Structure Of Amines

Amines are derivatives of ammonia ($NH_3$) where one, two, or all three hydrogen atoms are replaced by alkyl (R) or aryl (Ar) groups. They are classified as primary ($RNH_2$), secondary ($R_2NH$), or tertiary ($R_3N$) based on the number of alkyl/aryl groups attached to the nitrogen atom. The nitrogen atom in amines is $sp^3$ hybridized and has a lone pair of electrons, giving amines a pyramidal shape and Lewis basic character. The bond angle around nitrogen is typically less than the tetrahedral angle due to lone pair repulsion.

Classification

Amines are classified based on the number of hydrogen atoms in ammonia replaced by alkyl or aryl groups:

• Primary Amines (1°): One hydrogen replaced (R-NH₂ or Ar-NH₂).
• Secondary Amines (2°): Two hydrogens replaced ($R_2NH$ or $R-NH-R'$).
• Tertiary Amines (3°): All three hydrogens replaced ($R_3N$ or $RNR'R''$).

Simple amines have identical R/Ar groups, while mixed amines have different groups.

Nomenclature

Common Names: Derived by naming the alkyl/aryl group(s) followed by 'amine' (e.g., methylamine, diethylamine). For secondary/tertiary amines with different groups, prefixes 'N-' and 'N,N-' indicate substituents on the nitrogen.

IUPAC Names: Alkanes are named 'alkanamines' by replacing '-e' with 'amine'. For polyamines, locants and prefixes (di-, tri-) are used. For secondary and tertiary amines, substituents on nitrogen are denoted by 'N-' (e.g., N-methylethanamine for $CH_3NHCH_2CH_3$). Aromatic amines like aniline are named benzenamine in IUPAC, with substituents using locants or ortho-/para- prefixes.

Preparation Of Amines

Amines are synthesized through several methods:

• Reduction of Nitro Compounds: Nitroalkanes and nitroarenes are reduced to primary amines using $LiAlH_4$, catalytic hydrogenation ($H_2$/Ni, Pd, Pt), or metals in acid (e.g., Fe/HCl, Sn/HCl).
• Ammonolysis of Alkyl Halides: Reaction of alkyl halides with ammonia yields primary amines, which can further react to form secondary, tertiary amines, and quaternary ammonium salts. Excess ammonia favors primary amine formation.
• Reduction of Nitriles: Reduction with $LiAlH_4$ or catalytic hydrogenation converts nitriles ($R-C \equiv N$) to primary amines, extending the carbon chain by one.
• Reduction of Amides: $LiAlH_4$ reduction of amides ($R-CONH_2$) yields amines with the same number of carbon atoms.
• Gabriel Phthalimide Synthesis: A method for preparing primary amines, involving the alkylation of phthalimide anion followed by hydrolysis. It avoids over-alkylation and is suitable for primary amines but not aromatic ones.
• Hofmann Bromamide Degradation: Primary amines are formed from amides by reaction with bromine and a strong base ($NaOH$). This reaction involves rearrangement and results in an amine with one less carbon atom than the parent amide.

Physical Properties

Lower aliphatic amines (up to C3) are gases with fishy odours. Higher members are liquids or solids. Lower amines are soluble in water due to hydrogen bonding between N-H and water molecules. Solubility decreases with increasing hydrocarbon chain length. Tertiary amines lack N-H bonds, so they cannot form intermolecular hydrogen bonds with themselves, resulting in lower boiling points than primary and secondary amines of similar molecular mass. Primary amines have higher boiling points than secondary amines due to more extensive hydrogen bonding.

Chemical Reactions

Amines exhibit reactivity due to the lone pair of electrons on nitrogen and the presence of N-H bonds (in primary and secondary amines).

Basic Character Of Amines

Amines are basic due to the lone pair of electrons on nitrogen, readily accepting protons from acids to form ammonium salts. Aliphatic amines are stronger bases than ammonia due to the electron-donating (+I) effect of alkyl groups, increasing electron density on nitrogen. However, in aqueous solutions, factors like solvation and steric hindrance modify the basicity order (which isn't a simple +I effect order). Aromatic amines are weaker bases than ammonia because the lone pair on nitrogen is delocalized into the benzene ring via resonance, making it less available for protonation.

Alkylation

Amines react with alkyl halides via nucleophilic substitution ($S_N2$) to form secondary, tertiary amines, and quaternary ammonium salts.

