In 1904, Franz Knoop outlined a scheme for the biological oxidation of fatty acids that was shown \(-50\) years later \(-\) to be correct. In his key experiments, he fed rabbits fatty acids of formula \(\mathrm{C}_{6} \mathrm{H}_{5}\left(\mathrm{CH}_{2}\right)_{\mathrm{n}}\) COOH. When the side chain \((\mathrm{n}+1)\) contained an even number of carbons, a derivative of phenylacetic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{COOH}\), was excreted in the urine; an odd number, and a derivative of benzoic acid was excreted. What general hypothesis can you formulate from these results?

Based on the given results, we can hypothesize that the biological oxidation of fatty acids depends on the number of carbons in the side chain. When a fatty acid with an odd number of \(CH_2\) units in the side chain (or an even number of carbons including the terminal carbon) undergoes biological oxidation, a derivative of phenylacetic acid (\(C_6H_5CH_2COOH\)) forms. Conversely, when a fatty acid with an even number of \(CH_2\) units in the side chain (or an odd number of carbons including the terminal carbon) undergoes biological oxidation, a derivative of benzoic acid (\(C_6H_5COOH\)) forms. This indicates that the process of biological oxidation of fatty acids may vary depending on the number of carbons in their side chains, and different derivatives are formed accordingly.