Amines & Diazonium Salts – Chemistry Study Notes

Classification and Structure of Amines

Amines are broadly classified based on the number of hydrogen atoms of ammonia replaced by alkyl or aryl groups. When one hydrogen atom is replaced, we get a primary ($1^\circ$) amine ($R-NH_2$). The replacement of two hydrogen atoms yields a secondary ($2^\circ$) amine ($R_2NH$), and replacing all three gives a tertiary ($3^\circ$) amine ($R_3N$).

The nitrogen atom in amines undergoes $sp^3$ hybridization. Due to the presence of a lone pair of electrons on the nitrogen atom, the geometry is pyramidal, closely resembling that of ammonia with a bond angle slightly distorted from the tetrahedral angle (around $107^\circ$).

Amines can also be classified into aliphatic amines, where nitrogen is directly attached to an alkyl carbon, and aromatic amines, where nitrogen is bonded directly to an aryl carbon ring.

When naming amines according to the IUPAC system, primary aliphatic amines are named as alkanamines. For example, $CH_3CH_2NH_2$ is ethanamine. In secondary and tertiary amines, the largest alkyl group is chosen as the parent chain, while smaller alkyl groups attached to nitrogen are designated as N-substituents, such as *N-methylethanamine*.

Methods of Preparation of Amines

Several standard laboratory and industrial routes exist for synthesizing amines, each with distinct advantages and yield considerations. One classic method is the reduction of nitro compounds. Nitroalkanes and nitroarenes can be catalytically reduced using hydrogen in the presence of palladium, platinum, or nickel, or chemically reduced using metals like iron or tin in an acidic medium.

Another fundamental route is ammonolysis of alkyl halides, where an alkyl halide is treated with an ethanolic solution of ammonia. This nucleophilic substitution reaction yields a mixture of primary, secondary, tertiary amines, and quaternary ammonium salts.

To selectively prepare a pure primary amine, the Gabriel Phthalimide Synthesis is preferred. In this sequence, potassium phthalimide is alkylated with an alkyl halide followed by alkaline hydrolysis or hydrazinolysis to yield a primary aliphatic amine without secondary or tertiary contamination.

“The Gabriel Phthalimide synthesis is exclusively used for preparing primary aliphatic amines because aryl halides do not undergo nucleophilic substitution with potassium phthalimide under mild conditions.”

Additionally, amines with one carbon atom more than the starting material can be synthesized via the Hofmann Bromamide Degradation reaction. Here, an amide is treated with bromine and an aqueous or ethanolic solution of sodium hydroxide, resulting in the degradation of the amide to a primary amine with the elimination of a carbonyl carbon as carbonates.

Physical Properties and Basic Character of Amines

The physical properties of amines are heavily influenced by intermolecular hydrogen bonding. Primary and secondary amines engage in intermolecular association due to N-H bonds, leading to higher boiling points compared to non-polar compounds of comparable molecular mass, though lower than those of corresponding alcohols.

Tertiary amines, lacking hydrogen atoms on the nitrogen, cannot form intermolecular hydrogen bonds among themselves. This results in lower boiling points than isomeric primary and secondary amines.

Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules. However, solubility decreases as the hydrophobic alkyl chain length increases. Aromatic amines are generally insoluble in water due to the large hydrophobic benzene ring.

Amines act as Lewis bases because of the unshared electron pair on the nitrogen atom. The basic strength of amines can be measured by their $K_b$ or $pK_b$ values.

In the gaseous phase, basicity follows the inductive effect order: $3^\circ \text{ amine} > 2^\circ \text{ amine} > 1^\circ \text{ amine} > NH_3$. However, in aqueous solutions, solvation effects and steric hindrance alter this trend.

For methyl-substituted amines in water, the order is $2^\circ > 1^\circ > 3^\circ$, while for ethyl-substituted amines, it is $2^\circ > 3^\circ > 1^\circ$.

Distinction Between Primary, Secondary, and Tertiary Amines

Differentiating among $1^\circ$, $2^\circ$, and $3^\circ$ amines is a frequent testing point in competitive examinations. The most reliable chemical method is the Hinsberg Test, which employs benzene sulphonyl chloride ($C_6H_5SO_2Cl$), commonly known as Hinsberg’s reagent.

• Primary Amines: React with Hinsberg’s reagent to form an N-alkylbenzenesulphonamide. This sulphonamide contains a strongly acidic hydrogen attached to nitrogen, making it soluble in aqueous alkali ($\text{NaOH}$).
• Secondary Amines: React to form an N,N-dialkylbenzenesulphonamide. Because this product lacks an acidic hydrogen on the nitrogen atom, it remains insoluble in aqueous alkali.
• Tertiary Amines: Do not react with Hinsberg’s reagent at all because they lack a replaceable hydrogen atom on the nitrogen.

