Q.How are primary, secondary and tertiary nitroalkanes distinguished using $HNO_2$?

Five distinct routes build a nitroalkane, and exam questions typically test recognising WHICH route gives a given product. (1) FROM ALKYL HALIDES (laboratory method): an alkyl bromide/iodide + ethanolic KNO2, SN2 mechanism, e.g. ethyl bromide gives nitroethane + KBr; fails for nitrobenzene (a ring-bonded halogen resists this substitution). (2) VAPOUR-PHASE NITRATION OF ALKANES (industrial method): alkane + HNO3 vapour through a red-hot silica tube at 675 K; only methane gives a single clean product (nitromethane); every larger alkane up to n-hexane gives a MIXTURE via C-C cleavage (ethane gives 73% nitroethane + 27% nitromethane), separated by fractional distillation. (3) FROM AN ALPHA-HALOCARBOXYLIC ACID: alpha-chloroacetic acid + aqueous NaNO2 (boil), SN2, gives nitromethane + CO2 + NaCl. (4) OXIDATION OF A TERTIARY-ALKYL …