Question

Which of the following will highly increase acidity of phenol? (A) Alkylation of phenol (B) Nitration of phenol (C) Chlorination of phenol (D) Acylation of phenol

Short answer

The reaction that highly increases the acidity of phenol is (B) Nitration of phenol. This is because the -NO2 group has a strong electron-withdrawing effect, making it easier for phenol to donate its proton.

Step-by-step solution

Identify the Effects of Alkylation on Phenol

Alkylation is the process of adding an alkyl group (R) to phenol. When an alkyl group (R) is attached to the phenol, it results in a negative inductive effect. This means that the alkyl group pulls electron density away from the phenol. The electron-donating ability of the alkyl group makes it difficult for the oxygen to lose a proton, resulting in a decrease of acidity.

Identify the Effects of Nitration on Phenol

Nitration is the process of adding a nitro group (-NO2) to phenol. The presence of the nitro group has a strong electron-withdrawing effect through resonance, which means that it pulls electron density away from the phenol. This makes it easier for the oxygen to lose its proton, resulting in an increase in acidity.

Identify the Effects of Chlorination on Phenol

Chlorination is the process of adding a chlorine atom (Cl) to phenol. Chlorine is moderately electron-withdrawing due to its electronegativity, which means that it slightly decreases the electron density of the phenol. However, it is not as strong of an electron-withdrawing effect as the nitro group and, therefore, would not greatly increase the acidity of phenol.

Identify the Effects of Acylation on Phenol

Acylation is the process of adding an acyl group (RCO-) to phenol. The acylation of phenol will not have much effect on the acidity, because the presence of carbonyl (-CO) group does not have a strong electron- withdrawing effect.

Determine Which Reaction Causes the Greatest Increase in Acidity

Based on the effect of each reaction on the electron density of phenol and its ability to donate a proton, we can determine that nitration of phenol (option B) has the most significant impact on acidity due to its strong electron-withdrawing effect of the -NO2 group. Therefore, the answer is (B) Nitration of phenol.

Key concepts

Phenol Acidity

Phenol is a special type of alcohol where a hydroxyl group (-OH) is directly attached to a benzene ring. Its acidity is a key characteristic.
Phenol can donate a proton from the hydroxyl group, forming phenoxide ion. This ability to lose a proton determines its acidity in comparison to other alcohols.
In organic chemistry, acidity is often associated with the stability of the anion produced. For phenol, the phenoxide ion is stabilized by resonance. The negative charge is distributed over the aromatic ring, making it more stable than other alkoxide ions.
The acidity of phenol makes it more reactive in chemical processes and is a crucial factor when controlling reactions.

Electron-Withdrawing Effects

Electron-withdrawing groups are key players in organic chemistry as they pull electrons away from other areas of a molecule.
These groups can either stabilize or destabilize areas of the molecule they are attached to.
When an electron-withdrawing group, like a nitro group (-NO2), is added to phenol, it increases acidity.
This is because the electron-withdrawing group pulls electrons away from the hydroxyl group. This makes it easier for the oxygen to lose its proton.
Better proton donation means higher acidity, so when nitration occurs and a nitro group is added, phenol's acidity significantly increases.

Nitration Reaction

Nitration is a powerful chemical reaction that involves introducing a nitro group (-NO2) into an organic compound. For phenol, nitration notably increases acidity.
The nitro group acts as a strong electron-withdrawing group. By pulling electrons away from the phenolic oxygen, it enhances the strength of the phenoxide ion.
The nitro group's resonance effects distribute the negative charge, thereby improving stability. This facilitates easier proton loss, meaning phenol can behave as a stronger acid.
Nitration is highly valuable in industrial chemistry, forming more acidic phenols that serve as important building blocks for medicines, dyes, and other chemicals.

Inductive Effect

The inductive effect in organic chemistry involves the transfer of charge through a chain of atoms in a molecule.
It arises from the differing electronegativities of atoms or groups along that chain.
This effect causes a shift in electron density, influencing molecular properties, like acidity.
In phenol, introducing groups like alkyl groups can result in a negative inductive effect. Such groups push electron density toward the hydroxyl group, making it less acidic.
Conversely, electron-withdrawing inductive effects from groups like chlorine or acyl groups can stabilize negative charge, thus subtly affecting acidity.
Understanding inductive effects helps fine-tune reaction conditions and predicts changes in reactivity and acidity.