Haloarenes General methods of preparation, properties, and reactions; Nature of C-X bond; Mechanisms of substitution reactions.: NEET MCQs with Answers

Explanation: Nucleophilic aromatic substitution is accelerated by electron-withdrawing groups placed ortho or para to the leaving group. Such groups pull electron density away from the ring, making the carbon bearing chlorine more electrophilic, and they stabilise the negatively charged intermediate by delocalising the charge onto their own oxygen atoms. 2,4,6-Trinitrochlorobenzene carries three nitro groups, two ortho and one para to the chlorine. All three are perfectly placed to stabilise the intermediate, so it hydrolyses most readily, under mild conditions. 4-Nitrochlorobenzene has only one such group and needs harsher conditions; chlorobenzene has none and requires extreme conditions. The dimethylamino group is electron-donating, so it deactivates the ring towards nucleophilic attack and makes hydrolysis harder still.

Explanation: Take the three steps in order. (i) Cu₂Br₂/HBr replaces the diazonium group by bromine (Sandmeyer reaction), converting benzenediazonium chloride into bromobenzene. (ii) Magnesium in dry ether inserts into the C—Br bond to give the Grignard reagent, phenylmagnesium bromide. (iii) Water immediately protonates the strongly basic carbon of the Grignard reagent: C₆H₅MgBr + H₂O → C₆H₆ + Mg(OH)Br. The final product is therefore benzene. Phenylmagnesium bromide is only the intermediate after step (ii), not the product after step (iii).

Explanation: Toluene first undergoes side-chain chlorination to give C₆H₅CCl₃, a strongly electron-withdrawing group that directs the incoming bromine to the meta position. Zinc and HCl then reduce the –CCl₃ group back to –CH₃, leaving m-bromotoluene.

Explanation: Both methyl groups of m-xylene are ortho/para directors and they reinforce each other at C-4 and C-6. Bromination therefore occurs mainly at those activated positions rather than at the sterically crowded C-2 between the two methyls.

Explanation: 2-Chlorotoluene is an aryl halide: the lone pair on chlorine is delocalised into the ring, giving the C–Cl bond partial double-bond character, and the carbon is sp², so the bond is short and strong and hydrolysis is very difficult. Benzyl and allyl chlorides ionise readily to resonance-stabilised cations and ethyl chloride reacts by S_N2, so all three are far faster.

Explanation: In 2-chlorotoluene the chlorine is bonded directly to an sp² ring carbon. Its lone pair is in resonance with the ring, which gives the C–Cl bond partial double-bond character, and the electron-rich ring repels the incoming nucleophile. Alkyl, allyl and benzyl chlorides have Cl on an sp³ carbon and are substituted readily.

Explanation: Chlorine on a benzene ring is normally very unreactive towards nucleophiles. Three nitro groups at the ortho and para positions strongly withdraw electrons and stabilise the anionic intermediate, so even warm water can replace –Cl by –OH. The nitro groups are not removed, and the product is 2,4,6-trinitrophenol, picric acid.

Explanation: An aryl halide alone with sodium in dry ether joins two aryl groups together, which is the Fittig reaction, so bromobenzene gives biphenyl. A mixture of an alkyl halide and an aryl halide under the same conditions gives an alkylarene, which is the Wurtz-Fittig reaction. Bromobenzene and bromoethane therefore give ethylbenzene.

Explanation: In cyclohexyl chloride the C–Cl carbon is sp³, while in chlorobenzene it is sp² with greater s-character and so is more electronegative, which reduces the polarity of the C–Cl bond. Resonance also moves a lone pair of chlorine towards the ring, giving the bond partial double-bond character and opposing the C→Cl dipole. Both effects lower the dipole moment of chlorobenzene.

Explanation: Melting depends on how well molecules fit in the crystal as well as on intermolecular attraction. The symmetrical para isomer packs closely, so more energy is needed to break the lattice. Boiling involves no lattice, so the three isomers boil at similar temperatures. The para isomer actually has zero dipole moment, and all three have the same molecular mass.