Question

Fluorobenzene$$(C_6{H}_{5}F)$$ can be synthesized in the laboratory:
(A) By heating phenol with $$HF$$ and $$KF$$
(B) From aniline by diazotization followed by heating the diazonium salt with $$HB{F}_4$$.
(C) By direct fluorination of benzene with $$F_2$$ gas
(D) By reacting bromobenzene with $$NaF$$ solution

Answer 1

Hint: As we know that the fluorine atom is very small in size and also is an electron rich atom and$$F_2$$ gas is very explosive in nature so if we use fluorine atom in direct substitution it could be very harmful in laboratory.

Complete step by step answer:
If we use fluorine gas directly it can be very explosive due to its very small size and more electrons around the fluorine nucleus.
So in laboratory we synthesis $$(C_6{H}_{5}F)$$ indirectly, which is known as Balz-Schiemann reaction in which we will take aniline as a substrate which will react with $$HN{O}_2$$ (nitrous acid) in the presence of fluoroboric acid and gives a product diazonium salt. As the diazo group is a good leaving group so by reacting diazo-benzene with $$HB{F}_4$$,this fluoroboric acid attacks the diazo group and replaces diazo with fluorine atom. The $$HB{F}_4$$ gives fluoride ion as a nucleophile which attacks on diazo group. So the final product we will get as $$(C_6{H}_{5}F)$$.
The mechanism is shown as below.

Therefore, the correct option is option (B).
Note:
Diazotization can also occur in the presence of other acids such as sulphuric acid, hydrochloric acid.