Another classical distinction is the Carbylamine Test, used exclusively for primary amines. When a primary amine is heated with chloroform and ethanolic potassium hydroxide, it produces foul-smelling isocyanides (carbylamines). Secondary and tertiary amines do not give this test.

Preparation and Synthetic Utility of Benzene Diazonium Chloride

Diazonium salts have the general formula $Ar-N_2^+ X^-$, where $X^-$ is $\text{Cl}^-$, $\text{HSO}_4^-$, etc. Benzene diazonium chloride is prepared by the diazotization reaction, treating aniline with nitrous acid ($\text{HNO}_2$, generated *in situ* from $\text{NaNO}_2$ and $\text{HCl}$) at low temperatures ranging from $273\text{ K}$ to $278\text{ K}$. Maintaining this low temperature is critical to prevent the decomposition of the unstable diazonium salt into phenol.

The synthetic utility of benzene diazonium chloride lies in its ability to undergo two major classes of reactions: replacement of the diazo group liberating nitrogen gas, and coupling reactions retaining the $-\text{N}=\text{N}-$ linkage.

1. Replacement by Halogen or Cyano Group (Sandmeyer Reaction): Treatment with cuprous chloride ($\text{CuCl}$), cuprous bromide ($\text{CuBr}$), or cuprous cyanide ($\text{CuCN}$) dissolved in the corresponding halo-acid introduces chloro, bromo, or cyano groups.
2. Gattermann Reaction: A modification of the Sandmeyer reaction using copper powder in the presence of $\text{HCl}$ or $\text{HBr}$ to yield aryl halides.
3. Replacement by Iodine: Simply shaking the diazonium salt solution with potassium iodide ($\text{KI}$) yields iodobenzene.
4. Replacement by Fluorine (Schiemann Reaction): Treating benzene diazonium chloride with fluoroboric acid gives fluorobenzene diazonium fluoroborate, which on thermal decomposition yields fluorobenzene.
5. Coupling Reactions: Benzene diazonium chloride reacts with electron-rich aromatic compounds like phenol and aniline in mild acidic or alkaline medium to form brightly colored azo dyes (e.g., p-hydroxyazobenzene, an orange dye).

Key Points to Remember

• Amine basicity in aqueous solution depends on inductive effect, steric hindrance, and solvation stabilization of the protonated cation.
• Aniline does not undergo Friedel-Crafts alkylation or acylation because the Lewis acid ($\text{AlCl}_3$) coordinates with the lone pair on nitrogen, forming an inactive complex.
• Hinsberg’s reagent ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$) converts primary amines into alkali-soluble sulphonamides.
• Diazotization must be performed strictly between $0^\circ\text{C}$ and $5^\circ\text{C}$ ($273-278\text{ K}$).
• Gabriel Phthalimide synthesis yields exclusively primary aliphatic amines without contamination.
• Hofmann Bromamide degradation results in the loss of a carbonyl carbon atom, forming a primary amine with one carbon less.
• Carbylamine test is a definitive chemical test for primary aliphatic and aromatic amines.

Important Facts / Formulas

Previous Year Question Hints

1. Question Hint: When an organic compound reacts with benzenesulphonyl chloride to form a precipitate soluble in alkali, look for a primary amine. If insoluble, look for secondary; if no reaction, look for tertiary.
2. Question Hint: Questions on basicity order often test whether the medium is gaseous or aqueous, and whether the alkyl group is methyl or ethyl. Remember the classic exceptions driven by steric hindrance and hydration energy.
3. Question Hint: Synthetic conversion chains starting from nitrobenzene usually require reduction to aniline, diazotization to benzene diazonium chloride, followed by Sandmeyer or coupling reactions to construct complex bi-aryl architectures.

Quick Revision Summary

• Amines are classified as $1^\circ$, $2^\circ$, or $3^\circ$ depending on the replacement of hydrogen atoms in ammonia.
• Basic strength in water for methyl amines follows $2^\circ > 1^\circ > 3^\circ$, contrasting with gas-phase basicity trends.
• Hinsberg test separates amine classes based on the aqueous alkali solubility of their respective sulphonamides.
• Gabriel synthesis is specific to primary aliphatic amines, avoiding poly-alkylation side products.
• Hofmann bromamide degradation shortens the carbon chain by one unit via an isocyanate intermediate.
• Diazonium salts are versatile synthetic intermediates for introducing $-\text{Cl}$, $-\text{Br}$, $-\text{I}$, $-\text{CN}$, $-\text{F}$, and $-\text{OH}$ groups onto aromatic rings.
• Azo coupling reactions of diazonium salts proceed via electrophilic aromatic substitution with activated benzene rings like phenols and anilines.
• Low-temperature maintenance ($0-5^\circ\text{C}$) is mandatory during diazotization to avoid decomposition into phenols.