Question

Which of the following orders is true regarding the acidic nature of phenol? (A) phenol \(>o\)-cresol \(o\)-cresol \(>o\)-nitrophenol (D) phenol \(0\)-nitrophenol

Short answer

The correct order of acidity is phenol \(< o\)-cresol \(< o\)-nitrophenol, which corresponds to option (B).

Step-by-step solution

Identify the conjugate bases

To determine the acidity order, it's crucial to identify the conjugate base of each compound. The conjugate bases are formed by removing the acidic hydrogen from the compound. In this case, the acidic hydrogen is the -OH group. So, we have the following conjugate bases: - Phenoxide ion (from phenol) - o-Cresoxide ion (from o-cresol) - o-Nitrophenoxide ion (from o-nitrophenol)

Analyze the impact of substituents

Now, let's analyze the effect of substituents on the stability of the conjugate bases. In general, electron-withdrawing groups (EWG) stabilize conjugate bases (making the parent compound more acidic), while electron-donating groups (EDG) destabilize conjugate bases (making the parent compound less acidic). - Phenoxide ion: no additional substituents. - o-Cresoxide ion: has a -CH3 group, which is an electron-donating group due to the inductive effect. - o-Nitrophenoxide ion: has a -NO2 group, which is a strong electron-withdrawing group due to resonance and inductive effects.

Compare the stability of conjugate bases

Based on the impact of substituents, let's analyze the stability of these conjugate bases: - Phenoxide ion: intermediate stability - o-Cresoxide ion: less stable than phenoxide ion due to the electron-donating effect of the -CH3 group - o-Nitrophenoxide ion: more stable than phenoxide ion due to the electron-withdrawing effect of the -NO2 group

Determine the acidity order

Since more stable conjugate bases lead to more acidic parent compounds, we can determine the acidity order: - phenol: intermediate acidity - o-cresol: less acidic than phenol - o-nitrophenol: more acidic than phenol

Choose the correct answer choice

Based on our acidity order, the correct answer is as follows: _phenol < o-cresol < o-nitrophenol_ This corresponds to option (B). So, the correct answer is: (B) phenol \(< o\)-cresol \(< o\)-nitrophenol

Key concepts

Conjugate Bases

In the context of acidity, a conjugate base is what's left of a molecule after donating a proton (H+). Take phenol for instance; its conjugate base is the phenoxide ion. It's formed when phenol loses an acidic hydrogen atom from its hydroxyl (-OH) group. Understanding the nature of conjugate bases is essential as they often dictate the strength of the original acid. A more stable conjugate base typically originates from a stronger acid.

Let's look more closely at the phenoxide ion derived from phenol. It carries a negative charge which, due to resonance, is delocalized over the aromatic ring. This delocalization spreads the charge over a larger area, stabilizing the ion and consequently making phenol itself more acidic. Hence, the chemical nature and structure of a conjugate base greatly influence the parent compound's acidity.

Substituent Effects on Acidity

Substituents attached to a compound can significantly affect its acidity. How? They alter the stability of the conjugate base through either electron-donating or electron-withdrawing effects.

Consider o-cresol and o-nitrophenol. Both are derivatives of phenol with different substituents that influence the electron density of the phenoxide ion. A basic rule of thumb is that substituents that can withdraw electrons from the conjugate base increase its stability (as seen with -NO2 in o-nitrophenol), which enhances the compound's acidity. On the flip side, substituents pushing electrons towards the conjugate base (-CH3 in o-cresol) reduce its stability, thereby weakening the parent compound's acidity.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs), such as nitro (-NO2), cyano (-CN), and carbonyls like ketones and aldehydes, pull electron density away from the rest of the molecule, especially the conjugate base. This withdrawal effect can occur through resonance or inductive effects. Inductive effects involve the 'pull' of electron density through sigma bonds owing to electronegativity differences, while resonance effects involve the delocalization of electron density through pi systems.

For o-nitrophenol, the -NO2 group is an exemplar EWG. It stabilizes the negative charge on the o-nitrophenoxide ion through resonance, effectively spreading the charge over a wider area. This increased stability of the conjugate base contributes to the heightened acidity of o-nitrophenol compared to phenol without such a group.

Electron-Donating Groups

On the other side of the stability equation, we've got electron-donating groups (EDGs) such as alkyl groups (-CH3) or methoxy (-OCH3), which push electrons towards the rest of the molecule. Instead of stabilizing the negative charge like EWGs, EDGs destabilize it by increasing electron density around the conjugate base. This additional electron density can force the charge to be less dispersed, resulting in a less stable conjugate base.

An example is o-cresol which has a -CH3 group positioned ortho to the hydroxyl (-OH) group. This methyl group donates electrons via the inductive effect, leading to less stability of the o-cresoxide ion compared to the phenoxide ion. It gives o-cresol a lower acidity since its conjugate base is not as stable.

Stability of Conjugate Bases

Assessing the stability of conjugate bases is key to understanding the acidic nature of different phenolic compounds. Stability is influenced by the ability to handle or disperse the negative charge left after deprotonation. The more effectively a conjugate base can manage this charge, the more stable it is and, by extension, the more acidic its parent acid.

Between phenoxide, o-cresoxide, and o-nitrophenoxide ions, we notice a spectrum of stability. The o-nitrophenoxide ion is most stable due to the strong electron-withdrawing -NO2 group, making o-nitrophenol the most acidic. Phenoxide has intermediate stability with no additional substituents affecting it. Lastly, o-cresoxide is destabilized by the electron-donating -CH3 group, rendering o-cresol less acidic than both phenol and o-nitrophenol. The right understanding of these principles guides students to correctly rank the acidity levels of different phenolic compounds.