Chemistry · Ch 13 — Organic Nitrogen Compounds
Isomerism in Nitroalkanes
Isomerism in Nitroalkanes
Nitroalkanes show chain isomerism (differing in the length/branching of the carbon chain, e.g. 1-nitrobutane vs 2-methyl-1-nitropropane), position isomerism (differing only in where -NO2 sits on an otherwise identical chain, e.g. 1-nitrobutane vs 2-nitrobutane), and functional isomerism with alkylnitrites (a nitroalkane R-NO2 and the alkyl nitrite R'-O-N=O of the same molecular formula are functional isomers, e.g. 1-nitrobutane and butyl nitrite, both C4H9NO2). A nitro compound with the molecular formula C4H9NO2 illustrates all three at once. Primary and secondary nitroalkanes -- those carrying an alpha-hydrogen -- additionally show a special tautomerism between a 'nitro' form and an 'aci' form: nitromethane's nitro form (CH3-NO2, the familiar structure) sits in equilibrium with its aci form (CH2=N(=O)-OH, where the alpha-H has migrated onto an oxygen and a C=N double bond has formed). Tertiary nitroalkanes cannot tautomerise this way since they have no alpha-hydrogen to move. The nitro and aci forms differ measurably: the aci form is more acidic, dissolves in NaOH instantl …
What this figure shows. Two structures linked by a 'Tautomerises' arrow: the nitro form of nitromethane, CH3-NO2 (drawn with the carbon bonded to three H and the N=O/N-O nitro group), and the aci form, CH2=N(-OH)=O (drawn with one H having migrated to an oxygen, leaving a C=N double bond and an -OH on nitrogen). …
| S.No. | Nitro form | Aci form |
|---|---|---|
| 1 | Less acidic | More acidic |
| 2 | Dissolves in NaOH slowly | Dissolves in NaOH instantly |
| 3 | Decolourises FeCl3 solution | With FeCl3 gives reddish-brown colour |
Worked out. Book's practice box (no printed solution): write all possible isomers for (i) C2H5NO2 and (ii) C3H7NO2. Working through them: (i) C2H5NO2 has exactly two constitutional isomers by formula -- the nitroalkane nitroethane (CH3-CH2-NO2) and the functionally isomeric alkyl nitrite, ethyl nitrite (CH3-CH2-O-N=O); no chain/position isomerism is possible on only two carbons. (ii) C3H7NO2 has four isomers: the two position isomers of nitropropane (1-nitropropane, CH3-CH2-CH2-NO2, and 2-nitropropane, CH3-CH(NO2)-CH3) plus the two corresponding functionally-isomeric propyl nitrites (n-propyl nitrite, CH3-CH2-CH2-O-N=O, and isopropyl nitrite, (CH3)2CH-O-N=O) -- own s …