Unit 15
Organic Chemistry
Class 12 Chemistry
Nitro Compounds
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NEB/CDC syllabus scope:
Unit 15 is a 3-teaching-hour chapter divided into aliphatic nitro compounds (nitroalkanes) and nitrobenzene. The required scope includes nomenclature and isomerism of nitro compounds; preparation of nitroalkanes from haloalkanes and alkanes; physical properties and reduction of nitroalkanes; preparation and physical properties of nitrobenzene; reduction of nitrobenzene in different media; electrophilic substitution reactions of nitrobenzene—nitration, sulphonation and bromination; and important uses of nitro compounds.
1. Introduction to Nitro Compounds
Definition
Nitro compounds are organic compounds in which one or more nitro groups, –NO₂, are bonded directly to a carbon atom.
General representation: R–NO₂ or Ar–NO₂
Examples:
CH₃NO₂ = nitromethane CH₃CH₂NO₂ = nitroethane C₆H₅NO₂ = nitrobenzene
Nitro compound is not nitrite ester
In a true nitro compound the carbon atom is bonded directly to nitrogen: R–NO₂. In a nitrite ester, carbon is bonded through oxygen: R–O–N=O.
2. Structure and Nature of the Nitro Group
The nitro group is strongly polar and is best represented by resonance structures in which the N–O bonds share partial double-bond character.
Diagram 1: Resonance description of –NO₂
Important consequence
The strong electron-withdrawing nature of –NO₂ makes nitrobenzene less reactive than benzene toward electrophilic substitution and directs new electrophiles mainly to the meta position.
3. Classification of Nitro Compounds
| Class | General idea | Example |
|---|---|---|
| Aliphatic nitro compound | –NO₂ attached to an aliphatic carbon | CH₃CH₂NO₂ |
| Aromatic nitro compound | –NO₂ attached directly to aromatic ring | C₆H₅NO₂ |
| Mononitro compound | One –NO₂ group | CH₃NO₂ |
| Polynitro compound | More than one –NO₂ group | Dinitro aromatic compounds |
3.1 Primary, Secondary and Tertiary Nitroalkanes
Nitroalkanes may also be classified according to the type of carbon atom carrying the –NO₂ group.
| Type | General structure | Example |
|---|---|---|
| Primary (1°) | RCH₂NO₂ | CH₃CH₂NO₂ |
| Secondary (2°) | R₂CHNO₂ | CH₃CH(NO₂)CH₃ |
| Tertiary (3°) | R₃CNO₂ | (CH₃)₃CNO₂ |
Diagram 2: Primary, secondary and tertiary nitroalkanes
4. Nomenclature of Nitro Compounds
The nitro group is generally treated as the substituent prefix nitro-. Number the parent chain or ring so that substituents receive the lowest appropriate locants.
| Formula / structure | IUPAC name |
|---|---|
| CH₃NO₂ | Nitromethane |
| CH₃CH₂NO₂ | Nitroethane |
| CH₃CH(NO₂)CH₃ | 2-Nitropropane |
| CH₃CH₂CH₂NO₂ | 1-Nitropropane |
| C₆H₅NO₂ | Nitrobenzene |
| 1,3-(NO₂)₂C₆H₄ | 1,3-Dinitrobenzene |
5. Isomerism in Nitro Compounds
5.1 Chain Isomerism
Nitro compounds may have different carbon skeletons for the same molecular formula.
5.2 Position Isomerism
The –NO₂ group may occupy different positions on the same carbon skeleton or aromatic ring.
Example: C₃H₇NO₂
CH₃CH₂CH₂NO₂ = 1-nitropropane
CH₃CH(NO₂)CH₃ = 2-nitropropane
These are position isomers.
5.3 Functional Isomerism with Alkyl Nitrites
Nitroalkanes and alkyl nitrites may have the same molecular formula but differ in connectivity:
R–NO₂ = nitro compound R–O–N=O = alkyl nitrite6. Nitro Compound vs Alkyl Nitrite
| Feature | Nitro compound | Alkyl nitrite |
|---|---|---|
| Connectivity | R–N | R–O |
| Functional structure | R–NO₂ | R–O–N=O |
| Example | CH₃NO₂, nitromethane | CH₃ONO, methyl nitrite |
| Preparation from haloalkane | Favoured with AgNO₂ | Favoured with ionic nitrites such as NaNO₂/KNO₂ in simple substitution |
Diagram 3: Nitro–nitrite functional isomerism
7. Preparation of Nitroalkanes from Haloalkanes
Haloalkanes react with silver nitrite, AgNO₂, to give nitroalkanes.
