In the presence of very strong acid, phenol is brominated in the meta position. Why do you suppose this is the case? When phenol is brominated in slightly basic conditions, tribromophenol is the major product. Why would basic conditions favor polybromination?

Electrophilic aromatic substitution (EAS) is a fundamental reaction mechanism in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. This process is highly significant because it allows the introduction of various functional groups into aromatic compounds, facilitating further chemical transformations.
In EAS, the aromatic ring acts as a nucleophile, reacting with an electrophile (a positively charged or neutral species with a vacant orbital). The general mechanism involves three steps:

• Formation of the electrophile: In many cases, a catalyst (e.g., a lewis acid) is required to generate a sufficiently reactive electrophile.
• Formation of the arenium ion: The aromatic ring donates a pair of electrons to the electrophile, creating a positively charged intermediate (arenium ion or sigma complex).
• Deprotonation: The arenium ion loses a proton, restoring the aromaticity of the ring.

These steps highlight the importance of the ring's electron density, which can be influenced by substituents present on the ring.
Understanding EAS is key to explaining phenol bromination behavior. The -OH group in phenol usually directs the bromine to the ortho and para positions due to its electron-donating properties.

In electrophilic aromatic substitution reactions, substituents on the aromatic ring can significantly influence the rate and position of substitution by either activating or deactivating the ring.
Activating groups are those that increase the electron density on the aromatic ring, making it more reactive towards electrophiles. Examples include -OH, -NH2, and -OCH3 groups. These groups are typically electron-donating due to resonance or inductive effects. For instance, in phenol, the -OH group donates electron density through resonance, creating a higher electron density at the ortho and para positions.

• The increased electron density stabilizes the transition state of the EAS reaction, lowering the activation energy and increasing the reaction rate.
• The resonance effect creates negative charge densities, particularly at the ortho and para positions, favoring substitution at these sites.

However, reaction conditions can modify this behavior. In acidic conditions, the -OH group can become protonated, reducing its electron-donating ability and making even the meta position more favorable.

The behavior of substituted aromatic compounds in EAS can be dramatically altered by the reaction conditions, such as pH and the presence of catalysts.
Under very strong acidic conditions, phenol can become protonated to form phenol cation. This protonation diminishes the electron-donating effects of the -OH group, as described earlier, altering the usual ortho-para directing influence.

• In strongly acidic environments, the protonated phenol cation becomes an electron-poorer species, directing electrophiles to the meta position as the ortho and para sites are less favorable due to reduced electron density.

In contrast, under slightly basic conditions, the phenol might be deprotonated to form the phenoxide ion. This increases the electron-donating ability of the -OH group, enhancing overall electron density on the aromatic ring:

• Enhanced electron density leads to increased reactivity towards bromine, resulting in multiple bromination at the ortho and para positions, often leading to polybrominated products like tribromophenol.
• Basic conditions stabilize reactive intermediates, shifting the equilibrium towards products.

This demonstrates why different reaction conditions lead to different substitution patterns and product distributions in phenol bromination.