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MHT-CET Chemistry · Amines

Nomenclature and Classification of Amines

An amine is primary, secondary or tertiary by how many carbon groups sit on the nitrogen (not on the carbon), simple or mixed by whether those groups match, aliphatic or aromatic by whether one is an aryl ring; IUPAC names use -amine on the longest chain with N-prefixes for the other groups.

All Amines notesMHT-CET Chemistry strategy

Why this matters

19 PYQs, none HARD. Twelve are classification — which named or drawn compound is primary, secondary, tertiary, an aromatic mixed 3° amine, or a molecular formula (cyclohexylamine C₆H₁₃N, p-toluidine C₇H₉N); seven are IUPAC names (allylamine, ethylmethylisopropylamine, a drawn pentenamine, sulphanilic acid) and the C₄H₁₁N isomer counts. Two cards.

Concept 1 of 2

Primary, Secondary, Tertiary; Simple and Mixed; Aliphatic and Aromatic

Intuition

Count the carbons on NITROGEN. One — primary (R–NH₂), two — secondary (R₂NH), three — tertiary (R₃N). tert-Butylamine is a PRIMARY amine: the tertiary carbon is beside the point. Same groups — simple; different — mixed. Any aryl group directly on N makes it aromatic; benzylamine (PhCH₂NH₂) is aliphatic because the ring is one carbon away.

Definition

  • Primary: phenylmethanamine (benzylamine), 4-bromobenzenamine, propan-2-amine, ethane-1,2-diamine, cyclohexanamine, butan-1-amine.
  • Secondary: N-methylethanamine, N-methylmethanamine, N-phenylbenzenamine (diphenylamine), N-methylaniline.
  • Tertiary: N,N-dimethylaniline, N-ethyl-N-methylpropan-2-amine, N,N-dimethyl-1-butanamine, triphenylamine.
  • (C6H5)3N(\text{C}_6\text{H}_5)_3\text{N}: 3° aromatic SIMPLE; C6H5N(CH3)2\text{C}_6\text{H}_5\text{N(CH}_3)_2: 3° aromatic MIXED (aryl + two methyls); (C2H5)3N(\text{C}_2\text{H}_5)_3\text{N}: 3° aliphatic simple.
  • Formulas: cyclohexylamine C6H11NH2=C6H13N\text{C}_6\text{H}_{11}\text{NH}_2 = \text{C}_6\text{H}_{13}\text{N}; p-toluidine CH3C6H4NH2=C7H9N\text{CH}_3\text{C}_6\text{H}_4\text{NH}_2 = \text{C}_7\text{H}_9\text{N}. Amide grades count the CARBON groups on the amide N: primary R-CO-NH2\text{R-CO-NH}_2, secondary R-CO-NH-R′\text{R-CO-NH-R}' (one H left), tertiary R-CO-NR2′\text{R-CO-NR}'_2.

Degree of an amine

RNH2 (1∘),R2NH (2∘),R3N (3∘)— count groups on N, not on C\text{RNH}_2\ (1^\circ),\quad \text{R}_2\text{NH}\ (2^\circ),\quad \text{R}_3\text{N}\ (3^\circ) \quad — \text{ count groups on N, not on C}

Worked example

Classify N-ethylaniline, (CH₃)₃CNH₂ and N-methyl-N-phenylaniline by degree and by simple/mixed, aliphatic/aromatic.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1AminesMODERATE
Identify an aromatic, mixed, 3∘3^\circ amine among the following compounds.

[Q63 · 2nd May Shift 2 · 2023]

Grading the amine by its carbon

tert-Butylamine and propan-2-amine are PRIMARY amines — one carbon on N. The 1°/2°/3° label for amines counts nitrogen's substituents, unlike alcohols and halides.
Drill 11 more on primary, secondary, tertiary; simple and mixed; aliphatic and aromatic

Concept 2 of 2

IUPAC Names, N-Prefixes and Counting C₄H₁₁N Isomers

Intuition

Take the longest chain carrying N as the parent alkanamine, number so the amine carbon is lowest, and write the other groups on nitrogen as N-prefixes in alphabetical order. For isomer counts of C₄H₁₁N, list the four primary amines (four butyl skeletons), three secondary (diethyl, methylpropyl, methylisopropyl) and one tertiary (N,N-dimethylethanamine): eight in all.

Definition

  • Allylamine CH2=CH-CH2NH2\text{CH}_2\text{=CH-CH}_2\text{NH}_2 = prop-2-en-1-amine. CH3CH=CH-CH(NH2)CH3\text{CH}_3\text{CH=CH-CH(NH}_2)\text{CH}_3 = pent-3-en-2-amine.
  • Ethylmethylisopropylamine: the isopropyl chain is the parent → N-ethyl-N-methylpropan-2-amine.
  • C4H11N\text{C}_4\text{H}_{11}\text{N}: four 1°, three 2° (diethylamine, N-methylpropan-1-amine, N-methylpropan-2-amine), one 3° (N,N-dimethylethanamine).
  • Sulphanilic acid = 4-aminobenzenesulphonic acid (H2N-C6H4-SO3H\text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{H}), a zwitterion.
  • Reagents to keep straight: hydrazine NH2NH2\text{NH}_2\text{NH}_2, hydroxylamine NH2OH\text{NH}_2\text{OH}, semicarbazide NH2NHCONH2\text{NH}_2\text{NHCONH}_2, phenylhydrazine C6H5NHNH2\text{C}_6\text{H}_5\text{NHNH}_2 (one phenyl — C6H5NHNHC6H5\text{C}_6\text{H}_5\text{NHNHC}_6\text{H}_5 is wrong).

N-prefix naming

longest chain on N→alkan-k-amine;other N groups→N-alkyl prefixes, alphabetical\text{longest chain on N} \to \text{alkan-}k\text{-amine};\quad \text{other N groups} \to N\text{-alkyl prefixes, alphabetical}

Worked example

Name (CH3)2N-CH2CH2CH3(\text{CH}_3)_2\text{N-CH}_2\text{CH}_2\text{CH}_3 and CH3CH2-NH-CH(CH3)2\text{CH}_3\text{CH}_2\text{-NH-CH(CH}_3)_2.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2AminesMODERATE
What is IUPAC name of Ethylmethylisopropylamine?

[Q98 · 4th May Shift 1 · 2023]

Taking the ethyl as the parent chain

Isopropyl has three carbons to ethyl's two, so propan-2-amine is the parent and ethyl and methyl become N-prefixes. 'N-Methyl-N-isopropylethanamine' is the planted option.
Drill 6 more on iupac names, n-prefixes and counting c₄h₁₁n isomers

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

  • Primary, Secondary, Tertiary; Simple and Mixed; Aliphatic and Aromatic

    Degree of an amine

    RNH2 (1∘),R2NH (2∘),R3N (3∘)— count groups on N, not on C\text{RNH}_2\ (1^\circ),\quad \text{R}_2\text{NH}\ (2^\circ),\quad \text{R}_3\text{N}\ (3^\circ) \quad — \text{ count groups on N, not on C}
  • IUPAC Names, N-Prefixes and Counting C₄H₁₁N Isomers

    N-prefix naming

    longest chain on N→alkan-k-amine;other N groups→N-alkyl prefixes, alphabetical\text{longest chain on N} \to \text{alkan-}k\text{-amine};\quad \text{other N groups} \to N\text{-alkyl prefixes, alphabetical}

Watch out for (2)

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