Reaction of haloarenes

Worked out. When an aryl halide is reacted together with an alkyl halide and sodium metal in dry ether, the two couple to give a substituted aromatic compound; this reaction, an extension of the ordinary Wurtz reaction carried out on a mixed aryl+alkyl pair by the chemist Fittig, is called the Wurtz-Fittig reaction, and it is a way of alkylating an aryl halide -- for example bromobenzene + methyl bromide + 2 Na (dry ether) -> toluene + 2 NaBr. If only the aryl halide takes part (no alkyl halide present), two aryl halide molecules couple to each other instead, giving a biphenyl; this aryl-only variant is called the Fittig reaction on its own - …

Worked out. Aryl halides show low reactivity towards nucleophilic substitution for two combined reasons. First, resonance: one of the lone pairs of electrons on the halogen atom conjugates with the ring's pi-electron system, so chlorobenzene, for example, can be drawn as a set of resonance structures (I through V) in which structures II, III and IV place double-bond character directly on the carbon-chlorine bond; as a result the C-Cl bond in chlorobenzene is measurably stronger and shorter (169 pm) than the C-Cl bond in an alkyl chloride (178 pm), making it correspondingly harder to break. Second, hybridisation: the ring carbon carrying the halogen is sp2, not sp3. Together these rule out both possible substitution mechanisms -- a phenyl cation formed by self-ionisation of the haloarene would not be resonance-stabilised (unlike an alkyl carbocation), which rules out SN1, and backside attack of an incoming nucleophile is sterically blocked by the flat aromatic ring itself, which rules out SN2. Certain groups, however, placed at certain ring positions, can markedly activate the halogen towards substitution despite this general resistance: an electron-withdrawing group at the ortho and/or para position greatly increases reactivity (more such groups increase it further), while the same group at the meta position has practically no activating effect at all. The -I (inductive, electron-withdrawing) effect of the halogen itself is stronger than its own +R (resonance, …

Worked out. Nucleophilic substitution CAN occur at a reasonable rate when the ring carries a strong electron-withdrawing group at the ortho or para position, and this is explained by resonance stabilisation of the reaction's intermediate. For p-nitrochlorobenzene reacting with hydroxide, the incoming OH- first adds to the ring carbon bearing the chlorine, forming a negatively charged (carbanion) intermediate that can be drawn as several resonance structures; crucially, one of these resonance structures places the negative charge directly on the oxygen of the para-nitro group, extending the conjugation of the electron-withdrawing group all the way into where the new bond formed -- this delocalisation makes the intermediate markedly more stable and lowers the energy barrier of the (slow) addition step enough for the reaction to proceed; in the fast step that follows, chloride ion then leaves, restoring aromaticity and giving p-nitrophenol. Correspondingly, converting plain chlorobenzene (no activating group) to phenol by aqueous NaOH requires much harsher conditions -- roughly 623 K and 300 atmospheres of pressure; with one nitro group at the para position (p-nitrochlorobenzene) the same kind of substitution needs only about 433 K, with two nitro groups (2,4-dinitrochlorobenzene) only about 403 K, and with three (2,4,6-trinitrochlorobenzene) the substitution proceeds merely on warming with water -- eac …

Worked out. Aryl halides undergo electrophilic aromatic substitution reactions more slowly than benzene itself, because the halogen's strong electron-withdrawing inductive effect (-I) deactivates the ring even while its resonance donation (+R) directs the incoming electrophile to the ortho and para positions; in practice the para product usually predominates over the ortho product because of steric hindrance near the halogen at the ortho position. Four such reactions are covered by name: (i) halogenation, reacting the haloarene with a further equivalent of halogen in the presence of a ferric-salt Lewis-acid catalyst -- e.g. chlorobenzene + Cl2 (anhydrous FeCl3) gives mainly 1,4-dichlorobenzene plus a minor amount of 1,2-dichlorobenzene, plus HCl; (ii) nitration, heating the haloarene with concentrated HNO3 in the presence of concentrated H2SO4 -- e.g. chlorobenzene gives mainly 1-chloro-4-nitrobenzene plus a minor amount of 1-chloro-2-nitrobenzene; (iii) sulfonation, heating the haloarene with fuming sulphuric acid -- e.g. chlorobenzene gives a mixture of 2-chlorobenzenesulfonic acid and 4-chlorobenzenesulfonic acid; and (iv) Friedel-Crafts reaction, treating the haloarene with an alkyl chloride or an acyl chloride in the presence of anhydrous AlCl3 as catalyst -- e.g. chlorobenzene with methyl chloride (anhydrous AlCl …

Why the p-nitro group helps. The slow step adds OH⁻ to the carbon bearing chlorine, giving an sp3 intermediate whose negative charge spreads over the ring — and, in structure (III), right into the electron-withdrawing nitro group. That extra delocalization stabilizes the intermediate, so substitution that plain chlorobenzene resists becomes feas …