Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom. The o, p-directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown:
Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions. Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.
Assertion (A): Fluorobenzene is not prepared from benzene by electrophilic substitution with fluorine in the presence of a Lewis acid catalyst.
Reason (R): Among the carbon–halogen bonds, the C–F bond is the shortest.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): Fluorobenzene is not prepared from benzene by electrophilic substitution with fluorine in the presence of a Lewis acid catalyst. — True. Aryl chlorides and bromides are made this way, but fluoro compounds are not prepared by this method.
Reason (R): Among the carbon–halogen bonds, the C–F bond is the shortest. — True. Fluorine is the smallest halogen atom, so the C–F bond is the shortest: 139 pm in CH3F, against 178, 193 and 214 pm for the C–Cl, C–Br and C–I bonds.
Why R does not explain A?
R describes the size of the C–F bond in the product; it says nothing about how fluorine behaves when it reacts with benzene.
The correct reason is the high reactivity of fluorine, which makes this method unsuitable for fluoro compounds.
NCERT (2026-27 edition):
6.5 Preparation of Haloarenes – “Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride.” … “Fluoro compounds are not prepared by this method due to high reactivity of fluorine.”
6.3 Nature of C-X Bond – “…Fluorine atom is the smallest and iodine atom is the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 6.2.”
Table 6.2: Carbon-Halogen (C—X) Bond Lengths, Bond Enthalpies and Dipole Moments
