Physical and Chemical Properties — Explained

Detailed Explanation

Amines are fundamental organic compounds, essentially derivatives of ammonia (NH$_3$) where one, two, or all three hydrogen atoms are replaced by alkyl or aryl groups. Their properties are profoundly influenced by the nitrogen atom's lone pair of electrons and its electronegativity, leading to characteristic physical and chemical behaviors.

\n\nI. Physical Properties of Amines\n\n1. Physical State and Odour:\n * Lower aliphatic amines (e.g., methylamine, ethylamine) are gases at room temperature. They possess a characteristic 'fishy' or ammonia-like odour.

This odour becomes more pronounced and unpleasant with increasing molecular weight.\n * Primary amines with three or more carbon atoms are liquids. Higher amines are solids.\n * Aromatic amines, such as aniline, are typically colourless liquids or low-melting solids.

However, they are prone to oxidation upon exposure to air and light, which often causes them to turn dark or brownish.\n\n2. Boiling Points:\n * Amines generally have higher boiling points compared to non-polar compounds (like alkanes) of comparable molecular mass.

This is attributed to the presence of intermolecular hydrogen bonding.\n * Hydrogen Bonding: Primary ($1^\circ$) and secondary ($2^\circ$) amines have hydrogen atoms directly bonded to the electronegative nitrogen atom (N-H bonds).

This allows them to form intermolecular hydrogen bonds, albeit weaker than those formed by alcohols (O-H bonds) because nitrogen is less electronegative than oxygen. The strength of hydrogen bonding follows the order: Alcohols > Primary amines > Secondary amines.

\n * Tertiary ($3^\circ$) amines lack hydrogen atoms directly attached to nitrogen, so they cannot form intermolecular hydrogen bonds among themselves. Consequently, their boiling points are lower than those of $1^\circ$ and $2^\circ$ amines of comparable molecular mass.

However, $3^\circ$ amines can still act as hydrogen bond acceptors with protic solvents like water or alcohols.\n * Order of Boiling Points (for isomeric amines): $1^\circ > 2^\circ > 3^\circ$. For example, among C$_4$H$_9$NH$_2$ (butylamine), (C$_2$H$_5$)$_2$NH (diethylamine), and (CH$_3$)$_3$N (trimethylamine), butylamine will have the highest boiling point, followed by diethylamine, and then trimethylamine.

\n\n3. Solubility in Water:\n * Lower molecular weight amines (up to 5-6 carbon atoms) are appreciably soluble in water. This solubility is due to their ability to form hydrogen bonds with water molecules.

The nitrogen atom's lone pair can accept hydrogen bonds from water, and the N-H hydrogens (in $1^\circ$ and $2^\circ$ amines) can donate hydrogen bonds to water oxygen.\n * As the size of the hydrophobic alkyl or aryl group increases, the extent of hydrogen bonding with water becomes less significant compared to the non-polar interactions, leading to a decrease in water solubility.

For instance, aniline is only sparingly soluble in water.\n * All amines are generally soluble in common organic solvents such as alcohol, ether, and benzene, due to similar intermolecular forces.\n\n**II.

Chemical Properties of Amines\n\nChemical properties of amines are primarily dictated by the lone pair of electrons on the nitrogen atom, making them basic and nucleophilic.\n\n1. Basicity of Amines:**\n * Amines are Brønsted-Lowry bases (proton acceptors) and Lewis bases (electron pair donors) due to the presence of the lone pair of electrons on the nitrogen atom.

They react with acids to form ammonium salts.\n * Factors Affecting Basicity:\n * Inductive Effect: Electron-donating groups (like alkyl groups) increase electron density on nitrogen, making the lone pair more available for protonation, thus increasing basicity.

Electron-withdrawing groups decrease basicity.\n * Resonance Effect: In aromatic amines (e.g., aniline), the lone pair on nitrogen is delocalized into the benzene ring through resonance. This delocalization makes the lone pair less available for protonation, significantly reducing basicity compared to aliphatic amines or ammonia.

\n * Steric Hindrance: In the gas phase, basicity order is $3^\circ > 2^\circ > 1^\circ > NH_3$ due to the cumulative inductive effect. However, in aqueous solution, solvation effects play a crucial role.

The stability of the conjugate acid (ammonium ion) formed after protonation is enhanced by hydrogen bonding with water molecules. The more hydrogen atoms on the nitrogen of the ammonium ion, the more extensive the solvation.

This leads to a different order of basicity in aqueous solution:\n * For methyl-substituted amines: $2^\circ > 1^\circ > 3^\circ > NH_3$ (e.g., (CH$_3$)$_2$NH > CH$_3$NH$_2$ > (CH$_3$)$_3$N > NH$_3$)\n * For ethyl-substituted amines: $2^\circ > 3^\circ > 1^\circ > NH_3$ (e.

g., (C$_2$H$_5$)$_2$NH > (C$_2$H$_5$)$_3$N > C$_2$H$_5$NH$_2$ > NH$_3$)\n The exact order depends on the balance between inductive effect, steric hindrance (which hinders solvation), and solvation effects.

Generally, secondary amines are the strongest bases in aqueous solution.\n\n2. Alkylation (Reaction with Alkyl Halides):\n * Amines act as nucleophiles and react with alkyl halides (R-X) in a nucleophilic substitution (S$_N$2) reaction.

This reaction is known as Hofmann Ammonolysis.\n * A primary amine ($1^\circ$) reacts with an alkyl halide to form a secondary amine ($2^\circ$). The $2^\circ$ amine can further react to form a tertiary amine ($3^\circ$), and finally, a quaternary ammonium salt.

