Preparation & Properties of Nitroalkanes | chemca
Organic Chemistry
Preparation, Reduction, and Distinguishing Tests.
By chemca Team • Updated Jan 2026
Nitroalkanes ($R-NO_2$) are derivatives of alkanes where a hydrogen atom is replaced by a nitro group. They are isomeric with alkyl nitrites ($R-ONO$). The nitrite ion ($NO_2^-$) is an ambident nucleophile , meaning it can attack through Nitrogen or Oxygen.
1. Preparation Methods
From Alkyl Halides
A. With Silver Nitrite ($AgNO_2$):
Forms
Nitroalkanes (Major).
$$ R-X + AgNO_2 \xrightarrow{C_2H_5OH, \Delta} R-NO_2 + AgX \downarrow $$
Reason: Ag-O bond is covalent. Attack occurs through the lone pair on Nitrogen.
B. With Potassium Nitrite ($KNO_2$):
Forms
Alkyl Nitrites (Major).
$$ R-X + KNO_2 \xrightarrow{\Delta} R-O-N=O + KX $$
Reason: K-O bond is ionic. The negative charge is on Oxygen, which is the nucleophilic center.
C. Vapor Phase Nitration:
Reaction of alkanes with fuming $HNO_3$ at high temp (673 K). Gives a mixture of products.
$$ CH_3CH_3 + HNO_3 \xrightarrow{673K} CH_3CH_2NO_2 + CH_3NO_2 + H_2O $$
2. Reduction Reactions
Product Depends on Medium
A. Acidic Medium (Complete Reduction):
Forms
Primary Amines .
$$ R-NO_2 + 6[H] \xrightarrow{Sn/HCl \text{ or } Fe/HCl} R-NH_2 + 2H_2O $$
B. Neutral Medium (Partial Reduction):
Forms
N-Alkylhydroxylamines .
$$ R-NO_2 + 4[H] \xrightarrow{Zn/NH_4Cl} R-NHOH + H_2O $$
C. With $LiAlH_4$:
Forms
Primary Amines .
$$ R-NO_2 \xrightarrow{LiAlH_4} R-NH_2 $$
3. Reaction with Nitrous Acid ($HNO_2$)
Distinction between $1^\circ, 2^\circ, 3^\circ$ Nitroalkanes
Nitroalkanes react with $HNO_2$ (from $NaNO_2 + HCl$) based on the number of $\alpha$-hydrogens.
1. Primary Nitroalkanes ($R-CH_2-NO_2$):
React to form
Nitrolic Acid , which dissolves in alkali ($NaOH$) to give a
RED solution.
$$ R-CH_2-NO_2 + HONO \rightarrow R-C(=NOH)-NO_2 \xrightarrow{NaOH} \text{Red Solution} $$
2. Secondary Nitroalkanes ($R_2CH-NO_2$):
React to form
Pseudonitrole , which gives a
BLUE solution. It is insoluble in alkali.
$$ R_2CH-NO_2 + HONO \rightarrow R_2C(NO)-NO_2 \text{ (Blue)} $$
3. Tertiary Nitroalkanes ($R_3C-NO_2$):
No $\alpha$-hydrogen. No Reaction with $HNO_2$. Solution remains colorless.
4. Other Important Reactions
A. Halogenation
$\alpha$-Hydrogens are replaced by halogens in the presence of base.
$$ CH_3-NO_2 + 3Cl_2 + 3NaOH \rightarrow \underset{\text{Chloropicrin (Tear Gas)}}{CCl_3-NO_2} + 3NaCl + 3H_2O $$
B. Hydrolysis
Primary nitroalkanes hydrolyze with boiling HCl to form Carboxylic Acids and Hydroxylamine salt.
$$ R-CH_2-NO_2 + H_2O + HCl \xrightarrow{\Delta} R-COOH + NH_2OH \cdot HCl $$
Secondary nitroalkanes hydrolyze to Ketones ($Nef$ Carbonyl Synthesis).
Knowledge Check
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