Account for the fact that water-insoluble carboxylic acids ( \(\mathrm{p} K_{\mathrm{a}} 4-5\) ) dissolve in \(10 \%\) aqueous sodium bicarbonate \((\mathrm{pH} 8.5)\) with the evolution of a gas but that waterinsoluble phenols ( \(\mathrm{p} K_{\mathrm{a}} 9.5-10.5\) ) do not dissolve in \(10 \%\) sodium bicarbonate.

The concept of pKa values is crucial when studying acid-base chemistry. The pKa value signifies the acidity of a compound and is inversely related to its acid strength; the lower the pKa, the stronger the acid. Understanding pKa values helps explain why certain molecules, like carboxylic acids with pKa values between 4 and 5, are reactive in specific environments.

For instance, the effectiveness of a carboxylic acid to donate a proton to a base can be predicted by its pKa value. This is particularly noticeable when comparing carboxylic acids to phenols, which have higher pKa values ranging from 9.5 to 10.5. The significant difference in pKa values helps to explain their behavior in acid-base reactions, such as dissolving in solutions of different pH levels.

An acid-base reaction involves the exchange of protons between chemical species. Acids are proton donors, and bases are proton acceptors. The extent to which acids or bases can donate or accept protons depends on their pKa values and the pH of the reacting solutions.

When carboxylic acids, which are strong acids with low pKa values, are added to a bicarbonate solution with a higher pH, a reaction occurs where the bicarbonate acts as a base and accepts a proton from the acid. This generates a water-soluble carboxylate salt and leads to the evolution of carbon dioxide gas. On the other hand, phenols, with their higher pKa values, do not donate protons in this setting and thus don't react with sodium bicarbonate.

Solubility in organic chemistry refers to the ability of a substance to dissolve in a solvent. Water solubility is an important property for many organic compounds, and it is often influenced by the presence of functional groups and the compound's overall polarity.

Carboxylic acids can increase their solubility in water by reacting with bases to form more polar salts. The reaction with sodium bicarbonate is a classic case where the acid transforms into a polar carboxylate ion, which is then readily solvated by water molecules. This reaction highlights the importance of solubility when considering the fate and behavior of organic compounds in different environments.

The evolution of carbon dioxide (CO2) is a telltale sign of certain chemical reactions, particularly those involving carboxylic acids and carbonates or bicarbonates. In these reactions, CO2 is released as a gas, often observed as bubbling or effervescence.

During the reaction of carboxylic acids with sodium bicarbonate, the carbon dioxide produced is due to the formation of carbonic acid as an intermediate, which quickly decomposes into water and CO2 gas. This visible release of gas is evidence of the chemical reaction taking place, not only providing a visual cue but also creating a practical way to test for the presence or activity of carboxylic acids in a variety of scenarios in both laboratory and industrial contexts.