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ORGANIC CHEMISTRY

NITRO COMPOUNDS

1.What Are Nitro Compounds?

Nitro compounds are organic molecules that contain a nitro group (- NO ₂) attached to a carbon atom. The general formula is R-NO ₂, where R is any alkyl or aryl group (like a carbon chain or benzene ring).

Example:

Nitromethane → CH ₃ NO ₂

Nitrobenzene → C ₆ H ₅ NO ₂

The nitro group has one nitrogen atom bonded to two oxygen atoms — one by a double bond, the other by a single bond and a negative charge. The nitrogen itself carries a positive charge, so the group overall is neutral but highly polar and electron-withdrawing.

2. Nitro-Acid Tautomerism

Tautomerism is when a compound exists in two forms that can easily convert into each other by shifting a hydrogen atom and rearranging a double bond.

In nitro compounds — especially those having an alpha hydrogen (a hydrogen on the carbon next to the nitro group) — this tautomerism happens like this:

Basic Reaction Outline

Step 1: Formation of a Salt (aci-form)

We take a primary or secondary nitroalkane and treat it with a strong base like NaOH or KOH. This removes a hydrogen from the carbon next to the nitro group and forms a nitronate salt (also called the "aci-form").

Example:

CH ₃ CH ₂ NO ₂ + NaOH → CH ₃ CH=NO ₂⁻ (aci-form)

This form is resonance-stabilized and is crucial for the next step.

Step 2: Acid Hydrolysis (The Main Step)

Now we add dilute acid (like HCl or H ₂ SO ₄ ) to the above solution. Under acidic conditions, the nitronate ion gets protonated and then hydrolyzed.

This leads to the breaking of the C–N bond and formation of:

● An aldehyde (if we started with a primary nitroalkane)

● A ketone (if we started with a secondary nitroalkane)

And nitrous acid ( HNO ₂) as a by-product.

Example Reactions

● Primary Nitroalkane Example

Nitroethane ( CH ₃ CH ₂ NO ₂) → Acetaldehyde ( CH ₃ CHO )

CH ₃ CH ₂ NO ₂ + [ H ⁺] → CH ₃ CHO + HNO ₂

● Secondary Nitroalkane Example

2-Nitropropane ( CH ₃ CH(NO ₂ )CH ₃) → Acetone ( CH ₃ COCH ₃)

CH ₃ CH(NO ₂ )CH ₃ + [H ⁺ ] → CH ₃ COCH ₃ + HNO ₂

So, based on the structure of the starting nitro compound, you either get aldehydes or ketones.

Conditions Required for Nef’s Reaction

Reagents:

1. A base (like NaOH ) to form the nitronate (aci-form)
2. An acid (like HCl or H ₂ SO ₄) to hydrolyze the aci-form

Temperature: Usually warm to mild heating helps the reaction proceed.

Important Features of Nef’s Reaction

1. Specificity: Works only with primary and secondary nitroalkanes.
2. Useful Functional Group Conversion: Turns –CH₂NO₂ or –CH(NO₂)– into carbonyl groups.
3. One of the Few Reactions that directly uses nitroalkanes to give aldehydes or ketones.
4. No Strong Oxidizers Needed: Just base and acid — very mild and simple conditions.

Summary Table

Step Description Reactants Primary or secondary nitroalkanes Step 1 Treat with base → forms aci-form (nitronate ion) Step 2 Add dilute acid → protonation and hydrolysis Products Aldehyde or ketone + nitrous acid ( HNO ₂) Type of reaction Acidic hydrolysis of nitro compound via tautomer Importance Prepares useful carbonyl compounds under mild Conditions

3.Introduction: What is Nitrobenzene?

Nitrobenzene is an aromatic compound with the formula C ₆ H ₅ NO ₂. It is simply a benzene ring (C ₆ H ₆ ) with one of the hydrogen atoms replaced by a nitro group (-NO ₂ ).

Structure:

NO ₂ | Benzene ring

It’s a pale yellow oily liquid with a bitter almond smell and is toxic. But it's very important in chemistry because it’s the starting point to make many useful compounds, especially aniline, dyes, and drugs

What is Reduction?

Reduction means adding hydrogen (or removing oxygen) from a molecule. So, when we talk about the reduction of nitrobenzene, we mean converting the -NO ₂ group into other less-oxidized groups by adding hydrogen or using reducing agents.

The key idea:

Nitro group (- NO ₂) is in the highest oxidation state.

It can be reduced step by step into:

● Nitroso group (-NO)

● Hydroxylamine (-NHOH)

● Amine (- NH ₂)

Why Does Media Matters?

The reaction medium — whether acidic, basic, or neutral — changes what product we get.

A. Reduction in Acidic Medium

Most Common and Industrially Used

What Happens?

Nitrobenzene is completely reduced to aniline (C₆H₅NH₂).

This is a 6-electron reduction process.

