Aliphatic Nitro Compounds
By Anup Pokhrel
Organic compounds containing the nitro group ( – NO2) as a functional group are called nitro compounds.
Nitro compounds may be aliphatic or aromatic depending whether the nitro group ( – NO2) is bonded to an alkyl or aryl group.
ALIPHATIC NITROCOMPOUNDS OR NITROALKANES
Compounds in which nitro group( – NO2) is directly bonded to the carbon atom of an alkyl group are called aliphatic nitro compounds or simply nitroalkanes. These are also regarded as nitro derivatives of alkane
Type formula: R – NO2where R = any alkyl group like CH3, C2H5, etc.
Nitro compounds are classified as primary (10), secondary (20), tertiary (30) nitroalkanes according to the nitro group( – NO2) is bonded to primary (10), secondary (20), tertiary (30) carbon atom respectively.
→ No common name
→Prefix:”Nitro’ to the name of alkanes consisting of longest carbon chain and show its position on parent chain according to as IUPAC names.”
Isomerism in nitroalkane
Two or more compounds having same molecular formula but different properties are called isomers. And the phenomenon is known as isomerism. Isomers that arise due to different structural formulae are called structural isomers.
Nitroalkanes show following types of structural isomerism
i) Chain isomerism: Isomers that differ in carbon chain are called chain isomers. Nitroalkanes containing at least four carbon atoms show chain isomerism in which isomers differ in carbon chain.
ii) Positional isomerism: Isomers that differ in the position of a functional group or substituent on carbon chain are positional isomers. Nitroalkanes containing at least 3 carbon atoms show positional isomerism in which isomers differ in the position of the nitro group.
iii) Functional isomersim: Isomers that differ in the functional group are called functional isomers.
Nitroalkanes are functional isomers with alkyl nitrite.
The compounds in which NO2 is bonded to alkyl group to the nitrogen atom are called nitroalkanes whereas the compounds in which NO2 is bonded to the alkyl group through oxygen atom are called alkyl nitrites.
PREPARATION OF NITROALKANES
i) From haloalkanes
Haloalkanes react with alcoholic solution of silver nitrite (AgNO2) to form corresponding nitroalkanes.
Nucleophile:They are defined as the electron rich species which donates an electron pair to electron deficient center forming a new bond.
The substitution reaction involving having initial attack of nucleophile is known as nucleophilic substitution reaction.
Ambident nucleophiles: In nitrite ion (NO2–) , both Nitrogen and Oxygen have unshared pair of the electron which can donate to the electron-deficient site of the substrate ( i.e. haloalkane). So nitrite ion has two attacking sites i.e. oxygen and nitrogen atom through which it attacks electron deficient site i.e. haloalkane.
The attack through negative oxygen atom of NO2– on haloalkane forms alkyl nitrite .
The attack through the nitrogen atom of NO2– on haloalkane forms nitroalkanes.
Ambident nucleophile is defined as a nucleophile having two possible attack sites through which it attacks electron deficient species.
ii) Nitration of alkane
Nitroalkane can be prepared by passing the mixture of alkane and nitric acid vapor at 4000C.
The reaction involving substitution of hydrogen of alkane of nitro group is called nitration of alkane.
PROPERTIES OF NITROALKANE
i) Lower members are colorless liquids having the pleasant smell.
b) They are sparingly soluble in water.
c) As they are polar molecules, so there is the strong intermolecular force of attraction due to strong dipole-dipole interaction making their boiling point high.
i) Catalytic reduction
Nitroalkane is reduced to the corresponding amine by H2 in the presence of nickel or platinum as a catalyst and the reaction is the catalytic reduction.
ii) Acidic reduction
They are reduced to corresponding amines by metals like Zn, Fe, Sn.
iii) Reduction in neutral medium
Reduced to N- alkyl hydroxylamine by neutral reducing agents like Zn and NH4Cl