Discussion

Stiring with water for long, the newly generated diazonium salt may suffer from nucleophilic attacks in aqueous solution. This may result in the low yield in the 1st step.

Reference

[1] Furniss BS, Hannaford AJ, Smith PWG, Tatchell AR. Vogel's Practical Organic Chemistry [M] 5ed. Vol2, p939

[2] Flood DT, Fluorobenzene, Organic Synthesis [J] Vol2 p295 (1943)

(The End)