Question

Phenol, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\), is a stronger acid than methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), even though both contain an O-H bond. Draw the structures of the anions resulting from loss of \(\mathrm{H}^{+}\) from phenol and methanol, and use resonance structures to explain the difference in acidity.Phenol, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\), is a stronger acid than methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), even though both contain an O-H bond. Draw the structures of the anions resulting from loss of \(\mathrm{H}^{+}\) from phenol and methanol, and use resonance structures to explain the difference in acidity.

Short answer

Phenol is more acidic because its anion is resonance-stabilized, unlike methoxide.

Step-by-step solution

Identify the Anions

When phenol loses an \(\mathrm{H}^+\), the resulting anion is the phenoxide ion, represented as \(\mathrm{C}_6\mathrm{H}_5\mathrm{O}^-\). For methanol, the loss of \(\mathrm{H}^+\) results in the methoxide ion, represented as \(\mathrm{CH}_3\mathrm{O}^-\).

Draw the Anions

For the phenoxide ion, the benzene ring ( C_6H_5 ) is attached to an oxygen with a negative charge ( O^- ). For the methoxide ion, a methyl group ( CH_3 ) is bonded to oxygen with a negative charge ( O^- ).

Analyze Resonance in Phenoxide

Phenoxide ion can be stabilized through resonance. The negative charge on the oxygen is delocalized into the benzene ring, creating resonance structures where the negative charge can appear on various carbons in the ring, thus stabilizing the ion.

Explain Lack of Resonance in Methoxide

The methoxide ion does not benefit from resonance stabilization because the negative charge on oxygen remains localized, with no adjacent pi-bond system to delocalize the charge, making it less stable.

Compare the Stability of Anions

The resonance stabilization of phenoxide results in a more stable anion compared to methoxide. This increased stability makes phenol more likely to donate a proton ( H^+ ), hence a stronger acid than methanol.

Key concepts

Resonance Structures

In chemistry, resonance structures are different ways of depicting the same molecule by arranging electrons differently. Each structure represents a possible distribution of electrons, and the true form of the molecule is a combination of these structures.

For the phenoxide ion, created when phenol loses a proton, resonance plays a key role. In this ion, the negative charge initially on the oxygen atom can be spread across the benzene ring. This spreading, or delocalization, increases stability. How does this happen?

• First, the negative charge on the oxygen can form a double bond with the adjacent carbon atom in the benzene ring.
• This causes a shift in the location of double bonds within the ring, moving the negative charge around.
• As a result, the phenoxide ion has multiple resonance structures, all equally contributing to the overall hybrid.

The lack of a similar structure in methoxide makes it less stable, as there's no such ring or double bond to help spread the charge.

Anion Stability

The stability of an anion is crucial in determining the acidity of its parent molecule. A more stable anion means that the parent acid is more likely to lose a proton and thus be a stronger acid.

In the phenoxide ion, stability is vastly improved by resonance. The negative charge is not stuck in one place. It is shared across multiple atoms, primarily due to the structure of the benzene ring. This delocalization makes the phenoxide ion more stable than the methoxide ion, which lacks this stability.

The methoxide ion, on the other hand, does not have such stabilizing resonance forms. Its negative charge is confined to the oxygen atom. This confinement makes it energetically less favorable, and hence, methanol is a weaker acid compared to phenol.

Stability differences here directly affect how easily the acid can donate its hydrogen ion, impacting acidity.

Phenol and Methanol

Phenol and methanol are organic compounds that both feature an O-H group, but their acidic properties differ significantly. The ability of phenol to donate a proton is greater than that of methanol. But why is this the case?

When phenol loses a proton, it forms phenoxide, which is stabilized by resonance as discussed. This feature shifts the balance, favoring the dissociation of hydrogen ions.

With methanol, losing a proton results in methoxide. There's no resonance, and the resultant ion holds a concentrated negative charge on the oxygen, providing less incentive for methanol to give up its hydrogen ion.

Hence, even though both molecules have O-H bonds, their differing structures lead to phenol being a stronger acid. It is essential to understand these nuances to predict reactivity and strength of various acids effectively.