Chemistry · Ch 14 — Haloalkanes and Haloarenes
Chemical Properties
Chemical Properties
Haloarene chemistry splits cleanly into two groups. (A) Reactions involving the halogen atom itself -- aromatic nucleophilic substitution (only under forcing conditions) and reaction with metals (Wurtz-Fittig and Fittig coupling) -- where the C-X bond is the site of change. (B) Reactions involving the aromatic ring as a whole -- electrophilic substitution (the ring's own characteristic reactivity), reduction (removing the halogen with hydrogen), and Grignard-reagent formation (using the halogen to build a new organome …
Aromatic Nucleophilic Substitution Reaction
Haloarenes do not undergo nucleophilic substitution readily, because the C-X bond is short and strong (section 14.5.2) and because the aromatic ring is itself a centre of high electron density that repels an incoming nucleophile. Even so, at sufficiently high temperature and pressure the halogen CAN be displaced by OH-, NH2- or CN- with the right reagent: chlorobenzene + NaOH (623 K, 300 atm) gives phenol + NaCl -- this specific industrial route is called Dow's process; chlorobenzene + 2 NH3 (523 K, 50 atm) …
Worked out. Book's practice box (no printed solution): comment on why haloalkanes undergo nucleophilic substitution while haloarenes undergo electrophilic substitution. A haloalkane's C-X bond is a plain, weakly-polarised sp3 sigma bond, easily broken by an attacking nucleophile at the electron-poor carbon. A haloarene's C-X bond instead has partial double-bond character from resonance with the ring (section 14.5.2), making it short and strong and resistant to nucleophilic attack -- while that very resonance pushes extra electron density onto the ring's ortho/para positions, making the RING itself an attractive target for an electron-seeking electrophile instead (own solution, not printed in the textbook). …
Reaction with Metals
(a) Wurtz-Fittig reaction: a haloarene and a haloalkane heated together with sodium in dry ether give an alkylbenzene, e.g. chlorobenzene + chloroethane + 2 Na (ether) gives ethylbenzene + 2 NaCl. (b) Fittig reaction: two molecules of a haloarene heated with sodium in dry ether couple to give a biaryl, e.g. 2 chlorobenzene + 2 Na (ether) gives biphenyl + 2 NaCl -- the same metal-coupling idea as the Wurtz-Fittig re …
Electrophilic Substitution Reaction
Haloarenes DO undergo the ring's characteristic aromatic electrophilic substitution, but more slowly than benzene itself, because the halogen's -I (inductive, electron-withdrawing) effect nets out deactivating overall. At the same time, the halogen's lone pair conjugates (resonates) into the ring, raising electron density specifically at the ortho and para positions, so the halogen directs an incoming electrophile to ortho/para even while slowing the reaction down. Four reactions illustrate this: halogenation (chlorobenzene + Cl2/FeCl3, dark, gives a mixture of o- and p-dichlorobenzene, para major); nitration (chlorobenzene + HNO3/conc. H2SO4 gives o- and p-chloronitrobenzene, para major); sulphonation (chlorobenzene + fuming H2SO4 gives o- and p-chlorobenzenesul …
Reduction and Formation of Grignard Reagent
Reduction: a haloarene reduced with Ni-Al alloy in the presence of NaOH gives the parent arene, e.g. chlorobenzene + 2[H] gives benzene + HCl. Formation of Grignard reagent: unlike a simple ether, haloarenes need the higher-boiling solvent tetrahydrofuran (THF) to form a Grignard reagent with magnesium, e.g. c …