Physical & Chemical Properties of Amines

Physical Properties

Here are some essential physical properties of amines:

• Odour: Lower aliphatic amines have a fishy odor and exist in the gaseous state. Primary amines with 3-4 carbon atoms are liquid at room temperature, while those with more than four carbon atoms are solid.
• Colour: Aryl amines, like aniline, are mostly colorless but acquire color due to atmospheric oxidation when exposed.
• Solubility: Lower aliphatic amines can form hydrogen bonds with water molecules, making them soluble. However, as the size of hydrophobic alkyl groups increases, solubility in water decreases.
• Tertiary amines lack free hydrogen atoms, leading to an absence of intermolecular association.
• Intermolecular H-Bonding: Organic solvents such as benzene, ether, and alcohol can dissolve amines due to their lower polarity compared to alcohols, resulting in strong intermolecular hydrogen bonds.

Hydrogen Bonding between Primary Amine Molecules

• Both primary and secondary amines exhibit intermolecular associations through hydrogen bonding between hydrogen and nitrogen atoms.
• In primary amines, the intermolecular association is more prominent than in secondary amines, primarily due to the presence of two hydrogen atoms.
• Boiling Points: The boiling order of amines is: primary amines > secondary amines > tertiary amine

Chemical Properties

Basic nature

Amines are more basic than ammonia. The following reactions prove their basic nature.

(a) It forms ethyl ammonium hydroxide when dissolved in water.

Ethyl ammonium hydroxide ionizes to give OH^- ions.

C₂H₅NH₃OH  C₂H₅ + NH₃ + OH^-

(b) It reacts with acids to form salts.

C₂H₅NH₂ + HCl  C₂H₅NH₃Cl or C₂H₅NH₂.HCl
                                         Ethyl ammonium chloride
                                                 or
                                        Ethylamine hydrochloride

C₂H₅NH₂ + H₂SO₄  (C₂H₅NH₃)₂ SO₄
                                       Ethyl ammonium sulfate

(c) Its aqueous solution behaves like ammonium hydroxide. The aqueous solution of ethylamine precipitates iron, chromium, and aluminum as hydroxides when salts are treated with it.

(d) Its hydrochloride, like ammonium chloride, forms double salts with PtCl₄ and AuCl₃.

These double salts decompose on heating to pure metal and this method is used to determine the molecular mass of amines.

Reaction with Alkyl Halides (Alkylation) 

Ethylamine reacts with alkyl halides and forms secondary, tertiary amines, and quaternary ammonium salt.

Reaction of Ethylamine with Alkyl Halides

Reaction with Acetyl Chloride or Acetic Anhydride

Acetylation takes place when ethylamine combines with acetyl chloride or acetic anhydride.

Carbylamine Reaction

When you heat aliphatic and aromatic primary amines with chloroform and ethanolic potassium hydroxide, they produce foul-smelling substances known as isocyanides or carbylamines.

Nucleophilic RNH₂ attacks electrophilic intermediator [:CCl₂] dichlorocarbene.

This reaction is also known as Isocyanide Test and used for the detection of primary amines. Secondary and tertiary amines do not show this reaction. 

Mechanism-

Reaction with sodium 

Hydrogen is evolved when ethylamine is heated with sodium.

2C₂H₅NH₂ + 2Na  2C₂H₅NHNa + H₂

                                 Sodium derivative
                                    of ethylamine

Reaction with Grignard's reagent

Ethylamine reacts with Grignard's reagent to form alkanes.

Hofmann's Mustard Oil Reaction 

Carbon disulphide reacts with ethylamine in the presence of HgCl₂ to form ethyl isothiocyanate which has a mustard oil-like smell.

C₂H₅NH₂ + CS₂  + HgCl₂  C₂H₅N = C = S + 2HCl
                                                       Ethyl isothiocyanate

Reaction with Aldehydes 

Ethylamine reacts additively with aldehydes to form α-hydroxyl amines which are changed to Schiff bases with the elimination of water molecules.

Ethylamine Reaction with Aldehydes

Distinction between Primary, Secondary, and Tertiary Amines

General Chemical Properties of Aromatic Amines

Reaction with nitrous acid

Primary amines react with nitrous acid to produce diazonium ions as follows.

ArNH₂ + HNO₂  Ar - N⁺ ≡ N :

R - NH₂  + HNO₂  R - N⁺ ≡ N

However, the diazonium ions of aliphatic amines are very unstable and produce carbocation immediately, which can produce different products.

R - N⁺ ≡ N R⁺  (Carbocation)

Electrophilic Substitution

NH₂ - NHR and -NR₂ strongly activate the benzene ring toward electrophilic substitution.

1. Halogenation: For monohalogenation, -NH₂ is first acetylated because

is only moderately activating and is o and p-director in nature.

2. Sulfonation :

The dipolar ion structure of sulfanilic acid accounts for its (a) high melting point, (b) insolubility in H₂O and organic solvents, (c) solubility in aqueous NaOH, (d) insolubility in aqueous HCl.

will not exist as a dipolar ion since -COOH is too weakly acidic to transfer an H to the weakly basic -NH₂ attached to the electron-withdrawing benzene ring. When attached to an aliphatic C, the NH₂ is sufficiently basic to accept H^- from COOH.

The Hinsberg Test

This test can be used to demonstrate whether an amine is primary, secondary, or tertiary. Primary amines react with benzenesulfonyl chloride to form N-substituted benzenesulfonamides. These, in turn, undergo acid-base reactions with the excess potassium hydroxide to form water-soluble potassium salt. Acidification of this solution will cause the water-soluble in the next stage, cause the water-insoluble N-substituted sulfonamide to precipitate.

Secondary amies react with benzene sulfonyl chloride in aqueous potassium hydroxide to form insoluble, N - N-disubstituted sulfonamides that precipitate after the first stage. N₃N^-

Disubstituted sulfonamides do not dissolve in aqueous potassium hydroxide.

If the amine is a tertiary amine and if it is water-insoluble, no apparent change will take place in the mixture as we shake it with benzene sulfonyl chloride and aqueous KOH. When we acidify the mixture, the tertiary amine dissolves because it forms a water-soluble salt.