Final answer:
To isolate 4-bromoaniline from a solution of 4-bromoanilinium chloride, dilute NaOH should be added to deprotonate the anilinium ion, converting it into the insoluble free base, 4-bromoaniline, which can then precipitate out of the solution.
Step-by-step explanation:
To isolate 4-bromoaniline from an aqueous solution containing 4-bromoanilinium chloride, it is necessary to convert the 4-bromoanilinium ion back to the free base form, 4-bromoaniline, which is insoluble in water. This can be achieved by neutralizing the charge on the anilinium ion. The compound 4-bromoanilinium chloride is similar to anilinium chloride, which is a salt formed between a weak base (aniline) and a strong acid (hydrochloric acid). To convert the anilinium ion back to aniline, a base needs to be added to deprotonate the anilinium ion.
The best choice to accomplish this is dilute NaOH (sodium hydroxide). Sodium hydroxide is a strong base that will react with the 4-bromoanilinium ion to produce 4-bromoaniline, which is insoluble in water and can be separated out. When NaOH is added to an aqueous solution of 4-bromoanilinium chloride, it removes the proton from the anilinium ion, thereby neutralizing its charge and converting it to the free base, 4-bromoaniline. This process is known as 'deprotonation', and the resulting 4-bromoaniline precipitates out of the solution.