See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The acidic strength of phenol depends on the stability of the phenoxide ion (C6H5O-) formed after losing a proton. Electron-withdrawing groups (EWG) stabilize the phenoxide ion by dispersing the negative charge, thereby increasing acidity. Electron-donating groups (EDG) destabilize the phenoxide ion by increasing electron density on the oxygen, thereby decreasing acidity. Step 1: Identify the nature of each substituent. - -NO2 (nitro group): Strong electron-withdrawing group via both inductive and resonance effects. It stabilizes the phenoxide ion, increasing acidity. - -CN (cyano group): Electron-withdrawing group via inductive and resonance effects. It stabilizes the phenoxide ion, increasing acidity. - -CH3 (methyl group): Electron-donating group via hyperconjugation and inductive effect (+I effect). It destabilizes the phenoxide ion by increasing electron density, decreasing acidity. - -CHO (aldehyde group): Electron-withdrawing group via resonance and inductive effects. It stabilizes the phenoxide ion, increasing acidity. Step 2: Determine which substituent decreases acidic strength. Only -CH3 is an electron-donating group. It donates electrons toward the ring and ultimately toward the oxygen of the phenoxide ion, making it harder for the oxygen to bear the negative charge, thus destabilizing the phenoxide ion and decreasing the acidic strength of phenol. Step 3: Why other options fail. - -NO2, -CN, and -CHO are all electron-withdrawing groups that increase the stability of the phenoxide ion, thereby increasing (not decreasing) the acidic strength of phenol. Therefore, the correct answer is C.