Question

Write short notes on the following:
(A) Electrophilic substitution in halobenzene,
(B) Wurtz–Fittig reaction,
(C) Applications of Grignard’s reagent.

Hint:

Halogen = ortho/para directing but deactivating.

Answer 1

(A) Electrophilic substitution in halobenzene Step 1: Nature of halogen substituent.
In halobenzene, halogen atoms are deactivating due to the –I effect (electron withdrawing inductive effect). However, they also exhibit +R effect (electron donating by resonance), which increases electron density at ortho and para positions. Step 2: Directive influence.
Because of this dual effect, halogens are ortho/para directing but overall deactivate the ring towards electrophilic substitution. Step 3: Example reaction.
Chlorobenzene + Cl\(_2\) (in presence of FeCl\(_3\)): \[ C_6H_5Cl + Cl_2 \xrightarrow{FeCl_3} o\text{-}C_6H_4Cl_2 + p\text{-}C_6H_4Cl_2 \] Step 4: Conclusion.
Thus, halobenzene undergoes electrophilic substitution at ortho and para positions, but at a slower rate than benzene.