Cinnamic acid (like most organic acids) is soluble in aqueous sodium hydroxide solutions, but is insoluble in pure water, neutral solutions, or acidic solutions. Explain why.
The correct answer and explanation is:
Correct Answer:
Cinnamic acid is soluble in aqueous sodium hydroxide (NaOH) solutions because it undergoes a chemical reaction with the hydroxide ions to form a water-soluble salt (a carboxylate anion). However, it is insoluble in pure water, neutral solutions, or acidic solutions because it remains in its non-ionized, hydrophobic (water-repelling) form.
300-Word Explanation:
Cinnamic acid is an organic compound that contains a carboxylic acid functional group (-COOH). Like other weak organic acids, its solubility in water depends largely on whether it is in its ionized (charged) or non-ionized (neutral) form.
In pure water or neutral solutions, cinnamic acid is only slightly soluble. This is because in these conditions, it exists primarily in its non-ionized form. The non-ionized form is largely nonpolar due to the aromatic ring and alkene chain, making it poorly compatible with water, a polar solvent.
In acidic solutions, there is an excess of hydrogen ions (H⁺), which suppress the ionization of cinnamic acid (common ion effect). The acid remains in its protonated, neutral form, which is not significantly more water-soluble than in pure water. Thus, it still remains mostly undissolved.
In contrast, in basic solutions like aqueous sodium hydroxide (NaOH), the hydroxide ions (OH⁻) react with the carboxylic acid group of cinnamic acid. This neutralization reaction forms sodium cinnamate, the conjugate base of cinnamic acid (R-COO⁻Na⁺). The resulting carboxylate salt is ionic and highly polar, allowing it to dissolve readily in water due to favorable ion-dipole interactions.
The key reason behind this solubility behavior lies in the acid-base chemistry: cinnamic acid’s transformation into a polar, water-soluble ionic compound only occurs in basic environments. Therefore, cinnamic acid is soluble in aqueous NaOH but insoluble in pure, neutral, or acidic water due to differences in ionization and polarity.