Detailed conversion mechanisms for Amines and Carboxylic Acids. Master the techniques of Ascent of Series (Step Up) and Descent of Series (Step Down).
Board Exam
Class 12
(i) Ethanoic acid into methanamine
Step Down
$$ \text{CH}_3\text{COOH} \xrightarrow[\Delta]{\text{NH}_3} \text{CH}_3\text{CONH}_2 $$
$$ \text{CH}_3\text{CONH}_2 \xrightarrow{\text{Br}_2/\text{KOH}} \text{CH}_3\text{NH}_2 $$
Strategy: Convert carboxylic acid to amide. Then use the Hoffmann Bromamide Degradation to remove the carbonyl carbon and obtain an amine with one less carbon.
(ii) Hexanenitrile into 1-aminopentane
Step Down
$$ \text{C}_5\text{H}_{11}\text{CN} \xrightarrow[\text{Partial Hydrolysis}]{\text{H}_3\text{O}^+} \text{C}_5\text{H}_{11}\text{CONH}_2 $$
$$ \text{C}_5\text{H}_{11}\text{CONH}_2 \xrightarrow{\text{Br}_2/\text{KOH}} \text{C}_5\text{H}_{11}\text{NH}_2 $$
Strategy: Hexanenitrile (6C) needs to become pentanamine (5C). Hydrolyze the nitrile to amide, then degrade it using Hoffmann Bromamide reaction.
(iii) Methanol to ethanoic acid
Step Up
$$ \text{CH}_3\text{OH} \xrightarrow{\text{PCl}_5} \text{CH}_3\text{Cl} \xrightarrow{\text{KCN(alc)}} \text{CH}_3\text{CN} $$
$$ \text{CH}_3\text{CN} \xrightarrow{\text{H}_3\text{O}^+} \text{CH}_3\text{COOH} $$
Strategy: To add a carbon, convert alcohol to alkyl halide, then react with KCN to form a Nitrile. Complete hydrolysis of nitrile yields the acid.
(iv) Ethanamine into methanamine
Step Down
$$ \text{C}_2\text{H}_5\text{NH}_2 \xrightarrow{\text{HNO}_2} \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{KMnO}_4} \text{CH}_3\text{COOH} $$
$$ \xrightarrow[\Delta]{\text{NH}_3} \text{CH}_3\text{CONH}_2 \xrightarrow{\text{Br}_2/\text{KOH}} \text{CH}_3\text{NH}_2 $$
Strategy: Convert amine to alcohol (HONO), oxidize to acid, convert to amide, and finally use Hoffmann degradation to step down.
(v) Ethanoic acid into propanoic acid
Step Up
$$ \text{CH}_3\text{COOH} \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{PCl}_5} \text{CH}_3\text{CH}_2\text{Cl} $$
$$ \text{CH}_3\text{CH}_2\text{Cl} \xrightarrow{\text{KCN}} \text{CH}_3\text{CH}_2\text{CN} \xrightarrow{\text{H}_3\text{O}^+} \text{CH}_3\text{CH}_2\text{COOH} $$
Strategy: Reduce acid to alcohol, convert to halide, use KCN to add a carbon (Step Up), then hydrolyze.
(vi) Methanamine into ethanamine
Step Up
$$ \text{CH}_3\text{NH}_2 \xrightarrow{\text{HNO}_2} \text{CH}_3\text{OH} \xrightarrow{\text{PCl}_5} \text{CH}_3\text{Cl} $$
$$ \text{CH}_3\text{Cl} \xrightarrow{\text{KCN}} \text{CH}_3\text{CN} \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{NH}_2 $$
Strategy: Amine $\to$ Alcohol $\to$ Halide $\to$ Nitrile (add carbon) $\to$ Reduction to Amine (Mendius reaction).
(vii) Nitromethane into dimethylamine
Isocyanide Route
$$ \text{CH}_3\text{NO}_2 \xrightarrow{\text{Sn/HCl}} \text{CH}_3\text{NH}_2 $$
$$ \text{CH}_3\text{NH}_2 \xrightarrow[\text{KOH, }\Delta]{\text{CHCl}_3} \text{CH}_3\text{NC} \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{NHCH}_3 $$
Strategy: Reduce nitro to primary amine. Use Carbylamine Reaction to form Isocyanide. Reduction of isocyanide yields a secondary amine.
(viii) Propanoic acid into ethanoic acid
Step Down
$$ \text{C}_2\text{H}_5\text{COOH} \xrightarrow[\Delta]{\text{NH}_3} \text{C}_2\text{H}_5\text{CONH}_2 \xrightarrow{\text{Br}_2/\text{KOH}} \text{C}_2\text{H}_5\text{NH}_2 $$
$$ \text{C}_2\text{H}_5\text{NH}_2 \xrightarrow{\text{HNO}_2} \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{KMnO}_4} \text{CH}_3\text{COOH} $$
Strategy: Standard degradation: Acid $\to$ Amide $\to$ Amine (less carbon) $\to$ Alcohol $\to$ Acid.
Reaction Cheat Sheet
Br₂ / KOH
Hoffmann Bromamide (Step Down)
KCN (alc)
Nitrile Synthesis (Step Up)
HNO₂ (HONO)
Amine $\to$ Alcohol
CHCl₃ / KOH
Carbylamine (Isocyanide Test)
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