Step 1: Identify the reaction theme. This is about turning aniline into a diazonium salt and then into fluorobenzene. The fluorine step uses the Balz-Schiemann reaction.
Step 2: Find reagent X. Aniline is first changed into benzenediazonium chloride. This needs nitrous acid in hydrochloric acid, so X is HCl based diazotization, giving the diazonium chloride.
Step 3: Recall the fluorine introduction. To put a fluorine onto the ring, we cannot just use a simple fluoride. We use the special Balz-Schiemann route.
Step 4: Find reagent Y. The diazonium salt reacts with fluoroboric acid $HBF_4$ to make a stable salt, which is then heated. So Y is first $HBF_4$ and then heat $\Delta$.
Step 5: See the product. On heating, the salt breaks down to give fluorobenzene, releasing nitrogen and boron trifluoride.
Step 6: State the answer. So X is HCl and Y is first $HBF_4$ then $\Delta$. \[ \boxed{X = HCl,\ Y = (i)\,HBF_4\ (ii)\,\Delta} \]
