Chapter 21, Problem 1PP

PRACTICE PROBLEM

If we examine Table 21.1, we find that the methylphenols (cresols) are less acidic than phenol itself. For example,

This behavior is characteristic of phenols bearing electron-releasing groups. Provide an explanation.

Interpretation Introduction

Interpretation:

Methyl phenols are less acidic than phenols, due to the presence of an electron releasing group on them. The statement is to be explained.

Concept introduction:

$\text{p}{K}_{\text{a}}$ value is an index to express the acidity of a compound. Higher the $\text{p}{K}_{\text{a}}$ value, lower is the acidity of a compound.

Phenols are compounds of benzene, bearing a hydroxyl group. They are alcohol derivatives but have higher acidities than alkyl alcohols.

Methyl phenols are phenol derivatives, in which a hydrogen atom attached to the carbon ring is replaced by a methyl $\left({\text{CH}}_3\right)$ group. Another name for these compounds is cresol.

Answer to Problem 1PP

Solution: The $\text{p}{K}_{\text{a}}$ value of phenols is lower than that of methyl phenols. Hence, the acidity of phenols is higher.

Explanation of Solution

According to table $21.1$, the $\text{p}{K}_{\text{a}}$ value of methyl phenols is $10.2$ and for phenols it is $9.89$. As the $\text{p}{K}_{\text{a}}$ value is higher for methyl phenols, they are less acidic. Phenols show their acidic nature by releasing the proton attached to oxygen and forming a negatively charged phenoxide ion. The phenoxide ion is stabilized in phenols by resonance, but methyl phenols do not exhibit any such phenomenon. Also, methyl is an electron releasing group, it destabilizes the negative charge on phenoxide ion as the electron donating methyl group makes the benzene more electron rich and this +I effect of the alkyl group reduces the acidity of the phenols.

Conclusion

Phenol is more acidic, as the $\text{p}{K}_{\text{a}}$ value of phenols is less than that of methyl phenols.