Nitro Compounds

1. Introduction

Nitro compounds contain the nitro group $\ce{-NO2}$ bonded to a carbon atom. Their general formula is $\ce{R-NO2}$. It is important to note that the nitrogen (not oxygen) is joined to carbon; this distinguishes true nitroalkanes ($\ce{R-NO2}$) from their isomeric alkyl nitrites ($\ce{R-O-N=O}$), in which oxygen links to carbon. The nitro group is a resonance hybrid in which the negative charge and the double bond are shared equally over the two oxygen atoms, so both $\ce{N-O}$ bonds are identical.

Examples: nitromethane $\ce{CH3NO2}$, nitroethane $\ce{C2H5NO2}$, nitrobenzene $\ce{C6H5NO2}$.

2. Classification and Nomenclature

Classification by the carbon carrying $\ce{-NO2}$:

• Primary (1 degree): $\ce{R-CH2-NO2}$, e.g. nitroethane.
• Secondary (2 degree): $\ce{R2CH-NO2}$, e.g. 2-nitropropane.
• Tertiary (3 degree): $\ce{R3C-NO2}$, e.g. 2-methyl-2-nitropropane.

Also classified as aliphatic ($\ce{CH3NO2}$) or aromatic ($\ce{C6H5NO2}$).

Nomenclature: the $\ce{-NO2}$ group is named as the prefix nitro-: $\ce{CH3NO2}$ is nitromethane, $\ce{C6H5NO2}$ is nitrobenzene.

Isomerism: nitroalkanes show chain and position isomerism, and are functional isomers of alkyl nitrites ($\ce{C2H5NO2}$ = nitroethane or ethyl nitrite $\ce{C2H5ONO}$).