NEET Exam > NEET Notes > Chemistry Class 12 > Question Bank: Haloalkanes & Haloarenes
Question Bank: Haloalkanes & Haloarenes
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FAQs on Question Bank: Haloalkanes & Haloarenes
| 1. What's the difference between SN1 and SN2 reactions in haloalkanes? |  |
Ans. SN2 reactions are bimolecular, one-step processes where nucleophile attacks from the back side, inverting stereochemistry and favoring primary haloalkanes. SN1 reactions are unimolecular, two-step mechanisms forming carbocation intermediates, favouring tertiary haloalkanes with racemization. Substrate structure, solvent polarity, and nucleophile strength determine which mechanism dominates in haloalkane substitution reactions.
| 2. How do I identify whether a haloarene will undergo nucleophilic aromatic substitution? | |
Ans. Haloarenes undergo nucleophilic aromatic substitution when electron-withdrawing groups (like nitro groups) are ortho or para to the halogen, activating the aromatic ring. Without these activating groups, haloarenes are extremely unreactive toward nucleophiles due to aromatic stability. The presence and position of deactivating substituents determines reactivity in nucleophilic displacement reactions on benzene rings.
| 3. Why can't primary haloalkanes undergo elimination easily compared to tertiary ones? | |
Ans. Primary haloalkanes lack the stability needed for carbocation formation during elimination, making E1 mechanisms unfavourable. E2 elimination requires anti-periplanar geometry and strong bases; primary systems form less stable alkenes. Tertiary haloalkanes form highly stable tertiary carbocations, favouring E1 pathways and producing Zaitsev's product through rearrangement and dehydration mechanisms.
| 4. What's the most common mistake students make when predicting products of haloalkane reactions? | |
Ans. Students often ignore reaction conditions-assuming all nucleophilic displacements follow SN2 when polar aprotic solvents and secondary substrates favour SN1 instead. Forgetting stereochemistry inversion in SN2, overlooking carbocation rearrangement in tertiary systems, and misidentifying major products via Zaitsev's rule are frequent errors. Context matters: solvent, substrate type, nucleophile strength, and temperature together determine actual reaction pathways and product distributions.
| 5. How do electron-donating and electron-withdrawing groups affect haloarene reactivity in nucleophilic substitution? | |
Ans. Electron-withdrawing groups (nitro, cyano) activate haloarenes toward nucleophilic aromatic substitution by stabilising negative charge in the intermediate, increasing reactivity dramatically. Electron-donating groups (alkyl, amino) deactivate haloarenes, making them resistant to nucleophilic displacement. Group position relative to the halogen-ortho, meta, or para-determines activation strength and mechanism pathway in aromatic halogen substitution reactions.
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