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Loading home…Amines are organic derivatives of ammonia in which alkyl/aryl groups replace hydrogen.
The functional group of primary amines is –NH₂.
Methylamine, CH₃NH₂, is a primary amine.
Secondary amines have R₂NH; tertiary amines have R₃N.
Trimethylamine, (CH₃)₃N, is a tertiary amine with a fishy smell.
Aniline, C₆H₅NH₂, is the simplest aromatic amine.
Amines have a lone pair on nitrogen that can accept a proton.
They are Bronsted bases (proton acceptors) and Lewis bases (electron-pair donors).
Reaction with acids gives ammonium salts.
Aliphatic amines are generally stronger bases than ammonia in water.
Aniline is a weaker base than aliphatic amines because the lone pair conjugates with the ring.
Basicity is central to amine reactions and salt formation.
Aniline is the parent aromatic amine for many dye intermediates.
Diazotisation of aniline gives benzenediazonium salts.
Coupling of diazonium salts with phenols/amines produces azo dyes.
Azo dyes are brightly coloured and widely used on textiles.
Aniline itself is toxic and must be handled carefully.
Industrial aromatic amine chemistry grew with the synthetic dye industry.
Arenediazonium salts have Ar–N₂⁺ X⁻, formed from primary aromatic amines + HNO₂.
They are prepared cold (about 0–5 °C) to avoid decomposition.
Diazonium ions undergo substitution to introduce –OH, –X, –CN, etc.
Coupling reactions build azo compounds for dyes.
Aliphatic diazonium ions are usually too unstable for similar use.
Diazotisation is a highlight of aromatic amine chemistry.
Lower amines are gases or volatile liquids with fishy/ammoniacal odours.
Hydrogen bonding raises boiling points of 1° and 2° amines vs hydrocarbons.
Tertiary amines cannot H-bond to each other as donors, so often lower BP than isomers.
Amines form H-bonds with water and lower members are water-soluble.
Trimethylamine contributes to the smell of spoiled fish.
Aniline is a polar liquid with higher boiling point than comparable hydrocarbons.
With HCl: CH₃NH₂ + HCl → CH₃NH₃⁺ Cl⁻ (salt).
Salts are usually crystalline and water-soluble.
Acylation with CH₃COCl gives amides (N-substituted).
Acylation reduces basicity and is used in synthesis and protecting strategies.
Aniline acetylates to acetanilide, a common laboratory solid.
These reactions confirm the nucleophilic/basic nitrogen centre.
Amines are classified as primary (RNH₂), secondary (R₂NH), or tertiary (R₃N).
They may be aliphatic or aromatic depending on the groups attached to nitrogen.
Quaternary ammonium salts have four groups on N and a positive charge.
Lower aliphatic amines are gases (methylamine, trimethylamine).
Boiling points depend on hydrogen bonding and molar mass.
Primary and secondary amines form intermolecular N–H···N hydrogen bonds.
Tertiary amines lack N–H and generally boil lower than isomeric 1°/2° amines.
All amines can accept H-bonds from water via the nitrogen lone pair.
Solubility in water decreases as hydrophobic alkyl chains lengthen.
Odours are often fishy or ammoniacal for volatile amines.
Aniline has a characteristic smell and is only slightly soluble in cold water.
Salts of amines are more soluble in water than free bases often are.
Compare BP of isomeric amines of formula C₃H₉N in exam practice.
Aromatic amines have higher BP due to larger molar mass and polarity.
Write structures for CH₃NH₂, (CH₃)₂NH, (CH₃)₃N, and C₆H₅NH₂.
State that amine BP are usually lower than similar alcohols (O–H bonds stronger).
Physical properties help predict separation and purification methods.
Steam distillation is sometimes used for aniline historically.
Exam answers should define classes and link H-bonding to BP trends.
Mention trimethylamine’s association with fish odour as a memorable fact.
Clear classification prevents mistakes in naming and reaction prediction.
Basicity reflects availability of the nitrogen lone pair to accept a proton.
In water, aliphatic amines are generally stronger bases than ammonia.
Alkyl groups donate electron density by induction, stabilising the ammonium ion.
Order among simple methylamines is nuanced (solvation effects matter).
Aniline is a weaker base than ammonia and aliphatic amines.
The lone pair conjugates with the benzene π system, reducing availability.
Electron-withdrawing substituents on the ring further decrease aniline basicity.
Electron-donating substituents increase basicity of substituted anilines.
Amines form salts with strong acids; Kb or pKb compares strength.
Aromatic heterocyclic bases (pyridine) differ again in basicity patterns.
Write equations for salt formation with HCl.
Explain resonance in anilinium vs aniline when asked for reasons.
Practical consequence: aniline needs stronger conditions for some reactions.
Basicity also explains why amines dissolve in dilute acid and reprecipitate with base.
