Preparation of Nitroalkanes

Five distinct routes give nitroalkanes. (1) FROM ALKYL HALIDES (laboratory method): an alkyl bromide or iodide heated with an ethanolic solution of potassium nitrite (KNO2) displaces the halide via an SN2 mechanism, e.g. ethyl bromide + KNO2/ethanol gives nitroethane + KBr; this route fails for nitrobenzene since a halogen bonded directly to the benzene ring cannot be cleaved this way. (2) VAPOUR-PHASE NITRATION OF ALKANES (industrial method): a gaseous mixture of methane and nitric acid passed through a red-hot metal (silica) tube at 675 K gives nitromethane + water; every other alkane up to n-hexane instead gives a MIXTURE of nitroalkanes (via C-C bond cleavage during the vapour-phase reaction, e.g. ethane gives 73% nitroethane + 27% nitromethane), separated afterwards by fractional distillation. (3) FROM an ALPHA-HALOCARBOXYLIC ACID: alpha-chloroacetic acid boiled with aqueous sodium nitrite (an SN2 substitution) gives nitromethane + CO2 + NaCl. (4) OXIDATION OF TERTIARY-ALKYL AMINES: tert-butylamine oxidised with aqueous KMnO4 gives the tertiary nitroalkane 2-methyl-2-nitropropane + water -- the only general route to a TERTIARY nitroalkane, since the halide and nitrite ro …