The key chemical process in converting morphine base to heroin. Can be accomplished using either acetyl chloride or acetic anhydride. Acetyl chloride is flammable, irritating to the eyes, reacts violently with water or alcohol, and requires careful handling in laboratory processes. For these reasons, processors of heroin do not favor acetylation using acetyl chloride. Although acetic anhydride is corrosive and requires care in handling, it is less hazardous to the user than acetyl chloride and hence is the key chemical used in processing of heroin.