Methods of Preparation — Revision Notes

⚡ 30-Second Revision

Nitro Compounds \rightarrow Primary Amine — R-NO$_2$ \xrightarrow{Sn/HCl \text{ or } Fe/HCl \text{ or } H_2/Pd} R-NH$_2$ (Aromatic)\n- Nitriles \rightarrow Primary Amine: R-C\equiv N \xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-CH$_2$-NH$_2$ (+1 Carbon)\n- Amides \rightarrow Amine: R-CONH$_2$ \xrightarrow{LiAlH_4} R-CH$_2$-NH$_2$ (Retains Carbon, P/S/T Amine from P/S/T Amide)\n- Ammonolysis of Alkyl Halides: R-X + NH$_3$ \rightarrow R-NH$_2$ (Mixture of P, S, T amines)\n- Gabriel Phthalimide Synthesis: Phthalimide \xrightarrow{KOH} \text{K-Phthalimide} \xrightarrow{R-X} \text{N-Alkylphthalimide} \xrightarrow{H_2O/H^+ \text{ or } NH_2NH_2} R-NH$_2$ (Pure Primary Aliphatic Amine Only)\n- Hofmann Bromamide Degradation: R-CONH$_2$ \xrightarrow{Br_2/NaOH} R-NH$_2$ (-1 Carbon, Pure Primary Amine)\n- Reductive Amination: RCHO/R$_2$CO + NH$_3$/R'NH$_2$/R'$_2$NH \xrightarrow{\text{H}_2/Ni \text{ or } NaBH_3CN} \text{Amine}$ (P/S/T Amine)

2-Minute Revision

Quickly recall the main methods for preparing amines, focusing on the starting material, key reagents, and the type of amine produced. For nitro compounds, remember reduction with Sn/HCl or H$_2$/Pd yields primary aromatic amines.

Nitriles (R-C\equiv N) are reduced by LiAlH$_4$ or H$_2$/Ni to primary amines (R-CH$_2$-NH$_2$), notably adding one carbon. Amides (R-CONH$_2$) are reduced by LiAlH$_4$ to amines (R-CH$_2$-NH$_2$), retaining the carbon count and yielding primary, secondary, or tertiary amines based on the amide's substitution.

Be wary of ammonolysis of alkyl halides, as it often produces a mixture of amines due to over-alkylation. For pure primary aliphatic amines, the Gabriel phthalimide synthesis is key; remember it cannot be used for aromatic amines.

The Hofmann bromamide degradation is crucial for primary amines with one less carbon, using Br$_2$/NaOH. Finally, reductive amination of aldehydes/ketones with ammonia or amines followed by reduction is versatile for primary, secondary, or tertiary amines.

Pay attention to carbon count changes and the specific limitations of each method.

5-Minute Revision

A comprehensive review of amine preparation methods is essential for NEET. Start by categorizing reactions: Reduction reactions are prominent. Nitro compounds (R-NO$_2$) are reduced to primary aromatic amines (R-NH$_2$) using Sn/HCl or catalytic hydrogenation (H$_2$/Pd).

For example, nitrobenzene to aniline. Nitriles (R-C\equiv N) are reduced to primary amines (R-CH$_2$-NH$_2$) using LiAlH$_4$ or H$_2$/Ni; this method is unique as it adds a carbon atom to the chain. Amides (R-CONH$_2$) are reduced by LiAlH$_4$ to amines (R-CH$_2$-NH$_2$), maintaining the carbon count, and can yield primary, secondary, or tertiary amines depending on the amide's substitution.

\n\nNext, consider selective methods for primary amines. The Gabriel phthalimide synthesis is paramount for pure primary aliphatic amines. It involves phthalimide reacting with KOH, then a primary alkyl halide (R-X), followed by hydrolysis or hydrazinolysis.

Crucially, it does not work for aromatic amines. The Hofmann bromamide degradation is another vital method for primary amines, but it results in an amine with one carbon atom less than the starting primary amide (R-CONH$_2$ to R-NH$_2$) using Br$_2$/NaOH.

\n\nFinally, reductive amination offers a versatile route. Aldehydes or ketones react with ammonia (for primary amines), primary amines (for secondary amines), or secondary amines (for tertiary amines) to form an imine intermediate, which is then reduced (e.

g., with H$_2$/Ni or NaBH$_3$CN) to the corresponding amine. \n\nWorked Example: How would you synthesize propan-1-amine (CH$_3$CH$_2$CH$_2$NH$_2$) from an appropriate starting material?\n* Method 1 (Nitrile Reduction): Start with ethanenitrile (CH$_3$C\equiv N$).

