Chemical Properties of Nitroalkanes

Nitroalkanes undergo three principal reaction classes. (i) REDUCTION: the nitro group reduces stepwise -- nitro (R-NO2) to nitroso (R-N=O) to N-alkylhydroxylamine (R-NHOH) to the primary amine (R-NH2) -- with the final product depending on the reducing agent and the medium's pH. Under acidic conditions (Sn/HCl, 6[H]) nitromethane goes all the way to methylamine + 2H2O; under NEUTRAL conditions (Zn/NH4Cl, 4[H]) the reduction stops at the intermediate N-methylhydroxylamine + H2O. Alkyl nitrites reduce differently: ethyl nitrite with Sn/HCl (6[H]) gives ethanol + ammonia + water (the N-O bond, not a C-N bond, is what breaks). (ii) HYDROLYSIS: using concentrated HCl or concentrated H2SO4, PRIMARY nitroalkanes hydrolyse to a carboxylic acid + hydroxylamine (e.g. nitroethane, boiled with HCl/H2O, gives acetic acid + NH2OH), SECONDARY nitroalkanes hydrolyse to a ketone + nitrous oxide + water (e.g. 2-nitropropane gives acetone + N2O + H2O), and TERTIARY nitroalkanes give NO reaction at all (there is no alpha-H for the mechanism to remove). Alkyl nitrites, by contrast, hydrolyse (acid- or base-catalysed) straight back to the parent alcohol + nitrous acid. (iii) HALOGENATION: primary and secondary nitroalkanes, treated with Cl2 or Br2 in the presence of NaOH, undergo successive replacement of their alpha-hydrogens by halogen -- nitromethane + 3Cl2/NaOH gives chloropicrin (trichloronitromethane, CCl3-NO2) + 3HCl, a toxic compound used as an insecticide. (iv) NEF CARBONYL SYNTHESIS: the potassium salt of a primar …

Nitrobenzene's chemistry mirrors and extends the general nitroalkane reduction ladder, plus adds its own electrophilic aromatic substitution chemistry. REDUCTION STAGES (each stopping point depends on the reducing agent/medium): Sn/HCl (6[H], acid medium) reduces nitrobenzene all the way to aniline; Zn/NH4Cl (4[H], neutral medium) stops at phenylhydroxylamine; Fe/H2O steam (2[H]) stops at nitrosobenzene; and under ALKALINE conditions (Zn/NaOH), self-condensation of the partially-reduced intermediates gives azobenzene (2[H] further reduces this to hydrazobenzene, C6H5-NH-NH-C6H5). ELECTROLYTIC REDUCTION gives different products depending on the acidity of the medium: weakly acidic conditions give aniline directly, while strongly acidic conditions trap phenylhydroxylamine, which then undergoes an acid-catalysed rearrangement to p-aminophenol. CATALYTIC/METAL-HYDRIDE REDUCTION: nitrobenzene with Ni or Pt/H2 (or LiAlH4) gives aniline + 2H2O. SELECTIVE REDUCTION OF POLYNITRO COMPOUNDS: ammonium sulphide, (NH4)2S, selectively reduces just ONE of the two nitro groups in m-dinitrobenzene, giving m-nitroaniline (the other -NO2 survives). ELECTROPHILIC SUBSTITUTION: the -NO2 group is strongly deactivating and meta-directing, so nitrobenzene reacts only slowly and under vigorous conditions -- chlorination (Cl2/AlCl3) gives 3-chloronitrobenzene; nitration at 373 K gives 1,3-dinitrobenzene, and further nitration at 473 K gives 1,3,5-trinitrobenzene (TNB); sulphonatio …