Unit 15: Nitro Compounds

Nitroalkanes

Introduction

Nitroalkanes are organic compounds in which a nitro group (-NO₂) is attached to an alkyl carbon atom. The general formula is R‑NO₂, where R represents an alkyl chain (e.g., methyl, ethyl, propyl). The nitro group is a strong electron‑withdrawing substituent, influencing both the physical and chemical behavior of the molecule.

Nomenclature and Isomerism

According to IUPAC rules, nitroalkanes are named by adding the prefix “nitro‑” to the name of the parent alkane, indicating the position of the nitro group if necessary.

• CH₃‑NO₂ → nitromethane
• CH₃CH₂‑NO₂ → nitroethane
• CH₃CH₂CH₂‑NO₂ → 1‑nitropropane
• CH₃CH(NO₂)CH₃ → 2‑nitropropane (isomer of 1‑nitropropane)

Positional isomerism arises when the nitro group can occupy different carbon atoms in the chain (e.g., 1‑nitropropane vs. 2‑nitropropane). Functional group isomerism is not observed for nitroalkanes because the nitro group is distinct from other common groups like carbonyl or hydroxyl.

Preparation

From Haloalkanes (Alkyl Halides)

The most common laboratory method involves the nucleophilic substitution of an alkyl halide with silver nitrite (AgNO₂). Silver nitrite provides the nitrite ion (NO₂⁻) as a nucleophile, which attacks the electrophilic carbon of the halide.

R‑X + AgNO₂ → R‑NO₂ + AgX↓ (where X = Cl, Br, I)

Example: CH₃CH₂Br + AgNO₂ → CH₃CH₂NO₂ + AgBr↓ (nitroethane).

The reaction proceeds via an SN2 mechanism for primary halides; secondary halides may give lower yields due to competing elimination.

From Alkanes – Vapor‑Phase Nitration

Industrial production of simple nitroalkanes (especially nitromethane) is carried out by gaseous nitration of alkanes using concentrated nitric acid at high temperature (≈400 °C).

CH₄ + HNO₃ → CH₃NO₂ + H₂O

Higher alkanes give mixtures of nitro isomers; the reaction is radical‑based and requires careful temperature control to avoid over‑nitration or explosion.

Physical Properties

Nitroalkanes exhibit higher boiling points than their parent hydrocarbons due to the polar nitro group and its ability to engage in dipole‑dipole interactions.

The nitro group also imparts a characteristic pleasant odor to lower nitroalkanes.

Chemical Properties

The nitro group is susceptible to reduction, which is the most important transformation of nitroalkanes.

Reduction to Primary Amines

Under acidic or catalytic hydrogenation conditions, the nitro group is reduced stepwise to a primary amine (‑NH₂).

R‑NO₂ + 3[H] → R‑NH₂ + 2H₂O

Common reducing agents:

• Tin(II) chloride / hydrochloric acid (Sn/HCl) – classic laboratory reduction.
• Hydrogen gas with palladium on carbon (H₂/Pd‑C) – catalytic hydrogenation, milder conditions.

Example: CH₃CH₂NO₂ + 3[H] (Sn/HCl) → CH₃CH₂NH₂ + 2H₂O (nitroethane → ethylamine).

Other Reactions

Although less common, nitroalkanes can undergo:

• Nucleophilic substitution at the carbon bearing the nitro group (e.g., formation of nitroalkanes via the Victor Meyer test).
• Base‑catalyzed condensation (e.g., nitroalkane anion acts as a nucleophile in the Henry reaction).

Nitrobenzene (C₆H₅NO₂)

Introduction

Nitrobenzene is the simplest aromatic nitro compound, consisting of a benzene ring bearing a nitro group. It is a pale yellow oily liquid with a distinctive almond‑like odor and is highly toxic.

Preparation

Nitrobenzene is manufactured by nitration of benzene using a mixture of concentrated nitric and sulfuric acids (the “mixed acid”). Sulfuric acid acts as a catalyst and helps generate the nitronium ion (NO₂⁺), the electrophile.

C₆H₆ + HNO₃ (conc.) ⟶[H₂SO₄] C₆H₅NO₂ + H₂O

The reaction is carried out at 50‑60 °C to control exotherm and minimize dinitration.

Physical Properties

• Appearance: Pale yellow oily liquid
• Odor: Characteristic almond smell
• Boiling point: 210 °C
• Density: 1.20 g cm⁻³ (at 20 °C)
• Solubility: Slightly soluble in water (0.2 g L⁻¹), miscible with ethanol, ether, benzene, and most organic solvents.
• Toxicity: Harmful if inhaled, ingested, or absorbed through skin; methemoglobinemia risk.

Chemical Properties

Reduction Reactions

The nitro group in nitrobenzene can be reduced under different conditions to give a variety of products, which is crucial for synthetic planning.

1. Acidic medium (Sn/HCl) – complete reduction to aniline.

C₆H₅NO₂ + 6[H] (Sn/HCl) → C₆H₅NH₂ + 2H₂O

Example: Nitrobenzene + Sn/HCl → aniline (used in dye and rubber industries).

2. Alkaline medium (Zn/NaOH) – partial reduction leading to azobenzene via coupling of phenylhydroxylamine intermediates.

2 C₆H₅NO₂ + 3[H] (Zn/NaOH) → C₆H₅N=NC₆H₅ + 4 H₂O (azobenzene)

3. Neutral medium (SnCl₂/HCl) – formation of phenylhydroxylamine.

C₆H₅NO₂ + 2[H] (SnCl₂/HCl) → C₆H₅NHOH + H₂O

Electrophilic Aromatic Substitution (EAS)

The nitro group is a strong meta‑directing and deactivating substituent due to its –I and –M effects. Consequently, further electrophilic substitution occurs predominantly at the meta position.

• Nitration (mixed acid) → m‑dinitrobenzene (1,3‑dinitrobenzene).
• Sulphonation (fuming H₂SO₄) → m‑nitrobenzenesulfonic acid.
• Bromination (Br₂/FeBr₃) → m‑nitrobromobenzene.

C₆H₅NO₂ + Br₂ ⟶[FeBr₃] C₆H₄(Br)NO₂ (meta) + HBr

Due to deactivation, harsher conditions (higher temperature, longer reaction time) are required compared to benzene.

Uses of Nitro Compounds

Nitroalkanes and nitrobenzene serve as intermediates in many industrial processes:

• Explosives: Nitromethane is a component of high‑energy fuels; nitrobenzene is reduced to aniline, a precursor to TNT (trinitrotoluene) and other nitroaromatic explosives.
• Solvents: Nitromethane and nitroethane are polar aprotic solvents used in extractions, spectroscopy, and reaction media.
• Dyes and Pigments: Aniline derived from nitrobenzene reduction is the basis for azo dyes, indigo, and many synthetic colorants.
• Pharmaceuticals: Nitroalkanes are intermediates in the synthesis of certain antibiotics, analgesics, and anti‑inflammatory agents.
• Chemical Intermediates: Nitrobenzene undergoes further functionalization (e.g., reduction to aniline, then diazotization) to produce a wide range of aromatic compounds.

Summary

This chapter has provided a detailed account of nitroalkanes and nitrobenzene, covering their nomenclature, isomerism, preparation methods (both laboratory and industrial), physical characteristics, and rich chemistry—especially reduction reactions and electrophilic aromatic substitution behavior. Understanding these aspects is essential for appreciating the role of nitro compounds in synthetic organic chemistry and their widespread applications in explosives, solvents, dyes, and pharmaceuticals.