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Nitroalkanes

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).

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