Which of the following is formed as a by-product when arenediazonium fluoroborate decomposes to give fluorobenzene?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → BF3
Fluorine is introduced into the benzene ring through the diazonium salt. Benzenediazonium chloride with fluoroboric acid gives a precipitate of benzenediazonium fluoroborate:
\( \displaystyle \ce{C6H5N2^+Cl^- + HBF4 -> C6H5N2^+BF4^- v + HCl} \)
On heating, the dry salt decomposes. Nitrogen gas leaves, the ring takes one fluoride from \( \ce{BF4^-} \), and the rest of the ion is left as boron trifluoride:
\( \displaystyle \ce{C6H5N2^+BF4^-} \xrightarrow{\Delta} \ce{C6H5F + BF3 + N2} \)
The fluoroborate ion holds only boron and fluorine, so the boron by-product is \( \ce{BF3} \). \( \ce{BH3} \), \( \ce{H3BO3} \) and \( \ce{BN} \) would need hydrogen, oxygen or nitrogen bonded to boron, and none of these is formed in this step; the nitrogen leaves as \( \ce{N2} \) gas.
NCERT (2026-27 edition):
Reactions involving displacement of nitrogen