This acid–base behaviour is used in separation of organic mixtures.
Exam answers must state aliphatic > ammonia > aniline as a common school order.
Support with inductive and resonance arguments.
Avoid saying amines are acidic; they are basic with a weakly acidic N–H in 1°/2°.
Link basicity to biological amines and drugs that are protonated at physiological pH.
Summarise with a short comparative paragraph and one equation.
Correct basicity reasoning earns high marks in organic nitrogen questions.
Primary amines can be made by reduction of nitriles: RCN → RCH₂NH₂.
Reduction of nitro compounds (especially ArNO₂ → ArNH₂) is very important.
Aniline is prepared by reduction of nitrobenzene with Sn/HCl or catalytic hydrogenation.
Hoffmann bromamide degradation of amides gives primary amines with one fewer carbon.
Alkylation of ammonia with R–X gives amines but often mixtures (polyalkylation).
Excess ammonia favours primary amine in that route.
Gabriel phthalimide synthesis gives pure primary alkyl amines.
Reductive amination of carbonyls is another modern/school-mentioned path.
Write the nitrobenzene → aniline equation with reducing conditions.
State that aromatic primary amines are precursors to diazonium chemistry.
Industrial aniline production uses catalytic hydrogenation of nitrobenzene.
Purify aniline by steam distillation or as a salt then basify.
Safety: aromatic amines and nitro compounds are toxic.
Exam answers should give at least two distinct preparations for primary amines.
Include one specific preparation of aniline.
Mention ammonolysis of halogenoalkanes as a bridge to the previous chapter.
Chain extension via R–X → RCN → RCH₂NH₂ is a synthesis favourite.
Control of polyalkylation is a key discussion point for NH₃ + R–X.
Name reagents clearly (Sn/HCl, LiAlH₄, H₂/Ni, etc.).
Preparations set up the reaction platform for diazotisation next.
Always check whether the product is 1°, 2°, or 3° after alkylation.
Amines react with acids to form ammonium salts.
Alkylation with R–X successively can give 2°, 3°, and quaternary salts.
Acylation with acid chlorides/anhydrides gives amides.
Acetanilide formation protects aniline’s –NH₂ in some electrophilic substitutions.
With nitrous acid, primary aliphatic amines give nitrogen gas (quantitative in analysis).
Primary aromatic amines with HNO₂ at 0–5 °C give diazonium salts.
Secondary amines form N-nitrosamines with nitrous acid.
Tertiary aliphatic amines usually form salts without N₂ evolution.
These HNO₂ reactions help classify amines.
Write equations for salt formation, acetylation, and diazotisation.
State temperature control for diazotisation clearly.
Basicity explains solubility switches used in extraction.
Electrophilic aromatic substitution of aniline is activated by –NH₂.
Over-bromination of aniline occurs unless the amino group is protected.
Exam answers should organise reactions by reagent.
Link HNO₂ tests to laboratory identification schemes.
Mention that nitrosamines are hazardous — treat as theory/practical caution.
Acylation reduces the activating power of nitrogen on the ring.
Summarise: nitrogen lone pair drives almost all amine reactivity.
Practice predicting products for CH₃NH₂ vs C₆H₅NH₂ with the same reagent.
These reactions feed directly into dye chemistry via diazonium ions.
Diazotisation: ArNH₂ + NaNO₂ + 2HCl → ArN₂⁺ Cl⁻ + 2H₂O + NaCl (cold).
Benzenediazonium chloride is the parent example from aniline.
Sandmeyer and related reactions replace N₂⁺ by Cl, Br, CN, etc.
Warming aqueous diazonium salts gives phenols.
Coupling with activated aromatics (phenols, anilines) gives azo compounds Ar–N=N–Ar′.
Azo compounds are the basis of many synthetic dyes and indicators.
Colour arises from extended conjugation in the azo chromophore.
Aniline’s role in dye industry is historically and industrially central.
Biologically, amino groups appear in amino acids, proteins, and nucleic acid bases.
Many hormones, neurotransmitters, and drugs are amines or ammonium salts.
Protonation state at body pH affects drug absorption and receptor binding.
Alkaloids are natural nitrogen bases with strong physiological effects.
Exam answers should give the diazotisation equation and one coupling example.
State why aliphatic primary amines do not give stable diazonium salts.
Link dyes back to electrophilic attack of ArN₂⁺ on electron-rich rings.
Mention safety: many aromatic amines are toxic or carcinogenic.
Biological importance shows why nitrogen chemistry matters beyond the lab.
Summarise industrial (dyes) and biological (amino acids/drugs) strands together.
This closes the amines chapter by connecting mechanism to application.
Revise structures of aniline, diazonium ion, and a simple azo dye.
Clear cold-condition notes prevent common exam mistakes on diazotisation.