Reagents Used:

The process is clean and efficient. Acidic medium helps in protonating intermediates. This is the most commonly taught method in textbooks.

Reduction in Neutral Medium

Partial Reduction

What Happens?

Nitrobenzene is partially reduced to phenylhydroxylamine ( C ₆ H ₅ NHOH ). Reagents Used:

Zinc + Ammonium chloride ( NH ₄ Cl )

Mild heating is usually applied.

Reaction:

C ₆ H ₅ NO ₂ + 4[H] → C ₆ H ₅ NHOH + H ₂ O

Why Phenylhydroxylamine?

This compound is an intermediate that appears in many reactions like the Bamberger rearrangement, where it gives para-aminophenol (used to make Paracetamol).

Key Points:

Neutral medium avoids full reduction. Used when we need intermediates and not full amines.

Reduction in Alkaline Medium

Leads to Coupling Products (More Complex)

What Happens?

In alkaline medium, reduction becomes selective and often leads to products formed by coupling two molecules of nitrobenzene.

Possible Products:

1. Azobenzene ( C ₆ H ₅ –N=N–C ₆ H ₅) (deep red solid)
2. Azoxybenzene ( C ₆ H ₅ –N=N(O)–C ₆ H ₅) (yellow solid)
3. Hydrazobenzene ( C ₆ H ₅ –NH–NH–C ₆ H ₅ ) (white solid)

Reagents Used:

Zinc + NaOH

Glucose + NaOH

Sometimes even electrochemical methods

Reactions (simplified):

2C ₆ H ₅ NO ₂ + 4[H] → C ₆ H ₅ –N=N–C ₆ H ₅ + 2H ₂ O (Azobenzene) 2C ₆ H ₅ NO ₂ + 6[H] → C ₆ H ₅ –NH–NH–C ₆ H ₅ + 2H ₂ O (Hydrazobenzene)

● Industrial Production : Most aniline is made by reducing nitrobenzene.

● Dye Manufacturing : Products like azobenzene are used in making azo dyes.

● Medicinal Chemistry : Many drugs are made from these intermediates.

● Academic Interest : Great example to learn how reaction conditions affect the outcome.

Nitro Compounds as Explosives

Nitro compounds are organic molecules that have one or more nitro groups (- NO ₂) attached to them. A nitro group consists of one nitrogen atom bonded to two oxygen atoms — one with a double bond and one with a single bond.

General formula:

R–NO ₂, where R is usually a hydrocarbon chain or ring.

Examples of nitro compounds:

● Nitrobenzene ( C ₆ H ₅ NO ₂)

● Trinitrotoluene (TNT)

● Nitroglycerin

● PETN (Pentaerythritol tetranitrate)

● RDX (Research Department Explosive)

To understand why some nitro compounds are explosive, we need to understand a few key concepts:

A. They Contain Lots of Oxygen

The nitro group (- NO ₂) contains oxygen.

Oxygen is needed for burning (combustion).

In a closed molecule, if there’s fuel (carbon, hydrogen) and oxidizer (oxygen) in the same molecule, it can burn itself without outside oxygen.

B. They Are Unstable and Energetic

Structure: A benzene ring with three - NO ₂ groups and one - CH ₃ group.

Chemical formula: C ₆ H ₂ (NO ₂ ) ₃ CH ₃

Used in: Military bombs, grenades, industrial blasting.

Why it’s explosive:

Contains both fuel (carbon and hydrogen) and oxidizer (three NO ₂ groups). Stable enough to handle, but explodes when detonated. Releases heat.

B. Nitroglycerin

Made by nitrating glycerol (a 3-carbon alcohol). Used in: Dynamite (Alfred Nobel’s invention).

Why it’s explosive:

● Has multiple nitro groups.

● Liquid form is very sensitive to shock.

● Very powerful, but needs stabilizers for safe use.

C. PETN (Pentaerythritol tetranitrate)

A powerful military-grade explosive. Used in detonating cords, plastic explosives.

D. RDX (Cyclonite )

Full name: Research Department Explosive. Very powerful, used in military and plastic explosives like C-4.

More powerful than TNT.

Why Nitro Compounds Are Preferred in Explosives

Advantages:

Compact: Fuel and oxygen in one molecule. Fast: Explodes in microseconds. High-energy: Delivers more force per gram. Customizable: You can change their structure to make them more or less powerful.

Safety and Handling

● Nitro compounds, especially explosive ones, are dangerous. Here’s why:

● Some are very sensitive (like nitroglycerin).

● Others are more stable (like TNT), but still need care.

● They must be stored away from heat, flame, and friction.

That’s why experts use detonators or blasting caps to initiate explosions in a controlled way.

Real-Life Uses of Nitro Explosives

Military Use:

● Bombs, grenades, missiles, land mines.

● Used for controlled demolitions.

PETN Aliphatic with 4 NO₃

Military, plastic explosives

Very powerful

RDX Cyclic nitramine Military (C-4) Higher explosive power