Question

Which one of the following compounds is most acidic? (a) \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) (b) Oc1ccccc1 (c) O=[N+]([O-])c1ccccc1O (d) Cc1ccccc1O

Short answer

Compound (c) is the most acidic.

Step-by-step solution

Identify Functional Groups

The first step is to identify the functional groups present in each compound as these often dictate the compound's acidity.(a) \(\mathrm{Cl}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) contains an alcohol group \(\mathrm{-OH}\).(b) Phenol (\(\mathrm{Oc1ccccc1}\)) has a hydroxyl group attached to an aromatic ring.(c) The compound \(\mathrm{O=[N+]([O-])c1ccccc1O}\) contains a nitro group \(\mathrm{-NO_2}\) and a phenolic hydroxyl group.(d) Cresol \(\mathrm{Cc1ccccc1O}\), another phenol derivative, has a methyl group attached to the aromatic ring hosting the hydroxyl group.

Analyze Acidity Factors

Next, analyze factors that affect acidity such as electron-withdrawing groups and resonance stabilization.(a) Electron-withdrawing chlorine can increase acidity, but the effect is distant from the hydroxyl group.(b) In phenol, the electron delocalization in the aromatic ring stabilizes the negative charge formed after deprotonation.(c) The nitro group in \((c)\) is a strong electron-withdrawing group and enhances acidity significantly by stabilizing the phenoxide ion through resonance.(d) Cresol \((d)\) with a methyl group weakens acidity compared to plain phenol because the methyl group is electron-donating.

Determine the Most Acidic Compound

Compare the effects of these substituents on acidity:- Compound (c) has both a phenolic group and an electron-withdrawing nitro group, greatly enhancing its acidic character by stabilizing the conjugate base through resonance and electron-withdrawing effects.- Without additional electron-withdrawing groups, compound (b) phenol is less acidic than compound (c).- Compounds (a) and (d), being alcohols connected to either electron-withdrawing or electron-donating groups, are less acidic than phenolic compounds, especially (c).Thus, \(\mathrm{O=[N+]([O-])c1ccccc1O}\) is the most acidic due to the strong electron-withdrawing nitro group enhancing stability of the conjugate base.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have certain properties and react predictably. They play a crucial role in determining a compound's acidity.
For example, the hydroxyl group \(-OH\) is a common functional group found in alcohols and phenols. In alcohols, this group is generally attached to a saturated carbon atom, making alcohols weakly acidic. However, in phenols, the \(-OH\) is attached directly to an aromatic ring, significantly affecting its acidity.
Another example is the nitro group \(-NO_2\), which is highly relevant in altering a compound's acidity due to its electron-withdrawing nature. Identifying these groups is a fundamental step in analyzing a compound's acidic properties. They set the stage for understanding how the molecular structure will influence acidity.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are elements or functional groups that pull electron density away from other parts of a molecule. This effect can significantly enhance a compound's acidity.
These groups include nitro groups \(-NO_2\), halogens like chlorine (\(\mathrm{Cl}\)), and carbonyls. They stabilize the negative charge of a conjugate base after deprotonation, thus increasing the compound's acidity.

• The more electronegative the group, the stronger its electron-withdrawing ability.
• Distance plays a role too—the closer the EWG to the acidic site, the greater the effect.

In the exercise, the compound with a nitro group is the most acidic. This is because the powerful electron-withdrawing effect of the nitro group enhances the stability of the conjugate base, making it easier for the compound to donate a proton.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across a molecule. This electron delocalization can stabilize charged particles, such anions, which form when weak acids donate protons.
Phenolic compounds, like in our exercise, exhibit resonance stabilization. When a phenol loses a proton, the resulting negative charge is delocalized across the aromatic ring, enhancing the stability of the phenoxide ion.

• Resonance allows for multiple structures, spreading out charge and reducing localized strain.
• More resonance structures generally correlate with greater stability.

In compound (c) from the exercise, resonance is particularly effective because the presence of a nitro group not only withdraws electrons but also enhances resonance by allowing the negative charge to be efficiently delocalized through the aromatic ring.

Phenolic Compound

Phenolic compounds are characterized by an \(-OH\) group bound directly to an aromatic benzene ring, significantly affecting their acidity.
Phenol itself is more acidic than typical alcohols because the aromatic system allows for resonance stabilization of the phenoxide ion.

• Substituents on the aromatic ring can further influence acidity.
• Electron-withdrawing substituents increase acidity, while electron-donating groups decrease it.

In the context of the exercise, phenol, and cresol (a methylated derivative) are considered. Comparing their acidity shows the methyl group, an electron-donating substituent, decreases acidity. In contrast, a phenol with a nitro group exhibits increased acidity due to enhanced electron delocalization and stabilization, illustrating the interplay of substituents in altering phenolic compound acidity.