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Amines formulas

10 formulas and 10 common traps for MHT-CET Chemistry Amines, grouped by subtopic.

Full notes with worked examples

Nomenclature and Classification of Amines

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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}

Common traps

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.

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.

Physical Properties of Amines

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N–H Hydrogen Bonding: Boiling Point and Solubility

Two orders

b.p.: 1∘>2∘>3∘ (isomers);amine<alcohol<acid;solubility: alcohol>amine>alkane\text{b.p.: } 1^\circ > 2^\circ > 3^\circ \text{ (isomers)};\quad \text{amine} < \text{alcohol} < \text{acid};\qquad \text{solubility: alcohol} > \text{amine} > \text{alkane}

Common traps

Ranking amines above alcohols because N is 'more basic'

Basicity is not boiling point. O–H makes the stronger hydrogen bond, so alcohols boil higher and dissolve better; amines come second in both orders.

Preparation of Amines

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Reductions: Nitrile, Amide and Nitro to Amine

Three reductions

RCN→4[H]RCH2NH2;RCONH2→LiAlH4RCH2NH2;RNO2→6[H]RNH2\text{RCN} \xrightarrow{4[\text{H}]} \text{RCH}_2\text{NH}_2;\quad \text{RCONH}_2 \xrightarrow{\text{LiAlH}_4} \text{RCH}_2\text{NH}_2;\quad \text{RNO}_2 \xrightarrow{6[\text{H}]} \text{RNH}_2

Hofmann Bromamide Degradation, Gabriel Synthesis and Ammonolysis

Hofmann bromamide degradation

RCONH2→Br2, KOH (conc.)RNH2(one carbon fewer; −28 g mol−1)\text{RCONH}_2 \xrightarrow{\text{Br}_2,\ \text{KOH (conc.)}} \text{RNH}_2 \quad (\text{one carbon fewer; } -28\ \text{g mol}^{-1})

Common traps

Forgetting that cyanide adds a carbon

Methyl bromide ends as ETHYLamine — the CN carbon becomes CH₂. 'Methylamine' is the offered wrong answer in every version of the sequence.

Losing CO₂ (44) instead of CO (28)

The amide's carbonyl leaves as carbonate, but the MOLECULE loses C=O: 12 + 16 = 28. Amine mass = amide mass − 28.

Chemical Reactions and Basicity of Amines

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Basicity: the Aqueous Order and pKb

Aqueous basicity

R2NH>RNH2≷R3N>NH3≫C6H5NH2;low pKb=strong base\text{R}_2\text{NH} > \text{RNH}_2 \gtrless \text{R}_3\text{N} > \text{NH}_3 \gg \text{C}_6\text{H}_5\text{NH}_2;\qquad \text{low } pK_b = \text{strong base}

Acylation, the Carbylamine Test and Hinsberg's Reagent

Carbylamine reaction

R-NH2+CHCl3+3KOH→ΔR-N≡C+3KCl+3H2O\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH} \xrightarrow{\Delta} \text{R-N≡C} + 3\text{KCl} + 3\text{H}_2\text{O}

Exhaustive Methylation and Hofmann Elimination

Hofmann elimination

R3N+-CH2CH3 OH−→ΔR3N+CH2=CH2+H2O(least substituted alkene)\text{R}_3\text{N}^+\text{-CH}_2\text{CH}_3\ \text{OH}^- \xrightarrow{\Delta} \text{R}_3\text{N} + \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O} \quad (\text{least substituted alkene})

Common traps

Reading pKb as basic strength

pKb is −log Kb: the STRONGER the base, the SMALLER the number. 'Highest pKb' asks for the weakest base — aniline — and the strongest base has the lowest.

Giving the carbylamine test to a secondary amine

It needs two N–H hydrogens to form the isocyanide; secondary and tertiary amines give no smell. Acylation, by contrast, needs only ONE N–H — so secondary amines acylate but do not give carbylamine.

Eliminating the propyl group because it is 'bigger'

Hofmann elimination removes the β-hydrogen that is easiest to reach — on the LEAST substituted, least hindered alkyl. Ethyl beats propyl: ethene forms, propene does not.

Diazonium Salts and Aromatic Amine Reactions

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Diazotisation and the Replacement Reactions of the Diazonium Ion

Diazotisation and hydrolysis

ArNH2→NaNO2/HCl, 273 KArN2+Cl−→H2O, ΔArOH+N2+HCl\text{ArNH}_2 \xrightarrow{\text{NaNO}_2/\text{HCl},\ 273\text{ K}} \text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{H}_2\text{O},\ \Delta} \text{ArOH} + \text{N}_2 + \text{HCl}

Azo Coupling: Diazonium Ion Plus Phenol or Aniline

Azo coupling

ArN2++C6H5OH→OH−Ar-N=N-C6H4-OH (p)\text{ArN}_2^+ + \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{OH}^-} \text{Ar-N=N-C}_6\text{H}_4\text{-OH}\ (p)

Common traps

Stopping at the diazonium salt

The salt is A, the intermediate. When the sequence continues with warm water, B is PHENOL — 'benzenediazonium chloride' is offered as option (c) for B every time.

Coupling in strong acid

Strongly acidic medium keeps phenol as phenol, too weak a nucleophile for the diazonium ion; strongly basic medium destroys the diazonium ion. The window is mild alkali for phenol, mild acid for aniline.

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