitration reactions in a laboratory setting. Both reactions involve hazardous chemicals that require careful handling and appropriate protective equipment.

For bromination, bromine is a highly reactive and corrosive liquid that can cause severe burns on contact with skin and damage respiratory organs if inhaled. It’s vital to perform this reaction in a well-ventilated area, preferably under a fume hood. Safety goggles, gloves, and lab coats are essential to protect against splashes. Any spills should be neutralised and cleaned up immediately.

Nitration reactions involve concentrated acids, specifically nitric and sulphuric acids, which are highly corrosive. Direct contact can cause severe skin burns and eye damage, and their vapours can be harmful if inhaled. These reactions should also be conducted in a fume hood with appropriate personal protective equipment. Additionally, concentrated acids should be handled with extreme care, and any spills should be neutralised promptly.

Environmental considerations include proper disposal of waste products. These chemicals should not be poured down the drain as they can harm the environment. Used acids and bromine solutions must be neutralised and disposed of according to local hazardous waste disposal regulations. It’s also crucial to minimise the use of these chemicals to reduce waste and environmental impact.

In the nitration of phenol, ortho and para nitrophenols are the major products due to the directing effect of the hydroxyl group. The hydroxyl group is an activating and ortho/para-directing group, meaning that it not only makes the aromatic ring more reactive but also influences where the electrophile will add to the ring. The oxygen atom of the hydroxyl group donates electron density into the aromatic ring through resonance. This increased electron density is particularly pronounced at the ortho and para positions, making them more nucleophilic and hence more favorable sites for electrophilic attack. In contrast, the meta position does not benefit from this increased electron density and is less likely to be attacked. Therefore, when nitric acid reacts with phenol, the nitronium ion (NO₂⁺), which is the electrophile in this reaction, preferentially attacks the ortho and para positions, leading to the formation of ortho and para nitrophenols as the major products.

The mechanistic differences between the electrophilic aromatic substitution reactions of phenol and benzene primarily stem from the presence of the activating hydroxyl group in phenol.

In benzene, the mechanism involves the initial formation of an electrophile, which then attacks the aromatic ring to form a positively charged intermediate, known as the sigma complex or arenium ion. This intermediate is relatively unstable due to the disruption of the aromaticity of benzene. The reaction proceeds through a series of steps involving the loss of a hydrogen ion to reform the aromatic ring.

In phenol, the presence of the hydroxyl group significantly alters this mechanism. The hydroxyl group donates electron density into the aromatic ring through resonance, increasing the electron density, particularly at the ortho and para positions. This increased electron density makes the ring more reactive towards electrophiles, allowing the reaction to proceed more readily and often at milder conditions compared to benzene. The formation of the arenium ion intermediate is more stabilised in phenol due to the resonance interaction with the hydroxyl group. This stabilisation not only speeds up the reaction but also directs the electrophile predominantly to the ortho and para positions, leading to more specific substitution patterns compared to the relatively non-selective substitution seen in benzene.

Phenol can be brominated using just bromine water due to its activated aromatic ring, which results from the electron-donating effect of the hydroxyl group. This group increases the electron density of the phenol molecule, making it more nucleophilic and reactive towards electrophiles like bromine. As a result, phenol reacts readily with bromine in water, forming a white precipitate of 2,4,6-tribromophenol. In contrast, benzene's ring is relatively electron-poor and lacks such activating groups, rendering it less reactive towards electrophiles. Therefore, for the bromination of benzene, a halogen carrier like iron(III) bromide (FeBr₃) is needed. This carrier facilitates the formation of a more reactive electrophile, the bromonium ion (Br⁺), which can then attack the benzene ring. The halogen carrier essentially serves to increase the electrophilic nature of the bromine, compensating for the lower reactivity of the benzene ring compared to the activated ring of phenol.

The hydroxyl group in phenol significantly influences the stability of the arenium ion formed during electrophilic aromatic substitution. In these reactions, an electrophile attacks the aromatic ring, forming a positively charged intermediate known as the arenium ion. The stability of this ion is crucial for the reaction to proceed efficiently. In phenol, the oxygen atom of the hydroxyl group donates electron density into the aromatic ring through resonance. This delocalisation of electrons helps to stabilise the positive charge on the arenium ion, particularly when the electrophile attacks at the ortho or para positions relative to the hydroxyl group. This increased stability leads to a lower activation energy for the reaction and contributes to the enhanced reactivity of phenol compared to benzene, where such stabilisation is absent. Additionally, the hydroxyl group's ability to stabilise the intermediate explains why certain positions on the ring are favoured over others, leading to a preference for ortho and para products in electrophilic substitution reactions involving phenol.