________ of morphine yields an opiate used as an antitussive.

• A. Acetylation
• B. Methylation
• C. Esterification
• D. Acylation

Answer: (B)

Methylation of morphine’s phenolic hydroxyl group converts it into codeine, an opiate used as an antitussive. By adding a methyl group to the 3-OH, the molecule becomes codeine, which retains opioid activity but is typically used to suppress coughing rather than for strong analgesia. This specific modification—O-methylation—is what creates codeine from morphine.

Acetylation would lead toward diacetylmorphine (heroin), which has a different profile and is not used as an antitussive. Esterification or acylation describe broader types of chemical modifications and do not specifically yield the 3-O-methyl ether that forms codeine.