Basicity and Distinguishing Tests
Comparative basic strength in aqueous and gas phase; Hinsberg and carbylamine tests. (Chemistry › Amines and Diazonium Salts, NEET UG syllabus.)
What is Basicity and Distinguishing Tests?
The ability of an amine to accept a proton or donate its lone pair of electrons on the nitrogen atom, determined by the electron density on nitrogen and the stability of the conjugate acid.
Key formula / rule: Hinsberg Test (Primary Amine)
Key points
- Compare the basic strength of primary, secondary, tertiary amines, and ammonia in gaseous and aqueous phases.
- Explain the factors influencing the basicity of amines (inductive effect, steric hindrance, solvation).
- Describe the Hinsberg test, including reagents, reactions, and observations for 1°, 2°, and 3° amines.
- Describe the Carbylamine reaction, including reagents, conditions, and characteristic observations.
Common exam trap
Confusing the basicity order in gas phase with aqueous phase.
Definitions
- Term
Basicity of Amines
- Meaning
The ability of an amine to accept a proton or donate its lone pair of electrons on the nitrogen atom, determined by the electron density on nitrogen and the stability of the conjugate acid.
- Term
Hinsberg Reagent
- Meaning
Benzenesulphonyl chloride (C₆H₅SO₂Cl), used to distinguish between primary, secondary, and tertiary amines based on their reactivity and the solubility of the sulphonamide products.
- Term
Carbylamine Reaction
- Meaning
A chemical test specific for primary amines, where they react with chloroform and alcoholic KOH to form foul-smelling isocyanides (carbylamines).
- Term
Isocyanide
- Meaning
An organic compound containing the -NC functional group, characterized by a carbon atom double-bonded to a nitrogen atom, and often having a highly unpleasant odor.
Learning objectives
Compare the basic strength of primary, secondary, tertiary amines, and ammonia in gaseous and aqueous phases.
Explain the factors influencing the basicity of amines (inductive effect, steric hindrance, solvation).
Describe the Hinsberg test, including reagents, reactions, and observations for 1°, 2°, and 3° amines.
Describe the Carbylamine reaction, including reagents, conditions, and characteristic observations.
Identify primary, secondary, and tertiary amines using appropriate chemical tests.
Formulae
- Name
Hinsberg Test (Primary Amine)
- Note
The N-H proton is acidic due to the strong electron-withdrawing sulphonyl group.
- Expression
R-NH₂ + C₆H₅SO₂Cl → C₆H₅SO₂NHR (N-alkylbenzenesulphonamide) + HCl C₆H₅SO₂NHR + KOH(aq) → K⁺[C₆H₅SO₂NR]⁻ + H₂O (soluble salt)
- Name
Hinsberg Test (Secondary Amine)
- Note
No acidic hydrogen on nitrogen, hence insoluble in KOH.
- Expression
R₂NH + C₆H₅SO₂Cl → C₆H₅SO₂NR₂ (N,N-dialkylbenzenesulphonamide) + HCl C₆H₅SO₂NR₂ + KOH(aq) → No reaction (insoluble)
- Name
Hinsberg Test (Tertiary Amine)
- Note
Tertiary amines do not react with benzenesulphonyl chloride under normal conditions.
- Expression
R₃N + C₆H₅SO₂Cl → No reaction
- Name
Carbylamine Reaction (Isocyanide Test)
- Note
Specific for primary amines (aliphatic and aromatic); product has a foul smell.
- Expression
R-NH₂ + CHCl₃ + 3KOH(alc) → R-NC (alkyl isocyanide) + 3KCl + 3H₂O
Prerequisites
Basic understanding of organic nomenclature.
Concepts of inductive effect (+I effect).
Understanding of hydrogen bonding and solvation.
Knowledge of acid-base concepts (Lewis and Brønsted-Lowry theories).
General reactions of amines.
Common mistakes
Confusing the basicity order in gas phase with aqueous phase.
Incorrectly identifying the solubility of Hinsberg test products.
Applying the Carbylamine test to secondary or tertiary amines.
Forgetting the role of solvation in aqueous basicity.
Not recognizing the foul smell as a key indicator for Carbylamine test.
Keywords
Amines
Basicity
Inductive effect
Solvation effect
Steric hindrance
Hinsberg test
Benzenesulphonyl chloride
Carbylamine reaction
Isocyanide test
Primary amine
Secondary amine
Tertiary amine
N-alkylbenzenesulphonamide
N,N-dialkylbenzenesulphonamide
Isocyanide
Practice preview
Which of the following represents the correct order of basicity of methylamines in the gaseous phase?…
easy
What is the chemical name of the reagent used in the Hinsberg test?…
easy
Which of the following represents the correct order of basicity of methylamines in aqueous solution?…
medium