Amines Classification & Reactions — Chemistry Mnemonic
Target: mnemonic for amines classification reactions chemistry
Why is this hard to memorize?
Amines are nitrogen-containing organic compounds classified as primary (RNH₂), secondary (R₂NH), and tertiary (R₃N) based on the number of alkyl/aryl groups on nitrogen. Basicity order in aqueous solution is 2° > 1° > 3° > ArNH₂ (due to combined inductive, steric, and solvation effects). Key reactions include: Hofmann bromamide reaction, diazonium salt formation (1° aromatic amines + NaNO₂/HCl at 0-5°C), carbylamine test (1° amines only), and Hinsberg test to distinguish 1°/2°/3° amines. NEET and JEE test basicity comparisons and distinguishing tests extensively.
Classic mnemonics you should know
"1° = RNH₂ (one R on N). 2° = R₂NH (two Rs). 3° = R₃N (three Rs). 4° = R₄N⁺ (quaternary ammonium, always positive)"
Count the R groups on nitrogen: 1 = primary, 2 = secondary, 3 = tertiary. Don't confuse with alcohols — in amines, we count substituents on N, not on carbon. CH₃NH₂ = 1° amine, even though the carbon is primary too.
"Carbylamine: only 1° amines give isocyanide smell. Hinsberg: 1° dissolves in NaOH, 2° doesn't dissolve, 3° no reaction with sulfonyl chloride"
Carbylamine test: RNH₂ + CHCl₃ + KOH → R-NC (foul smell). Only 1° amines react. Hinsberg test uses benzenesulfonyl chloride: 1° gives sulfonamide (dissolves in alkali), 2° gives insoluble sulfonamide, 3° gives no reaction.
"In water: (C₂H₅)₂NH > C₂H₅NH₂ > (C₂H₅)₃N > NH₃ > C₆H₅NH₂"
2° > 1° because two alkyl groups push electrons to N (+I) AND the cation is still well-solvated. 3° is less basic than expected because 3 bulky groups hinder solvation of the conjugate acid. Aniline (ArNH₂) is weakest because the lone pair delocalizes into the ring.
The complete list
- 1° amine: RNH₂ (one R on nitrogen)
- 2° amine: R₂NH (two R groups)
- 3° amine: R₃N (three R groups)
- Basicity (aq): 2° > 1° > 3° > ArNH₂
- Carbylamine test: only for 1° amines
- Hinsberg test: distinguishes 1°/2°/3°
- Diazonium salt: 1° aromatic + NaNO₂/HCl at 0-5°C
- Aniline is weak base (lone pair delocalized)
Frequently asked questions
Why is the basicity order of amines 2° > 1° > 3° in water?
Three factors compete: (1) +I effect of alkyl groups increases electron density on N (favours more R groups). (2) Solvation of conjugate acid (R₂NH₂⁺ is better solvated than R₃NH⁺ because it has more N-H bonds for hydrogen bonding). (3) Steric hindrance reduces solvation for 3°. In water, solvation wins: 2° has good +I AND good solvation. In gas phase (no solvation), order is 3° > 2° > 1°.
What is the carbylamine reaction?
RNH₂ + CHCl₃ + 3KOH → R-N=C (isocyanide) + 3KCl + 3H₂O. Only PRIMARY amines give this reaction. The product R-N≡C (isocyanide/carbylamine) has a foul, nauseating smell that is the positive test indicator. Secondary and tertiary amines do NOT form isocyanides.
Why is aniline a weaker base than cyclohexylamine?
In aniline, the nitrogen lone pair is delocalized into the benzene ring (resonance), making it less available for protonation. In cyclohexylamine, the lone pair is localized on N (no resonance with the ring). Additionally, the sp² carbon of benzene is more electronegative than sp³ carbon of cyclohexane, pulling electrons away from N. Result: aniline pKb ≈ 9.4, cyclohexylamine pKb ≈ 3.3.
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