15.1 Aliphatic Nitro Compounds
After completion of the chapter, students should be able to:
- Describe the nomenclature and isomerism of nitro compounds.
- Show the preparation from haloalkanes and alkanes.
- State the physical properties of nitro compounds.
- Explain the chemical properties of nitro compounds, i.e., reduction.
R–X + AgNO2 → ?
R–X + KNO2 → ?
Organic compounds having a –NO2 functional group are called nitro compounds. Based on the hydrocarbon part, they can be classified as aromatic or aliphatic nitro compounds. The nitro group has the following structure, which should not be confused with the nitrite group.
R–NO2 = nitroalkane
R–ONO = alkyl nitrite
Based on the carbon atoms to which the nitro group is bonded, the aliphatic nitro compounds can be further classified into:
- Primary Nitroalkane — A nitroalkane in which the carbon attached to the –NO2 group is a primary carbon atom.
e.g., CH3–NO2 Nitromethane, CH3–CH2–NO2 Nitroethane etc.
- Secondary Nitroalkane — The nitrogen atom of the nitro group in this case is bonded to a secondary carbon atom.
e.g., CH3CH(NO2)CH3 2-nitropropane
- Tertiary Nitroalkane — In this case, the nitro group is attached to a tertiary carbon atom.
| CH3 | ||
| | | ||
| CH3 | — C — | CH3 |
| | | ||
| NO2 |
In nitroalkanes, the –NO2 group is treated as a substituent and is indicated by the prefix “nitro-”.
| Formula | Name |
|---|---|
| CH3NO2 | Nitromethane |
| CH3CH2NO2 | Nitroethane |
| CH3CH2CH2NO2 | 1-nitropropane |
| CH3-CH(NO2)-CH3 | 2-nitropropane |
| CH3CH2CH2CH2NO2 | 1-nitrobutane |
| CH3CH2CH(NO2)CH3 | 2-nitrobutane |
| CH3 | ||
| | | ||
| CH3 | — C — | CH3 |
| | | ||
| NO2 |
| CH3 | ||
| | | ||
| CH3 | — C — | CH2—NO2 |
| | | ||
| H |
- Chain Isomerism — Nitroalkanes having four or more carbon atoms can exhibit chain isomerism. e.g., 1-nitrobutane and 2-methyl-1-nitropropane.
- Positional Isomerism — Aliphatic nitro compounds with 3 or more carbons can exhibit such isomerism. e.g., 1-nitropropane and 2-nitropropane.
- Functional isomers — Nitroalkanes exhibit isomerism with alkyl nitrites. E.g., Nitromethane and methyl nitrite.
Write the structures of nitromethane and methyl nitrite.
1. From alkanes (with fuming conc. nitric acid at 400°C) — studied in class 11
When an alkane is heated with fuming nitric acid at 400°C (vapour phase nitration), a mixture of all possible nitroalkanes is formed by carbon chain fission reaction of alkanes.
2. From alkyl halides (with silver nitrite) — studied in haloalkanes
When primary alkyl halides (especially bromides and iodides) are heated with aqueous or (alcoholic) ethanolic silver nitrite solution, nitroalkane is obtained.
Haloalkanes with potassium nitrite and sodium nitrite give alkyl nitrite.
R–X + KNO2 → R–O–N=O + KX
- Lower nitroalkanes are generally colourless liquids. They are denser than water and have characteristic pleasant odours.
- Lower nitroalkanes are slightly soluble in water, while their solubility decreases as the carbon chain becomes longer. They are soluble in many organic solvents like benzene, ethanol, etc.
- Due to their polar nature, the boiling points of nitro compounds are higher than those of alkanes with comparable molecular mass.
1. Reduction
a. With Sn/HCl or Fe/HCl or Zn/HCl or H2/Ni or Pt or LiAlH4, primary amines are produced.
b. In a neutral medium, Zn/NH4Cl(aq) produces N-alkylhydroxylamine.