4-Methylphenol, \(\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{OH}\left(\mathrm{p} K_{\mathrm{a}} 10.26\right)\), is only slightly soluble in water, but its sodium salt, \(\mathrm{CH}_{3} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{O}^{-} \mathrm{Na}^{+}\), is quite soluble in water. In which solution(s) will 4-methylphenol dissolve? (a) Aqueous \(\mathrm{NaOH}\) (b) Aqueous \(\mathrm{NaHCO}_{3}\) (c) Aqueous \(\mathrm{Na}_{2} \mathrm{CO}_{3}\)

Understanding the acidity or basicity of a compound is easily conveyed through its pKa value. The pKa value represents the strength of an acid in terms of its ability to donate protons to a base. In simple terms, the lower the pKa, the stronger the acid, and vice versa. For instance, 4-methylphenol has a pKa of 10.26, meaning it's a relatively weak acid since its pKa is higher compared to strong acids like hydrochloric acid with a pKa of -6.3.

When considering acid-base reactions, acids with a lower pKa value will donate a hydrogen ion to the bases with a higher pKa value, moving towards equilibrium. This concept is crucial when determining in which solutions an organic compound like 4-methylphenol will dissolve. The comparison of pKa values dictates the direction of the reaction and the solubility of the compounds involved.

The solubility of organic compounds in water depends on their molecular structure and the presence of functional groups capable of interacting with water molecules. For example, 4-methylphenol contains a hydroxyl group (-OH), which can form hydrogen bonds with water; however, its benzene ring makes it less polar and less soluble in water.

Organic compounds often increase their solubility in water when they react to form salts. Sodium salts, in particular, are generally water-soluble due to the strong electrostatic interaction between the water molecules and the ionic components of the salt. This interaction outcompetes the cohesive forces within the ionic compound, allowing it to dissociate into ions that become hydrated and thus, soluble in the aqueous environment. In the case of 4-methylphenol, its sodium salt, sodium 4-methylphenoxide, is much more soluble than the compound itself due to these solvation effects.

Acid-base reactions are ubiquitous in chemistry and involve the exchange of protons between substances. Typically, an acid will donate a proton (H+) to a base, which accepts it. The soluble product of this reaction often dictates whether the initial substances will dissolve. For 4-methylphenol to dissolve, it must react with a base to form a water-soluble salt, as illustrated in the given exercise solutions.

In the context of 4-methylphenol, when reacting with a base like NaOH, a strong base, it readily donates its proton, thus increasing the solubility of 4-methylphenol because its sodium salt is formed. Understanding acid-base reaction principles allows one to predict solubility outcomes and optimize conditions for these reactions, which is an essential skill in chemistry.