Acylation

Primary and secondary amines react with acylating agents (acid chlorides, anhydrides) to form amides, involving nucleophilic acyl substitution.

Carbylamine Reaction

Primary amines (aliphatic and aromatic) react with chloroform ($CHCl_3$) and alcoholic KOH to form foul-smelling isocyanides (carbylamines). Secondary and tertiary amines do not give this reaction, making it a test for primary amines.

Reaction With Nitrous Acid

• Primary Aliphatic Amines: React with nitrous acid ($HNO_2$, prepared in situ from $NaNO_2$/$HCl$ at low temps) to form unstable diazonium salts, which decompose to yield nitrogen gas and alcohols.
• Primary Aromatic Amines: Form stable arenediazonium salts at low temperatures (273-278 K), crucial intermediates in synthesis.
• Secondary and Tertiary Amines: React differently, forming nitrosamines or undergoing other reactions.

Reaction With Arylsulphonyl Chloride

Benzenesulfonyl chloride (Hinsberg's reagent) reacts differently with primary, secondary, and tertiary amines. Primary amines form acidic sulfonamides soluble in alkali. Secondary amines form non-acidic sulfonamides insoluble in alkali. Tertiary amines do not react. This reaction distinguishes between the three classes of amines.

Electrophilic Substitution

The amino group (-NH₂) is a powerful activating and ortho-, para- directing group in electrophilic aromatic substitution reactions due to resonance delocalization of the lone pair into the ring. However, in acidic media, the amine gets protonated to form an anilinium ion ($Ar-NH_3^+$), which is deactivating and meta-directing. To control reactivity and achieve monosubstitution, the amino group is often protected by acetylation before undergoing reactions like nitration or halogenation.

Method Of Preparation Of Diazoniun Salts

Arenediazonium salts ($Ar-N_2^+ X^-$) are prepared from primary aromatic amines by diazotization – reaction with nitrous acid ($HNO_2$, generated in situ from $NaNO_2$ and $HCl$) at low temperatures (273-278 K). These salts are unstable in solution at higher temperatures but can be stabilized as fluoroborates ($Ar-N_2^+ BF_4^-$). They are key intermediates in aromatic synthesis.

Physical Properties

Lower aliphatic amines are gases with fishy odours. Higher aliphatic amines are liquids or solids. Arylamines are often colourless liquids but darken on exposure to air. Lower aliphatic amines are soluble in water due to hydrogen bonding. Solubility decreases with increasing molecular size. Tertiary amines have lower boiling points than primary and secondary amines of similar mass due to lack of intermolecular H-bonding. Aliphatic amines are generally stronger bases than ammonia, while aromatic amines are weaker due to resonance delocalization of the lone pair.

Chemical Reactions

Amines undergo reactions based on the basicity of the nitrogen atom and the presence of N-H bonds:

Reactions Involving Displacement Of Nitrogen

The diazonium group (-N₂⁺) is an excellent leaving group and is readily displaced by various nucleophiles, often catalyzed by copper(I) salts (Sandmeyer reaction). This allows introduction of halogens (-Cl, -Br), cyanide (-CN), hydroxyl (-OH), and nitro (-NO₂) groups onto the aromatic ring.

• Halogenation/Cyanation: Using $CuCl/HCl$, $CuBr/HBr$, or $CuCN/KCN$. Gattermann reaction uses copper powder.
• Iodination: Direct reaction with KI.
• Fluorination: Via diazonium fluoroborates ($Ar-N_2^+ BF_4^-$) upon heating.
• Reduction to Arenes: Using reducing agents like $H_3PO_2$ or ethanol.
• Hydrolysis to Phenols: Warming the diazonium salt solution with water.

Reactions Involving Retention Of Diazo Group

Azo Coupling Reactions: Arenediazonium salts react with phenols or aromatic amines (at ortho- or para- positions) in mild alkaline or acidic conditions, respectively, to form azo compounds ($Ar-N=N-Ar'$). These are important dyes due to their extended conjugated systems.

Importance Of Diazonium Salts In Synthesis Of Aromatic Compounds

Diazonium salts are highly versatile intermediates in organic synthesis. They allow the introduction of various functional groups (-Cl, -Br, -I, -CN, -F, -OH, -NO₂) onto an aromatic ring, often under milder conditions and with better yields than direct electrophilic substitution methods. They are particularly important for synthesizing aryl iodides, fluorides, cyanides, and phenols, which are difficult to obtain otherwise. They are also crucial for producing azo dyes through coupling reactions.

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