R - NO2 Aliphatic nitro compoundsAr - NO2 Aromatic nitro compound
The aliphatic nitro compounds may be further classified into primary, secondary or tertiary nitro compounds according as the nitro group is attached to primary, secondary or tertiary carbon atom respectively.
The nitro group is an ambident group and is capable of getting attached to carbon chain through nitrogen.
as well as through oxygen (-O - N = O) atom. The compound in which the -NO2 group is linked to the alkyl or aryl group through oxygen atom are called nitrites. Nitrites are isomeric with nitro compounds.
The position of nitro group and other substituents if any are indicated by arabic numerals.Examples:
CH3 NO2 - NitromethaneCH3 CH2 NO2 - Nitroethane
i) Vapour phase nitration of alkanes
Hydrocarbons when heated with forming nitric acid at 693 - 793 K, are converted to nitro alkenes. Nitration of methane end lower members can be carried out by this nitration technique.
This direct nitration of alkanes is relatively more difficult in comparison to the nitration of arenes. Hence this method has no significance as a laboratory method but may be of commercial importance in the bulk production of small nitro alkanesSince the process occurs at high temperature therefore, C - C bond of alkenes also undergo cleavage at this temperature and a mixture of nitro alkenes are formed.
ii) Treatment of alkyl halides with alcoholic silver nitrate
Iodo alkanes on treatment with alcoholic AgNO2 gives 80% of nitroalkanes and 20% alkyl nitrites.
This method is primarily used to prepare primary nitro compounds. Secondary and tertiary halides give major proportion of alkenes due to b-elimination reaction.Hence 2o and 3o nitro alkenes are not prepared by this method.
If alkyl bromides or iodides are treated with alkali nitrites, the major with alkali nitrites, the major product is alkyl nitrite with nitro alkanes as the minor product.
Even NaNO2 or KNO2 give a fairly good yield (60%) of nitro compound if the reaction is carried out in solvents like dimethyl formamide (DMH) or dimethyl sulphoxide (DMSO).
iii) Oxidation of t-alkyl amines with KMnO
In this reaction, the amine must be primary and -NH2 group should be attached to a tertiary carbon.