Amines: Class 12 Chemistry Notes, Chemical Reactions and Diazonium Salts

According to the NCERT textbook, “Amines can be considered as derivatives of ammonia, obtained by replacement of one, two, or all three hydrogen atoms by alkyl and/or aryl groups.”

Simply, it can be explained as:

Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups.

They keep the basic nitrogen personality but gain new characteristics from their organic attachments. From the proteins in your muscles to the neurotransmitters in your brain, amines play crucial roles in most biological processes. 

Amines have a unique combination of properties. The nitrogen atom carries a lone pair of electrons, which gives amines their basic nature of attracting to protons and electron-deficient species, which explains why amines are so reactive and versatile. The general formula of Amines is- R₃N, where R can be hydrogen, alkyl, or aryl groups.

Amines are originally ammonia (NH₃) molecules where one or more hydrogen atoms have been replaced by organic groups. 

The nitrogen atom in amines is at the center with its lone pair of electrons. This lone pair is what gives amines their basic nature, that is, to attract electrons-deficient molecules.

The nitrogen atom in amines is sp³ hybridized, creating a pyramidal shape. This is a three-dimensional shape.

Basic Structure of Amines

The structure of an amine can be written as:

• Primary amine: R-NH₂
• Secondary amine: R-NH-R’
• Tertiary amine: R-N(R’)-R’’

Where R, R’, and R’’ represent alkyl or aryl groups.

Classification of amines depends on how many organic groups are attached to the nitrogen atom. This classification directly affects their properties and reactivity.

Based on the Number of Alkyl/Aryl Groups, Amines are classified as Primary Amines, Secondary Amines, and Tertiary Amines.

Primary Amines (1°)

The structure of primary amines is R-NH₂, with examples such as Methylamine (CH₃NH₂) and ethylamine (C₂H₅NH₂). In a primary amine, two hydrogen atoms are still attached to nitrogen.

Secondary Amines (2°)

The structure is R-NH-R’ or R₂NH, examples such as Dimethylamine ((CH₃)₂NH), Diethylamine ((C₂H₅)₂NH). In a secondary amine, a hydrogen atom is still attached to nitrogen

Tertiary Amines (3°)

The structure is R-N-R’-R’’ or R₃N. For example-Trimethylamine ((CH₃)₃N), Triethylamine ((C₂H₅)₃N). No hydrogen atoms attached to nitrogen

Based on the Type of Carbon Attachment:

Aliphatic Amines are organic compounds in which a hydrogen atoms are replaced by alkyl groups and attached to nitrogen

Examples: Methylamine, Ethylamine, Propylamine

Aromatic Amines are organic compounds in which NH₂ is attached directly to an aromatic ring.

Examples: Aniline (C₆H₅NH₂), N-methylaniline

Naming amines is like learning a new language; once you understand the patterns, naming becomes much more straightforward and follows specific rules that help communicate clearly about these compounds..

IUPAC Nomenclature

For Primary Amines

Replace the ‘e’ in the alkane name with ‘amine’

-Examples:

• CH₃NH₂ → Methanamine
• C₂H₅NH₂ → Ethanamine
• C₃H₇NH₂ → Propanamine

For Secondary and Tertiary Amines

The largest alkyl group is considered the parent chain, and other groups are treated as N-substituents

Examples:

• (CH₃)₂NH → N-methylmethanamine
• (C₂H₅)₂NH → N-ethylethanamine
• (CH₃)₃N → N, N-dimethylmethanamine

Common Names

Many amines have traditional names that are still used in practice. These names often reflect the compound’s origin or properties:

• Aniline: C₆H₅NH₂ (from indigo plant)
• Toluidine: CH₃C₆H₄NH₂ (methylaniline)
• Benzylamine: C₆H₅CH₂NH₂
• Putrescine: H₂N(CH₂)₄NH₂ (found in decaying matter)

In the process of preparing amines, many compounds are used as intermediates in the making of many other important molecules. We can prepare amines through the following methods.

Reduction of Nitriles

This method involves reducing nitriles, which are compounds with the -CN group, to primary amines.

R-CN + 4[H] → R-CH₂-NH₂

Reducing agents used are lithium aluminium hydride (LiAlH₄), hydrogen with nickel catalyst (H₂/Ni), Sodium in ethanol (Na/C₂H₅OH).

Example: CH₃CN + 4[H] → CH₃CH₂NH₂ (ethylamine from acetonitrile)

Reduction of Nitro Compounds

Nitro compounds can be reduced to give primary amines, useful for aromatic amines.

R-NO₂ + 6[H] → R-NH₂ + 2H₂O

Example: C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O (nitrobenzene to aniline)

Gabriel Phthalimide Synthesis

This method is used specifically for producing primary amines without any secondary or tertiary amine contamination.

Phthalimide reacts with KOH to form potassium phthalimide. Alkyl halide displaces potassium to form N-alkylphthalimide. Hydrolysis releases the primary amine.

