Introduction and nomenclature of aliphatic nitrocompounds

Functional groups containing nitrogen are present in a variety of naturally occurring and man made organic compounds. These functional groups impart physico- chemical characteristics to these molecules. These groups are responsible for their unique chemical reactivity patterns and play crucial roles in the preparation of drugs, agrochemicals, dyes and molecules of life. There are many functional groups, which contain one or more nitrogen atoms. Some categories of compounds based on these functional groups include nitro compounds, amines, cyanides, isocyanides and diazo compounds.

Nitro compounds

Nitro compounds are characterized by the presence of nitro group (-NO₂) in their molecules. They may be aliphatic or aromatic compounds according to whether the nitro group is attached to alkyl or aryl groups.

R - NO₂ Aliphatic nitro compounds

Ar - 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.

Nomenclature of nitro compounds

Nitro compounds are named by using the prefix nitro before the name of parent hydrocarbon. Aliphatic nitro compounds are called nitroalkanes while aromatic nitro compounds are called nitroarenes.

The position of nitro group and other substituents if any are indicated by arabic numerals.

Examples:

CH₃ NO₂ - Nitromethane

CH₃ CH₂ NO₂ - Nitroethane

Aliphatic nitro compounds

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 alkanes

Since 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.

For e.g.,

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 2^o and 3^o 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

₄

In this reaction, the amine must be primary and -NH₂ group should be attached to a tertiary carbon.