Reduce it with LiAlH$_4$ or H$_2$/Ni. CH$_3$C\equiv N \xrightarrow{LiAlH_4} CH$_3$CH$_2$NH$_2$. (Wait, this gives ethanamine. To get propan-1-amine, start with propanenitrile: CH$_3$CH$_2$C\equiv N \xrightarrow{LiAlH_4} CH$_3$CH$_2$CH$_2$NH$_2$).

\n* Method 2 (Amide Reduction): Start with propanamide (CH$_3$CH$_2$CONH$_2$). Reduce it with LiAlH$_4$. CH$_3$CH$_2$CONH$_2$ \xrightarrow{LiAlH_4} CH$_3$CH$_2$CH$_2$NH$_2$.\n* Method 3 (Gabriel Synthesis): Start with 1-bromopropane (CH$_3$CH$_2$CH$_2$Br).

React with potassium phthalimide, then hydrazinolysis. This yields pure propan-1-amine. \n\nRemember to always consider the carbon count and the type of amine required.

Prelims Revision Notes

1
Reduction of Nitro Compounds — \n * Starting Material: Aromatic nitro compounds (e.g., nitrobenzene). \n * Reagents: Sn/HCl, Fe/HCl, or H$_2$/Pd. \n * Product: Primary aromatic amines (e.g., aniline). \n * Carbon Count: Retained. \n2. Reduction of Nitriles: \n * Starting Material: Nitriles (R-C\equiv N). \n * Reagents: LiAlH$_4$ or H$_2$/Ni. \n * Product: Primary amines (R-CH$_2$-NH$_2$). \n * Carbon Count: Increases by one. \n3. Reduction of Amides: \n * Starting Material: Primary (R-CONH$_2$), secondary (R-CONHR'), or tertiary (R-CONR'R'') amides. \n * Reagents: LiAlH$_4$. \n * Product: Corresponding primary (R-CH$_2$-NH$_2$), secondary (R-CH$_2$-NHR'), or tertiary (R-CH$_2$-NR'R'') amines. \n * Carbon Count: Retained. \n4. Ammonolysis of Alkyl Halides: \n * Starting Material: Alkyl halides (R-X) + Ammonia (NH$_3$). \n * Product: Mixture of primary, secondary, tertiary amines, and quaternary ammonium salts. \n * Limitation: Poor selectivity for pure primary amines. \n5. Gabriel Phthalimide Synthesis: \n * Starting Material: Phthalimide, KOH, primary alkyl halide (R-X). \n * Reagents: KOH, R-X, then H$_2$O/H$^+$ or NH$_2$NH$_2$. \n * Product: Pure primary aliphatic amines. \n * Limitation: Cannot prepare aromatic amines (aryl halides unreactive). \n * Carbon Count: Retained (from R-X). \n6. Hofmann Bromamide Degradation: \n * Starting Material: Primary amides (R-CONH$_2$). \n * Reagents: Br$_2$/NaOH (or KOH). \n * Product: Primary amines (R-NH$_2$). \n * Carbon Count: Decreases by one. \n7. Reductive Amination: \n * Starting Material: Aldehydes/Ketones + NH$_3$/Primary Amine/Secondary Amine. \n * Reagents: H$_2$/Ni, NaBH$_3$CN. \n * Product: Primary, secondary, or tertiary amines depending on the amine used. \n * Mechanism: Imine formation followed by reduction. \n\nKey Points for NEET: \n* Identify reagents for specific transformations. \n* Predict product structure, including carbon count changes. \n* Know the limitations of Gabriel synthesis and ammonolysis. \n* Distinguish between methods that increase, decrease, or retain carbon count.

Vyyuha Quick Recall

Great Hydrogen Nitrogen All Reactions Are Mine!\n\n* Gabriel: Pure Primary Aliphatic (No Aryl)\n* Hofmann: Primary, -1C (Br$_2$/NaOH)\n* Nitro: Primary Aromatic (Sn/HCl)\n* Nitrile: Primary, +1C (LiAlH$_4$)\n* Amide: P/S/T, 0C (LiAlH$_4$)\n* Ammonolysis: Mixture (R-X + NH$_3$)\n* Reductive Amination: P/S/T (Aldehyde/Ketone + Amine + Reducer)