Phthalimide + KOH → K⁺ phthalimide⁻

K⁺ phthalimide⁻ + R-X → N-alkylphthalimide + KX

N-alkylphthalimide + H₂O/H⁺ → R-NH₂ + phthalic acid

Hofmann Bromamide Degradation

This reaction converts amides to primary amines with one less carbon atom.

R-CONH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

Mechanism:

• An amide reacts with bromine in an alkaline medium, forming N-bromoamide intermediate
• Rearrangement occurs to give isocyanate
• Hydrolysis produces an amine

Example: CH₃CONH₂ + Br₂ + 4NaOH → CH₃NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

The physical properties of amines are determined by their ability to form hydrogen bonds and by their molecular structure. Physical properties make amines unique among organic compounds and are used to explain their practical applications.

 

• State and Odor

 

Lower amines (C₁-C₄):

Gaseous at room temperature, smells strongly fishy- ammonia-like odor. Highly soluble in water.

Medium amines (C₅-C₁₂):

Liquids at room temperature have less intense odor. Moderate water solubility.

Higher amines (>C₁₂):

Solids at room temperature with little to no odor are Insoluble in water.

 

• Boiling Points

 

Amines generally have higher boiling points than corresponding hydrocarbons but lower than alcohols with similar molecular weight.

Order of boiling points:

Primary > Secondary > Tertiary amines

Primary amines can form more hydrogen bonds than secondary, and secondary more than tertiary.

Examples:

• Methylamine (CH₃NH₂): -6°C
• Dimethylamine ((CH₃)₂NH): 7°C
• Trimethylamine ((CH₃)₃N): 3°C

 

• Solubility

 

Considering water solubility, lower aliphatic amines are highly soluble due to hydrogen bonding.

• Solubility decreases as carbon chain length increases
• Aromatic amines are less soluble due to the hydrophobic benzene ring

Solubility in organic solvents:

All amines are soluble in organic solvents like ethanol and ether. Organic solvents are useful for extraction and purification.

Hydrogen Bonding

Amines can act as both hydrogen bond donors (through N-H bonds) and acceptors (through lone pair on nitrogen).

Hydrogen bonding effects result in an increase in boiling point, an increase in water solubility, and affect density and viscosity.

Amines participate in many chemical reactions due to their basic nature and the presence of lone pair electrons on nitrogen.

The lone pair of electrons on nitrogen makes amines nucleophilic (they’re attracted to electron-deficient centers). This nature drives most of the chemical reactions of amines you read below:

Basic Nature of Amines

Amines are bases because they can accept protons (H⁺) from acids, forming salts.

R₃N + HCl → R₃NH⁺Cl⁻

• Basicity order in gas phase: Tertiary > Secondary > Primary > NH₃
• Basicity order in aqueous solution: Secondary > Primary > Tertiary > NH₃

Electron-releasing groups (alkyl groups) will increase the basicity, and Electron-withdrawing groups (aryl groups) will decrease the basicity.

Alkylation Reactions

Amines can react with alkyl halides to form higher amines through nucleophilic substitution.

Primary amine reaction: R-NH₂ + R’-X → R-NH-R’ + HX

Problem with alkylation: Over-alkylation occurs, producing mixtures of products:

R-NH₂ + R’-X → R-NH-R’ (secondary amine)

R-NH-R’ + R’-X → R-N(R’)₂ (tertiary amine)

R-N(R’)₂ + R’-X → R-N⁺(R’)₃X⁻ (quaternary ammonium salt)

Acylation Reactions

Amines react with acid chlorides or anhydrides to form amides.

With acid chlorides: R-NH₂ + R’-COCl → R-NH-CO-R’ + HCl

With acid anhydrides: R-NH₂ + (R’-CO)₂O → R-NH-CO-R’ + R’-COOH

Advantages of acylation are- no over-acylation occurs, clean reaction with a single product, and is used to protect amino groups during synthesis.

Carbylamine Reaction

This is a specific test for primary amines using chloroform and alcoholic KOH.

R-NH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O

Carbylamine formed: R-NC 

It has a foul odor as one of its characteristics.

Only primary amines give a positive carbylamine test

Reaction with Nitrous Acid

Different amines react differently with nitrous acid (HNO₂), So, this test is useful to distinguish between them.

• Primary aliphatic amines:

R-NH₂ + HNO₂ → R-OH + N₂ + H₂O (nitrogen gas evolved)

• Primary aromatic amines:

Ar-NH₂ + HNO₂ + HCl → Ar-N₂⁺Cl⁻ + 2H₂O (diazonium salt formation)

• Secondary amines:

R₂NH + HNO₂ → R₂N-NO + H₂O (N-nitrosoamine formation, yellow oil)

• Tertiary amines:

R₃N + HNO₂ → R₃NH⁺NO₂⁻ (salt formation, no other reaction)

Hofmann Elimination

Quaternary ammonium hydroxides undergo elimination when heated to form alkenes.

R-CH₂-CH₂-N⁺(CH₃)₃OH⁻ → R-CH=CH₂ + (CH₃)₃N + H₂O

Hofmann’s rule: The least substituted alkene is the major product.