Q3. Why is acetic anhydride usually preferred to acetyl chloride for this acetylation? (Compare Lab Manual Unit 10, Discussion Question (i), page 113.)
Acetyl chloride fumes strongly in air and must be stored dry, and pyridine must be dispensed in an efficient fume cupboard and distilled before use, because it absorbs moisture and the reaction does not take place under moist conditions (Lab Manual, pages 104–105). Our safety advice: any drying or distillation is a separate operation for the instructor. Acetyl chloride also gives HCl, which turns unreacted aniline into its salt (amines react with acids to form salts, NCERT page 266); the –NH3+ group of the salt has no lone pair to attack the acyl group, so a base such as pyridine must remove the HCl (NCERT page 270; reasoned). Acetic anhydride works with glacial acetic acid, so that method avoids HCl generation and pyridine use; both routes still require controlled handling (reasoned).
Q4. Why is the acetylation mixture poured into ice-cold water?
Acetanilide is only slightly soluble in water (Wikipedia, “Acetanilide”, gives less than 0.56 g per 100 mL at 25 °C), so it comes out as a solid, while the acetic acid formed and any excess anhydride stay in, or react with, the water (reasoned from Lab Manual, page 105).
Q5. How would you turn acetanilide into p-nitroaniline?
Nitrate it with concentrated HNO3 and H2SO4 to p-nitroacetanilide, then hydrolyse the amide with acid or base: the –NHCOCH3 group becomes –NH2 again (NCERT page 273).
Q6. At least what mass of HNO3 is needed to nitrate the Lab Manual’s 2 g of acetanilide completely? (H = 1, N = 14, O = 16.)
2 g of acetanilide is 2/135 = 0.01481 mol (Example 2), and one mole of HNO3 is needed per mole. M(HNO3) = 1 + 14 + 48 = 63 g mol−1: 0.01481 × 63 = 0.93 g (reasoned).