See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Nucleophilic substitution (SN2 or SN1) reactivity at a carbon bearing a leaving group depends on the availability of the carbon to attack and the nature of the C-Cl bond. Vinyl and aryl halides (where Cl is directly on sp2 carbon with resonance delocalization into the double bond/ring) are very unreactive toward nucleophilic substitution because the lone pairs on Cl donate into the pi system, strengthening the C-Cl bond and making the carbon less electrophilic. Step 1: Classify each compound: (a) CH2=CH-Cl (vinyl chloride): Cl is on sp2 carbon; C-Cl has partial double bond character due to resonance (Cl lone pair -> pi bond). Very unreactive toward SN reactions. (b) C6H5Cl (chlorobenzene): Cl is on aromatic sp2 carbon; C-Cl strengthened by resonance with ring. Very unreactive toward SN reactions. (c) CH3CH=CHCl (1-chloropropene / allylic? No - Cl is on the vinylic carbon): Cl is on sp2 carbon (vinylic), so again unreactive toward nucleophilic substitution. (d) ClCH2-CH=CH2 (allyl chloride, 3-chloropropene): Cl is on sp3 carbon that is allylic (adjacent to C=C). The allylic position makes it highly reactive toward SN reactions because: (i) the sp3 carbon is accessible to nucleophilic attack, (ii) the allylic carbocation or transition state is stabilized by resonance with the adjacent double bond, lowering the activation energy significantly. Step 2: Compare reactivities: - Options (a), (b), (c) all have Cl on sp2 (vinylic or aryl) carbons - these are the least reactive toward nucleophilic substitution. - Option (d) has Cl on an sp3 allylic carbon - allylic halides are among the most reactive toward nucleophilic substitution due to resonance stabilization of the transition state/intermediate. Step 3: Why others fail: (a) Vinyl chloride: vinylic C-Cl, resonance makes it resistant to SN. (b) Chlorobenzene: aryl C-Cl, highly resonance-stabilized, resistant to SN. (c) CH3CH=CHCl: also vinylic C-Cl, resistant to SN. (d) Allyl chloride: allylic sp3 C-Cl with resonance stabilization of TS, most reactive. Therefore, the correct answer is D.