Nitro Compounds – NEB Class 12 Notes – Organic Chemistry

Chapter 15: Nitro Compounds

NEB Class 12 Chemistry Unit 15 Nitro Compounds Notes, Exercise, Question Answer, PDF 2081-2082 (2024) Academic Year Notes.

Nitro Compound Syllabus

• 15.1 Nitroalkanes
• 15.1.1 Introduction, nomenclature, and isomerism
• 15.1.2 Preparation from haloalkane and alkane
• 15.1.3 Physical properties
• 15.1.4 Chemical properties: Reduction
• 15.2 Nitrobenzene
• 15.2.1 Preparation from benzene
• 15.2.2 Physical properties
• 15.2.3 Chemical properties
• 15.2.4 Reduction in different media
• 15.2.5 Electrophilic substitution reactions (nitration, sulphonation & bromination)
• 15.2.6 Uses of nitro-compounds

Introduction to Nitro Compounds

Organic compounds containing nitro group as a functional group are called nitro compounds. They may be aliphatic as well as aromatic.

Types of Nitro Compounds

• Aliphatic Nitro Compounds
• Aromatic Nitro Compounds

Aliphatic Nitro Compounds:

When nitro groups are attached to the aliphatic hydrocarbons, such nitro compounds are called aliphatic nitro compounds.

Aliphatic Nitro Compound Types:

Aliphatic nitro compounds are classified into three categories.

(a) Primary nitro compounds (1^° nitro compounds.)
(b) Secondary nitro compounds (2° nitro compounds.)
(c) Tertiary nitro compounds (3° nitro compounds.)

The chemical properties of nitro alkane and alkyl nitrite (isomeric compound) are quite different and nitro alkanes are much more stable than alkyl nitrite.

Preparation of aliphatic nitroalkanes:

Nitroalkanes can be prepared by the following methods:

(a) From alkanes (From Nitration of Alkanes)

When higher alkanes are heated with fuming nitric acid at 250°C – 400°C under pressure give nitro alkanes. By this process it is very difficult to produce pure alkanes because there is random cleavage of covalent bonds to give different nitro alkanes.

(b) From alkyl halide:

Nitroalkanes can generally be prepared from alkylhalide (Generally bromide and iodide) when treated with alcoholic solution of silver nitrite.

2° and 3° nitro alkanes can more easily be prepared. The order of ease of abstraction of hydrogen by nitro group is given as;

3° > 2° > 1°

Alkali metal nitrite is not used because instead of nitroalkanes, alkyl nitrite is formed.

Physical Properties of Aliphatic Nitroalkanes:

1. Physical Appearance and Odor: Aliphatic nitroalkanes are colorless liquids with a pleasant aroma.

2. Solubility: These compounds exhibit slight solubility in water but are readily soluble in organic solvents.

3. Polarity and Boiling Points: As polar molecules, aliphatic nitroalkanes experience strong intermolecular attractions due to dipole-dipole interactions, which result in high boiling points.

Chemical Properties of Aliphatic Nitroalkanes:

Nitro compounds may be reduced to variety of compounds depending up on the nature of reducing agent and medium. The reduction proceeds through the following stages.

(a) Reduction in acidic medium:

Both aliphatic and aromatic nitro compounds can be reduced to primary amines using various reducing agents such as Sn/HCl, Zn/HCl, Fe/HCl, or LiAlH4 in an acidic medium.

Excessive use of HCl during the reduction process can lead to the formation of a salt with the resulting amine product.

(b) Reduction in Neutral medium:

In a neutral medium, both aliphatic and aromatic nitro compounds are reduced to hydroxylamines.

(c) Catalytic Reduction Process:

Nitroalkanes can be reduced to primary amines in the presence of hydrogen (H₂) and a catalyst such as nickel (Ni), platinum (Pt), or palladium (Pd).

(d) Reduction with LiAlH₄:

Nitroalkanes can also be reduced to primary amines using lithium aluminum hydride (LiAlH₄) as a reducing agent.

Uses of Aliphatic Nitro Compounds

1. Solvents: Lower nitroalkanes are used as solvents for fats, oils, and dyes.

2. Explosives and Chemical Intermediates: Nitro compounds serve as explosives and chemical intermediates.

3. Insecticides: Chloropicrin (CCl₃NO₂) is a significant insecticide.

4. Synthesis of Other Compounds: Nitro compounds are used to prepare important organic compounds such as hydroxylamine, amines, and nitrosoamines.