\n * Example: $RNH_2 + R'X \rightarrow RNHR' + HX \rightarrow R_2NR' + HX \rightarrow R_3NR' + HX \rightarrow R_3N^+R'X^-$ (Quaternary ammonium salt)\n * This reaction often leads to a mixture of products ($2^\circ, 3^\circ$ amines, and quaternary salts), making it less suitable for preparing specific higher amines.

To favor the formation of a primary amine, a large excess of ammonia can be used.\n\n3. Acylation (Reaction with Acid Chlorides, Anhydrides, Esters):\n * Primary and secondary amines react with acid chlorides, acid anhydrides, or esters to form amides.

This reaction involves the nucleophilic attack of the amine nitrogen on the carbonyl carbon, followed by the elimination of a leaving group.\n * Reaction with Acid Chlorides: $RNH_2 + R'COCl \rightarrow RNHCOR' + HCl$ (Amide)\n * Reaction with Acid Anhydrides: $RNH_2 + (R'CO)_2O \rightarrow RNHCOR' + R'COOH$\n * Tertiary amines do not undergo acylation because they lack a replaceable hydrogen atom on the nitrogen atom.

\n * Aromatic amines also undergo acylation. For example, aniline reacts with acetyl chloride to form acetanilide.\n\n4. Carbylamine Reaction (Isocyanide Test):\n * This reaction is a characteristic test for primary amines (both aliphatic and aromatic).

When a primary amine is heated with chloroform (CHCl$_3$) and an alcoholic solution of potassium hydroxide (KOH), it forms an isocyanide (or carbylamine), which has a highly offensive smell.\n * $RNH_2 + CHCl_3 + 3KOH \xrightarrow{\text{heat}} R-NC + 3KCl + 3H_2O$\n * Secondary and tertiary amines do not give this reaction.

\n\n5. **Reaction with Nitrous Acid (HNO$_2$):**\n * Nitrous acid is unstable and is usually generated *in situ* by mixing sodium nitrite (NaNO$_2$) and dilute hydrochloric acid (HCl) at low temperatures (0-5 $^\circ$C).

\n * Primary Aliphatic Amines: React with nitrous acid to form highly unstable aliphatic diazonium salts, which immediately decompose to form alcohols with the evolution of nitrogen gas. This is a useful method for converting $1^\circ$ amines to alcohols.

\n $RNH_2 + HNO_2 \xrightarrow{0-5^\circ C} [R-N_2^+Cl^-] \rightarrow ROH + N_2 \uparrow + HCl$\n * Primary Aromatic Amines: React with nitrous acid at low temperatures (0-5 $^\circ$C) to form relatively stable aromatic diazonium salts.

These diazonium salts are important intermediates in organic synthesis (e.g., Sandmeyer reaction, Gattermann reaction, coupling reactions).\n $ArNH_2 + HNO_2 \xrightarrow{0-5^\circ C} Ar-N_2^+Cl^- + 2H_2O$\n * Secondary Amines (Aliphatic and Aromatic): React with nitrous acid to form N-nitrosoamines (yellow oily compounds).

These compounds are generally carcinogenic.\n $R_2NH + HNO_2 \rightarrow R_2N-N=O + H_2O$\n * Tertiary Aliphatic Amines: React with nitrous acid to form trialkylammonium nitrite salts.\n $R_3N + HNO_2 \rightarrow R_3NH^+NO_2^-$ (salt)\n * Tertiary Aromatic Amines: Undergo electrophilic substitution at the para position (if available) to form p-nitroso-N,N-dialkylaniline.

\n\n6. Electrophilic Substitution (for Aromatic Amines):\n * The amino group ($-NH_2$) in aromatic amines (like aniline) is a powerful activating group and an ortho-para director due to the resonance effect, which increases electron density at these positions.

\n * Bromination: Aniline reacts with bromine water at room temperature to give 2,4,6-tribromoaniline as a white precipitate. The activating effect is so strong that all three ortho and para positions are substituted.

\n $C_6H_5NH_2 + 3Br_2 \xrightarrow{H_2O} C_6H_2(Br)_3NH_2 + 3HBr$\n * To obtain monobrominated aniline, the activating effect of the amino group must be reduced. This is achieved by acetylation (acylation with acetic anhydride) to form acetanilide.

The lone pair on nitrogen is then delocalized into both the benzene ring and the carbonyl group, reducing its activating power. After bromination, the acetyl group can be hydrolyzed back to the amino group.

\n * Nitration: Direct nitration of aniline with nitrating mixture (conc. HNO$_3$ + conc. H$_2$SO$_4$) yields a mixture of ortho, meta, and para products, with a significant amount of meta-nitroaniline (around 47%) due to the formation of anilinium ion ($C_6H_5NH_3^+$) in strongly acidic medium, which is meta-directing and deactivating.

To obtain para-nitroaniline as the major product, the amino group is protected by acetylation first.\n * Sulphonation: Aniline reacts with concentrated sulphuric acid to form anilinium hydrogen sulphate, which on heating to 453-473 K produces sulphanilic acid (p-aminobenzenesulphonic acid).

Sulphanilic acid exists as a zwitterion.\n\n7. Oxidation:\n * Amines are susceptible to oxidation. Primary and secondary amines can be oxidized to various products depending on the oxidizing agent and conditions.

For example, primary amines can be oxidized to nitroso compounds, nitro compounds, or even carboxylic acids.\n * Aromatic amines, especially aniline, are readily oxidized in air, leading to the formation of coloured products (often dark brown or black) due to complex polymerization reactions.

This is why aniline should be stored in dark, airtight bottles.