R–X + AgNO₂ → R–NO₂ + AgXExample
C₂H₅Br + AgNO₂ → C₂H₅NO₂ + AgBr
Exam Important
Use AgNO₂ when the required product is a nitroalkane. Nitrite ion is ambident, and different nitrite reagents can favour different bonding modes.
Diagram 4: Preparation of nitroalkane using AgNO₂
8. Preparation of Nitroalkanes from Alkanes
Alkanes can undergo vapour-phase nitration with nitric acid at elevated temperature, replacing a hydrogen by –NO₂.
R–H + HNO₃ → R–NO₂ + H₂O (high temperature)Simple example
CH₄ + HNO₃ → CH₃NO₂ + H₂O
Important limitation
Higher alkanes may give mixtures because substitution can occur at more than one carbon and radical-chain processes may also cause side reactions. For exam purposes, focus on the general conversion alkane → nitroalkane.
9. Physical Properties of Nitroalkanes
| Property | General behaviour |
|---|---|
| Polarity | Strongly polar because of the nitro group. |
| State | Lower nitroalkanes are generally colourless liquids. |
| Boiling point | Relatively high compared with similar non-polar hydrocarbons because of strong dipole–dipole interactions. |
| Water solubility | Lower members have limited/moderate solubility; solubility generally decreases as the hydrocarbon portion grows. |
| Organic-solvent solubility | Usually soluble in many organic solvents. |
Diagram 5: Polar character of nitro compounds
10. Chemical Property of Nitroalkanes: Reduction
Nitroalkanes are reduced to corresponding primary amines under sufficiently strong reducing conditions.
RNO₂ + 6[H] → RNH₂ + 2H₂OExample
CH₃CH₂NO₂ + 6[H] → CH₃CH₂NH₂ + 2H₂OCommon school-level reducing systems include metal/acid combinations or catalytic hydrogenation.
Diagram 6: Nitroalkane reduction to primary amine
11. Preparation of Nitrobenzene from Benzene
Nitrobenzene is prepared by nitration of benzene using a nitrating mixture of concentrated nitric acid and concentrated sulphuric acid.
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (conc. H₂SO₄)11.1 Electrophile Formation
Sulphuric acid helps generate the nitronium ion, NO₂⁺, which acts as the electrophile.
HNO₃ + H₂SO₄ ⇌ NO₂⁺ + HSO₄⁻ + H₂ODiagram 7: Nitration of benzene
12. Physical Properties of Nitrobenzene
| Property | Description |
|---|---|
| Appearance | Pale yellow to nearly colourless oily liquid when pure. |
| Odour | Characteristic almond-like aromatic odour; odour must never be used as a safety test. |
| Water solubility | Only slightly soluble in water. |
| Organic solvents | Soluble in many organic solvents. |
| Density | Denser than water. |
| Polarity | Strongly polar because of –NO₂. |
Safety
Nitrobenzene is toxic and can be absorbed through skin or inhaled. Its characteristic smell is not a safe way to identify or handle it.
13. Reduction of Nitrobenzene in Different Media
The reduction product of nitrobenzene depends strongly on the reducing agent and reaction medium.
13.1 Acidic Medium — Aniline
Strong reduction in acidic medium converts the nitro group completely into an amino group.
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂OTypical school-level reagent system: Sn/HCl or Fe/HCl, followed by liberation of the free amine.
13.2 Neutral / Mild Reduction — Phenylhydroxylamine
Controlled mild reduction can stop at phenylhydroxylamine.
C₆H₅NO₂ + 4[H] → C₆H₅NHOH + H₂O13.3 Alkaline Medium — Coupled Reduction Products
In alkaline medium, partial reduction can lead through coupled N–N products such as azoxybenzene, azobenzene and hydrazobenzene as reduction proceeds further.
Nitrobenzene → Azoxybenzene → Azobenzene → HydrazobenzeneDiagram 8: Reduction of nitrobenzene in different media
High-yield memory line
Acidic strong reduction → aniline; mild/neutral reduction → phenylhydroxylamine; alkaline partial reduction → coupled azo/azoxy/hydrazo products.
14. Electrophilic Substitution Reactions of Nitrobenzene
The nitro group withdraws electron density from the aromatic ring through both inductive and resonance effects. Therefore nitrobenzene is less reactive than benzene toward electrophilic substitution, and –NO₂ is a meta-directing group.
14.1 Nitration
C₆H₅NO₂ + HNO₃ → m-C₆H₄(NO₂)₂ + H₂O (conc. H₂SO₄)The major product is 1,3-dinitrobenzene (m-dinitrobenzene).
