MHT-CET Chemistry · Amines

Preparation of Amines

Amines are made by reducing nitriles (Mendius, Na/ethanol, one carbon MORE than the halide), amides (LiAlH₄, same carbons) and nitro compounds (Sn/HCl), by the Hofmann bromamide degradation of an amide (one carbon FEWER) and by Gabriel phthalimide synthesis (primary only), or by ammonolysis and alkylation of halides.

All Amines notes MHT-CET Chemistry strategy

Why this matters

15 PYQs, none HARD. Ten are reductions — CH₃Br → KCN → Na/EtOH → ethylamine (the chapter's most repeated sequence), which amide LiAlH₄ turns into ethanamine, how many H atoms reduce a nitrile (4) or a nitro compound (6), acetic acid → SOCl₂ → NH₃ → acetamide; five are Hofmann degradation (loss of CO, 28 g mol⁻¹; acetamide → methylamine), Gabriel (what is never formed), and ammonolysis followed by methylation. Two cards.

Concept 1 of 2

Reductions: Nitrile, Amide and Nitro to Amine

Intuition

Count carbons before and after. A nitrile made from R–X has one carbon more than R, and reducing it (Mendius, Na/ethanol or LiAlH₄, 4 H atoms) keeps them all: CH₃Br → CH₃CN → CH₃CH₂NH₂. An amide reduced by LiAlH₄ keeps its carbons: acetamide → ethanamine. A nitro compound needs 6 H atoms: nitroethane → ethylamine. Nothing is lost in any of these.

Definition

Mendius: $\text{RCN} + 4[\text{H}] \xrightarrow{\text{Na/C}_2\text{H}_5\text{OH or LiAlH}_4} \text{RCH}_2\text{NH}_2$ . No carbon lost. Acetonitrile needs 4 H atoms per mole.
Sequence: $\text{CH}_3\text{Br} \xrightarrow{\text{KCN}} \text{CH}_3\text{CN} \xrightarrow{\text{Na/EtOH}} \text{CH}_3\text{CH}_2\text{NH}_2$ (ethylamine). Butanenitrile comes from n-propyl chloride + alcoholic KCN.
Amide: $\text{CH}_3\text{CONH}_2 \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{NH}_2$ — A for ethanamine is acetamide, not propanamide or C₂H₅CN (which gives propanamine).
Nitro: $\text{C}_2\text{H}_5\text{NO}_2 + 6[\text{H}] \xrightarrow{\text{Sn/HCl}} \text{C}_2\text{H}_5\text{NH}_2 + 2\text{H}_2\text{O}$ — 6 H atoms.
Making the amide: acetic acid $\xrightarrow{\text{SOCl}_2}$ acetyl chloride $\xrightarrow{\text{NH}_3}$ acetamide. A Grignard with NH₃ is just protonated: EtMgCl + NH₃ → ethane.

Three reductions

\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

Worked example

Starting from ethyl bromide, make propan-1-amine, and say how many H atoms the reduction step consumes.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1AminesMODERATE

Identify the product 'B' in the following sequence of reactions.

\text{CH}_3\text{Br}\xrightarrow{\text{KCN}}\text{A}\xrightarrow{\text{Na/C}_2\text{H}_5\text{OH}}\text{B}

[Q97 · 14th May Shift 1 · 2024]

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.

Drill 9 more on reductions: nitrile, amide and nitro to amine

Concept 2 of 2

Hofmann Bromamide Degradation, Gabriel Synthesis and Ammonolysis

Intuition

Hofmann degradation is the one route that LOSES a carbon: an amide with Br₂ and concentrated KOH gives the amine with one carbon fewer, the CO leaving as carbonate — a drop of 28 g mol⁻¹. Gabriel synthesis makes primary amines only: potassium phthalimide + R–X → N-alkylphthalimide → hydrolysis gives the amine and phthalate; phthalic acid itself never appears under the basic conditions. Ammonolysis of R–X gives a mixture that alkylates on.

Definition

Hofmann degradation: $\text{RCONH}_2 + \text{Br}_2 + 4\text{KOH} \to \text{RNH}_2 + \text{K}_2\text{CO}_3 + 2\text{KBr} + 2\text{H}_2\text{O}$ . Acetamide → methylamine. Loss in molar mass = CO = 28 g mol⁻¹.
Gabriel: phthalimide $\xrightarrow{\text{KOH}}$ potassium phthalimide $\xrightarrow{\text{R-X}}$ N-alkylphthalimide $\xrightarrow{\text{NaOH(aq)}}$ primary amine + sodium phthalate. NOT formed: phthalic acid. Aryl halides do not work.
Ammonolysis: benzyl chloride + NH₃ → benzylamine; then 2 CH₃I → $\text{C}_6\text{H}_5\text{CH}_2\text{N(CH}_3)_2$ (tertiary). Excess R–X goes on to the quaternary salt.
Hofmann DEGRADATION (amide → amine, one C fewer) is not Hofmann ELIMINATION (quaternary salt → alkene).

Hofmann bromamide degradation

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

Worked example

Which amide gives propan-1-amine by Hofmann degradation, and which nitrile gives it by Mendius reduction?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2AminesMODERATE

Which from following compounds is obtained when acetamide is warmed with bromine and excess conc.

KOH_{(aq)}

solution?

[Q67 · 21 April Shift II · 2025]

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.

Drill 4 more on hofmann bromamide degradation, gabriel synthesis and ammonolysis

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)

Reductions: Nitrile, Amide and Nitro to Amine
Three reductions
$\text{RCN} \xrightarrow{4[\text{H}]} \text{RCH}_2\text{NH}_2;\quad \text{RCONH}_2 \xrightarrow{\text{LiAlH}_4} \text{RCH}_2\text{NH}_2;\quad \text{RNO}_2 \xrightarrow{6[\text{H}]} \text{RNH}_2$
Hofmann Bromamide Degradation, Gabriel Synthesis and Ammonolysis
Hofmann bromamide degradation
$\text{RCONH}_2 \xrightarrow{\text{Br}_2,\ \text{KOH (conc.)}} \text{RNH}_2 \quad (\text{one carbon fewer; } -28\ \text{g mol}^{-1})$

Watch out for (2)

Forgetting that cyanide adds a carbon
→ Reductions: Nitrile, Amide and Nitro to Amine
Losing CO₂ (44) instead of CO (28)
→ Hofmann Bromamide Degradation, Gabriel Synthesis and Ammonolysis

Drill every past-year question on this subtopic

15 questions from the bank — paginated, with cart and Word-export support.

Drill the 15 questions

Drill every past-year question on this subtopic

Related notes