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

The most acidic compound is (c) p-Nitrophenol.

Step-by-step solution

Identify each compound

Let's write down each molecule starting from the given choices:(a) Chloroethanol: \( \text{Cl}-\text{CH}_2-\text{CH}_2-\text{OH} \) (b) Phenol: Given as SMILES Oc1ccccc1, which translates to \( \text{C}_6\text{H}_5\text{OH} \)(c) p-Nitrophenol: Given as SMILES O=[N+]([O-])c1ccccc1O, which translates to 4-nitrophenol, \( \text{C}_6\text{H}_4(\text{NO}_2)\text{OH} \)(d) p-Cresol: Given as SMILES Cc1ccccc1O, which translates to 4-methylphenol, \( \text{C}_7\text{H}_8\text{O} \)

Understand the acidity concept

Acidity of a compound depends on its ability to donate a proton, which is influenced by the stability of its conjugate base. The more stable the conjugate base, the stronger the acid. Factors such as resonance, electronegativity, and inductive effects play crucial roles in stabilizing the conjugate base.

Analyze the conjugate bases

- For Chloroethanol (a), the conjugate base would be \( \text{Cl}-\text{CH}_2-\text{CH}_2-\text{O}^- \). The chlorine atom might inductively stabilize the base slightly through its -I effect.- For Phenol (b), the conjugate base \( \text{C}_6\text{H}_5\text{O}^- \) is stabilized by resonance across the aromatic ring.- For p-Nitrophenol (c), the conjugate base \( \text{C}_6\text{H}_4(\text{NO}_2)\text{O}^- \) is even more stabilized by resonance due to the electron-withdrawing nitro group, which makes it the most stabilized.- For p-Cresol (d), the conjugate base \( \text{CH}_3\text{C}_6\text{H}_4\text{O}^- \) does have resonance, but the methyl group donates electrons slightly, making it less acidic than nitrophenol.

Determine which is most acidic

Analyzing the resonance and inductive effects, (c) p-Nitrophenol has the most stabilized conjugate base due to the strong electron-withdrawing effect of the nitro group. This makes p-Nitrophenol the most acidic among the options.

Key concepts

Resonance Effect

Resonance is a powerful concept in organic chemistry that explains the stabilization of molecules. It involves the delocalization of electrons within a molecule across multiple atoms, potentially including the entire structure. This electron delocalization creates resonance structures which contribute to the overall hybrid of the compound.

• When considering acids like p-Nitrophenol, resonance facilitates charge dispersion across the aromatic system.
• In this case, the conjugate base (anion) of p-Nitrophenol benefits from additional resonance provided by the nitro group, spreading the negative charge over a larger area.
• This extended resonance significantly stabilizes the conjugate base, making the compound more acidic.

Overall, resonating structures are crucial because they allow for more extensive electron distribution which directly affects the molecule's reactivity and acidity.

Inductive Effect

The inductive effect is a concept that describes how electronegative atoms or functional groups can influence the electron density of other parts of a molecule. It involves the polarization of sigma bonds within the molecule due to differences in electronegativity.

• Electronegative atoms, like chlorine in chloroethanol, exert a negative inductive effect (-I effect), pulling electron density toward themselves and away from adjacent atoms.
• This electron-withdrawing effect can help to increase the stability of a conjugate base by dispersing the negative charge.
• In chloroethanol, the chlorine atom slightly stabilizes the acetate anion, making the compound more acidic than it would otherwise be.

The inductive effect is always limited to atoms in close proximity; it decreases in strength with increasing distance from the electronegative group.

Conjugate Base Stability

Understanding the stability of a conjugate base is crucial in determining the acidity of a compound. A stable conjugate base means the parent acid will more readily donate a proton, increasing its acidity.

• Stability of the conjugate base is influenced by several factors, including resonance and the presence of electron-withdrawing groups.
• For example, in p-Nitrophenol, the conjugate base is remarkably stable due to both resonance and the electron-withdrawing nature of the nitro group.
• Electron-withdrawing groups stabilize negative charges by dispersing them, and resonance allows further distribution across the aromatic system.

This combination enhances the overall stability of the conjugate base, making the parent acid more acidic.

Aromatic Compounds

Aromatic compounds, sometimes known as arenes, possess a unique set of characteristics due to their conjugated ring systems. The term "aromatic" refers to the concept of stability derived from resonance and delocalization of pi electrons in cyclic structures.

• These compounds typically exhibit high stability, contributing to their chemical inertness.
• The resonance within the aromatic ring allows dispersal of electron density, which can stabilize various intermediates and derivatives formed during reactions.
• In phenolic compounds like phenol and p-Cresol, the aromatic system aids in delocalizing the negative charge formed after deprotonation.
• This delocalization increases the acidity compared to non-aromatic alcohols, as it stabilizes the resulting conjugate base.

The aromatic framework is fundamental to understanding the unique reactivity and properties of these compounds, including their acidic behaviors.