14.2 Sulphonation
C₆H₅NO₂ + H₂SO₄(fuming) → m-NO₂C₆H₄SO₃H + H₂O14.3 Bromination
C₆H₅NO₂ + Br₂ → m-BrC₆H₄NO₂ + HBr (FeBr₃)Diagram 9: Meta-directing effect of –NO₂
Diagram 10: Required electrophilic substitution reactions
15. Uses of Nitro Compounds
- Intermediates for amines: reduction of nitro compounds is an important route to primary amines such as aniline.
- Dye and pigment manufacture: aromatic nitro compounds are useful intermediates in colour-chemical industries.
- Pharmaceutical and fine-chemical synthesis: nitro groups are useful synthetic functional groups that can later be transformed into amino groups.
- Solvents and specialty chemicals: selected nitro compounds have specialized industrial uses.
- Industrial organic synthesis: nitro compounds serve as starting materials or intermediates in multi-step chemical manufacturing.
Safety and environmental note
Many nitro compounds require careful handling because toxicity, flammability or other hazards vary substantially by compound. Their industrial uses should not be interpreted as instructions for uncontrolled preparation or handling.
16. High-Yield Reaction Summary
| Topic | Reaction / key result | Exam point |
|---|---|---|
| Haloalkane → nitroalkane | RX + AgNO₂ → RNO₂ + AgX | Use AgNO₂ |
| Alkane nitration | RH + HNO₃ → RNO₂ + H₂O | High-temperature substitution |
| Nitroalkane reduction | RNO₂ + 6[H] → RNH₂ + 2H₂O | Gives primary amine |
| Nitrobenzene preparation | C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O | Conc. H₂SO₄ generates NO₂⁺ |
| Strong acidic reduction | C₆H₅NO₂ → C₆H₅NH₂ | Aniline |
| Mild reduction | C₆H₅NO₂ → C₆H₅NHOH | Phenylhydroxylamine |
| Alkaline partial reduction | Nitrobenzene → azoxy/azo/hydrazo products | Coupled products |
| Nitration of nitrobenzene | → m-dinitrobenzene | Meta substitution |
| Sulphonation | → m-nitrobenzenesulphonic acid | Meta substitution |
| Bromination | → m-bromonitrobenzene | Meta substitution |
17. Common Exam Mistakes
- Confusing a nitro compound, R–NO₂, with an alkyl nitrite, R–O–N=O.
- Using NaNO₂/KNO₂ when the required textbook product is specifically a nitroalkane from a haloalkane; the standard syllabus preparation uses AgNO₂.
- Forgetting the direct C–N bond in nitroalkanes.
- Calling nitrobenzene an amine because it contains nitrogen. Nitrobenzene contains –NO₂, not –NH₂.
- Writing –NO₂ as an ortho/para director. It is a strongly deactivating meta director.
- Writing nitrobenzene as more reactive than benzene toward electrophiles. It is less reactive.
- Forgetting concentrated H₂SO₄ in nitration of benzene.
- Confusing NO₂⁺ (nitronium ion) with NO₂⁻ (nitrite ion).
- Writing the same reduction product for every medium without stating conditions.
- Forgetting that strong acidic reduction of nitrobenzene ultimately gives aniline.
- Confusing azobenzene, azoxybenzene and hydrazobenzene in partial alkaline reduction.
- Writing para products as major products of nitration, sulphonation or bromination of nitrobenzene.
18. Worked Examples
Worked Example 1: Name CH₃CH(NO₂)CH₃
The parent chain is propane and the nitro group is on carbon 2.
Answer: 2-nitropropane.
Worked Example 2: Nitro or nitrite?
CH₃NO₂: carbon is directly attached to nitrogen → nitromethane.
CH₃ONO: carbon is attached to oxygen → methyl nitrite.
Worked Example 3: Prepare nitroethane from bromoethane
C₂H₅Br + AgNO₂ → C₂H₅NO₂ + AgBr
The nitro group replaces bromine.
Worked Example 4: Reduce nitroethane
C₂H₅NO₂ + 6[H] → C₂H₅NH₂ + 2H₂O
Product: ethylamine.
Worked Example 5: Predict nitration product of nitrobenzene
–NO₂ is strongly deactivating and meta directing.
Major product: 1,3-dinitrobenzene (m-dinitrobenzene).
19. Important Exam Questions
Short-Answer Questions
- Define a nitro compound with an example.
- Write the resonance structures of the nitro group.
- Differentiate aliphatic and aromatic nitro compounds.
- Classify primary, secondary and tertiary nitroalkanes.
- Give IUPAC names of CH₃NO₂, C₂H₅NO₂ and CH₃CH(NO₂)CH₃.
