Question

\(p\) -Nitrobenzyl alcohol is more acidic than benzyl alcohol, but \(p\) -methoxybenzyl alcohol is less acidic. Explain.

Short answer

p-Nitrobenzyl alcohol is more acidic due to electron-withdrawing stabilization, while p-methoxybenzyl alcohol is less acidic due to electron-donating destabilization.

Step-by-step solution

Understand the Concept of Acidity

The acidity of a compound is its ability to donate a proton (H+). In organic compounds, this often relates to the stability of the anion formed after the proton is lost. A more acidic compound has a more stable anion.

Examine Substituent Effects on the Benzyl Alcohol

In benzyl alcohol, the hydroxyl group (-OH) is attached to the benzene ring through a methylene (-CH2-) bridge. The acidity is affected by substituents on the benzene ring, which can either stabilize or destabilize the anion formed upon deprotonation.

Analyze the Effect of the Nitro Group

In p-nitrobenzyl alcohol, the nitro group (-NO2) is an electron-withdrawing group. It stabilizes the anion formed after the alcohol donates a proton by delocalizing the negative charge through resonance, making p-nitrobenzyl alcohol more acidic than benzyl alcohol.

Analyze the Effect of the Methoxy Group

In p-methoxybenzyl alcohol, the methoxy group (-OCH3) is an electron-donating group. It destabilizes the anion formed after proton donation by increasing electron density, making p-methoxybenzyl alcohol less acidic than benzyl alcohol.

Compare the Effects

The nitro group increases acidity due to its electron-withdrawing nature, stabilizing the anion. The methoxy group decreases acidity due to its electron-donating nature, destabilizing the anion. Thus, the electron withdrawal or donation by the groups affects acidity.

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are essential players in organic chemistry, especially when it comes to influencing acidity. These groups remove or withdraw electron density from the surrounding area through different mechanisms, such as inductive effects or resonance. By withdrawing electron density, they stabilize any negative charges that form during reactions.

In the context of acidity, when an organic compound loses a proton and forms an anion, EWGs help stabilize this anion. The stabilization occurs because the electron-withdrawing group distributes the negative charge more evenly over the molecule.

Consider the nitro group (-NO2) as a classic example of an EWG. Attached to organic structures, it can draw electron density toward itself and spread the negative charge across the molecule through resonance. This results in increased stability of the anion formed and, therefore, a higher acidity level. That's why p-nitrobenzyl alcohol is more acidic than plain benzyl alcohol— the nitro group enhances stability by withdrawing electrons.

Electron-Donating Groups

On the flip side of electron-withdrawing groups, we have electron-donating groups (EDGs). EDGs are substituents that release or push electron density towards other parts of the molecule. This can happen via resonance or inductive effects.

When discussing acidity, electron-donating groups have the opposite effect of electron-withdrawing groups. Instead of stabilizing an anion, EDGs tend to destabilize it by increasing electron density, particularly in areas where negative charges might congregate.

Take the methoxy group (-OCH3) as an example. This group is a well-known EDG, as it pushes electron density toward other parts of the molecule. As a result, when the compound becomes an anion, the extra electron density makes it less stable. This reduced stability leads to lower acidity in comparison to benzyl alcohol without the methoxy group. Hence, p-methoxybenzyl alcohol is less acidic than benzyl alcohol because the methoxy group donates electrons that destabilize the anion.

Substituent Effects on Acidity

Substituents attached to organic compounds can significantly impact acidity. These effects are largely due to the nature of the substituent: whether it acts as an electron-withdrawing or electron-donating group.

When you attach a substituent to a benzene ring, it alters the electronic distribution within the molecule. The resulting effect could be a change in acidity due to the stabilization or destabilization of the anion formed when the compound loses a proton.

• Electron-Withdrawing Substituents: These enhance acidity. By stabilizing the resulting anion, they make it easier for the compound to lose a proton.
• Electron-Donating Substituents: These reduce acidity. By introducing additional electron density, they destabilize the anion, making it less favorable for the compound to lose a proton.

Understanding the nature of the substituent is crucial in predicting the acidity of organic compounds. Each substituent shifts the acidity based on its ability to influence electron density and charge stability. This explains the differing acidities of compounds like p-nitrobenzyl alcohol and p-methoxybenzyl alcohol.