Question

Following are structural formulas for phenol and cyclohexanol along with the acid dissociation constants for each. Oc1ccccc1 OC1CCCCC1 Phenol \(\mathrm{p} K_{\mathrm{a}} 9.96\) Cyclohexanol Propose an explanation for the fact that phenol is a considerably stronger acid than cyclohexanol.

Short answer

Answer: Phenol is a stronger acid than cyclohexanol because its conjugate base, phenoxide ion, is more stable due to the delocalization of the negative charge via resonance structures in the benzene ring. In contrast, the conjugate base of cyclohexanol, cyclohexoxide ion, does not have the benefit of resonance structures for stabilization, making it less stable and cyclohexanol a weaker acid.

Step-by-step solution

Identify the conjugate bases for both phenol and cyclohexanol

First, we need to consider the conjugate bases formed after the acid dissociation. This involves losing the acidic hydrogen (H+) from the hydroxyl group of each compound. For Phenol: $$\mathrm{Phenol}\xrightarrow{\mathrm{deprotonation}}\mathrm{Phenoxide} + \mathrm{H^+}$$ For Cyclohexanol: $$\mathrm{Cyclohexanol}\xrightarrow{\mathrm{deprotonation}}\mathrm{Cyclohexoxide} + \mathrm{H^+}$$

Compare the stability of the conjugate bases

The strength of an acid can be related to the stability of its conjugate base. A more stable conjugate base makes the compound a stronger acid. So, by comparing the stability of phenoxide ion and cyclohexoxide ion, we can explain the difference in the acidity of phenol and cyclohexanol.

Examine electron delocalization and resonance in the conjugate bases

The stability of the conjugate bases can be attributed to the presence of resonance structures and electron delocalization. We need to determine whether the negative charge on the oxygen atom of the conjugate base is efficiently delocalized or not. In the case of phenoxide ion, the negative charge on the oxygen atom is delocalized throughout the benzene ring via resonance structures. This delocalization of the negative charge over the whole aromatic ring helps to stabilize the phenoxide ion. In contrast, the cyclohexoxide ion does not have any resonance structures, and therefore, the negative charge on the oxygen atom is localized, making it less stable compared to phenoxide ion.

Conclude and explain the difference in acidity

We can conclude that phenol is a stronger acid than cyclohexanol because its conjugate base, phenoxide ion, is more stable due to the delocalization of the negative charge via resonance structures in the benzene ring. On the other hand, for cyclohexanol, the conjugate base cyclohexoxide ion does not have the benefit of resonance structures for stabilization, which makes it less stable and cyclohexanol a weaker acid.