See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The solubility of carboxylic acids in water depends on their ability to form hydrogen bonds with water and the ratio of the hydrophilic (-COOH groups) to hydrophobic (carbon chain or aromatic ring) portions of the molecule. Step 1 - Identify the structures: - Phthalic acid: benzene-1,2-dicarboxylic acid (aromatic, 2 COOH groups, large hydrophobic ring) - Succinic acid: HOOC-CH2-CH2-COOH (4 carbons, 2 COOH groups, ratio 2 COOH per 4 C) - Malonic acid: HOOC-CH2-COOH (3 carbons, 2 COOH groups, ratio 2 COOH per 3 C / only 1 CH2 between two -COOH) - Salicylic acid: 2-hydroxybenzoic acid (aromatic ring, 1 COOH, 1 OH, large hydrophobic component) Step 2 - Assess hydrophilic-to-hydrophobic ratio: - Malonic acid has the highest ratio of hydrophilic groups (-COOH) to hydrophobic carbon atoms among the options. With only one -CH2- group flanked by two -COOH groups, the molecule is predominantly hydrophilic. - Succinic acid has 2 carbons in the chain between the two COOH groups, making it less hydrophilic per unit mass than malonic acid. - Phthalic acid and salicylic acid contain benzene rings, which are hydrophobic and significantly reduce water solubility. Step 3 - Compare solubilities: - Malonic acid solubility: ~139 g/100 mL at 20°C - Succinic acid solubility: ~8 g/100 mL at 25°C - Phthalic acid solubility: ~0.6 g/100 mL at 20°C - Salicylic acid solubility: ~0.2 g/100 mL at 20°C Step 4 - Why other options fail: - Phthalic acid: large aromatic hydrophobic ring greatly reduces solubility - Succinic acid: longer carbon chain than malonic acid, lower hydrophilic ratio - Salicylic acid: aromatic ring dominates, very low solubility Malonic acid, having the smallest hydrocarbon portion relative to its two carboxylic acid groups, forms the most hydrogen bonds per unit of hydrophobic character, giving it the highest water solubility among the choices. Therefore, the correct answer is C.