- What is functional isomerism between nitroalkanes and alkyl nitrites?
- Distinguish R–NO₂ from R–ONO.
- How is nitroethane prepared from bromoethane?
- How are nitroalkanes prepared from alkanes?
- State physical properties of lower nitroalkanes.
- What is obtained on reduction of a nitroalkane?
- How is nitrobenzene prepared from benzene?
- What is the electrophile in nitration of benzene?
- State physical properties of nitrobenzene.
- What is the product of strong acidic reduction of nitrobenzene?
- What is formed on mild reduction of nitrobenzene?
- Name the coupled products encountered in alkaline reduction of nitrobenzene.
- Why is nitrobenzene less reactive than benzene toward electrophiles?
- Why is –NO₂ meta directing?
- Write nitration of nitrobenzene.
- Write sulphonation of nitrobenzene.
- Write bromination of nitrobenzene.
- State important uses of nitro compounds.
Long-Answer Questions
- Explain nomenclature and isomerism of nitro compounds with examples.
- Describe preparation of nitroalkanes from haloalkanes and alkanes.
- Discuss physical properties and reduction of nitroalkanes.
- Describe preparation of nitrobenzene from benzene and explain the role of the nitrating mixture.
- Explain reduction of nitrobenzene in acidic, neutral/mild and alkaline media.
- Explain why the nitro group is strongly deactivating and meta directing.
- Describe nitration, sulphonation and bromination of nitrobenzene.
- Compare nitroalkanes and alkyl nitrites.
Conversion Questions
- Bromoethane → nitroethane.
- Nitroethane → ethylamine.
- Benzene → nitrobenzene.
- Nitrobenzene → aniline.
- Nitrobenzene → phenylhydroxylamine.
- Nitrobenzene → m-dinitrobenzene.
- Nitrobenzene → m-bromonitrobenzene.
- Nitrobenzene → m-nitrobenzenesulphonic acid.
Diagram Questions
- Draw resonance structures of the nitro group.
- Draw primary, secondary and tertiary nitroalkanes.
- Draw the structural difference between nitro compound and nitrite ester.
- Draw haloalkane → nitroalkane preparation.
- Draw nitroalkane → amine reduction.
- Draw benzene → nitrobenzene preparation.
- Draw reduction pathways of nitrobenzene in different media.
- Draw the meta-directing effect of –NO₂.
- Draw a reaction map for nitrobenzene electrophilic substitution.
Exam Strategy
This is a compact 3-hour unit. Prioritize four things: nitro vs nitrite connectivity, AgNO₂ preparation, nitrobenzene reduction in different media, and the strongly deactivating/meta-directing nature of –NO₂.
20. One-Minute Revision
- Nitro compounds contain a direct C–NO₂ bond.
- Nitroalkane = R–NO₂; alkyl nitrite = R–O–N=O.
- The nitro group is strongly polar and resonance stabilized.
- 1° nitroalkane = RCH₂NO₂; 2° = R₂CHNO₂; 3° = R₃CNO₂.
- 1-Nitropropane and 2-nitropropane are position isomers.
- RX + AgNO₂ gives RNO₂.
- Alkanes can be nitrated at high temperature with nitric acid.
- Reduction of RNO₂ gives RNH₂.
- Benzene + HNO₃/conc. H₂SO₄ gives nitrobenzene.
- The nitrating electrophile is NO₂⁺.
- Nitrobenzene is a strongly polar aromatic compound.
- Strong acidic reduction of nitrobenzene gives aniline.
- Mild controlled reduction can give phenylhydroxylamine.
- Alkaline partial reduction can produce azoxy-, azo- and hydrazo compounds.
- –NO₂ strongly withdraws electron density from benzene.
- Nitrobenzene is less reactive than benzene toward electrophilic substitution.
- –NO₂ is meta directing.
- Nitration gives mainly m-dinitrobenzene.
- Sulphonation gives mainly meta-substituted product.
- Bromination gives mainly m-bromonitrobenzene.
- Nitro compounds are useful intermediates for amines, dyes and fine chemicals.
21. Diagram Practice
Students should practice these diagrams for the NEB examination:
- Resonance structures of the nitro group.
- Primary, secondary and tertiary nitroalkanes.
- Nitro compound vs nitrite ester.
- Preparation of nitroalkane from haloalkane.
- Polarity of the nitro group.
- Reduction of nitroalkane to primary amine.
- Nitration of benzene to nitrobenzene.
- Reduction of nitrobenzene in different media.
- Meta-directing effect of –NO₂.
- Nitration, sulphonation and bromination reaction map.
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