You are given the mixture of two compounds A and B. A = CH3CH2CH2 NH2 and B = CH3CH2NHCH3 How would you separate their mixture by applying Hoffmann's method ? Give the nitrous acid test to…

Separation of Mixture of A and B Using Hoffmann’s Method

Step 1: Understanding the Compounds

• Compound A: $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{NH}{}_2$ (1-Propylamine, a primary amine)
• Compound B: $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{NHCH}{}_3$ (Ethylmethylamine, a secondary amine)

Hoffmann’s method exploits the difference in reactivity between primary and secondary amines with nitrous acid ($\mathrm{HNO}{}_2$).

Step 2: Reaction with Nitrous Acid

• Primary amine (A) reacts with $\mathrm{HNO}{}_2$ to form a diazotized product, which is unstable and decomposes to give alcohol, nitrogen gas ($\mathrm{N}{}_2$), and water: $$\mathrm{R}-\mathrm{NH}{}_2+\mathrm{HNO}{}_2\to \mathrm{R}-\mathrm{OH}+\mathrm{N}{}_2+\mathrm{H}{}_2\mathrm{O}$$ For A: $$\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{NH}{}_2+\mathrm{HNO}{}_2\to \mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{OH}+\mathrm{N}{}_2+\mathrm{H}{}_2\mathrm{O}$$ The alcohol ($\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{OH}$) is volatile and can be distilled off.

• Secondary amine (B) reacts with $\mathrm{HNO}{}_2$ to form a stable nitrosoamine (yellow oily liquid), which does not decompose easily: $$\mathrm{R}{}_2\mathrm{NH}+\mathrm{HNO}{}_2\to \mathrm{R}{}_2\mathrm{N}-\mathrm{N}=\mathrm{O}+\mathrm{H}{}_2\mathrm{O}$$ For B: $$\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{NHCH}{}_3+\mathrm{HNO}{}_2\to (\mathrm{CH}{}_3\mathrm{CH}{}_2)(\mathrm{CH}{}_3)\mathrm{N}-\mathrm{N}=\mathrm{O}+\mathrm{H}{}_2\mathrm{O}$$

Step 3: Separation Process

1. Add sodium nitrite ($\mathrm{NaNO}{}_2$) and hydrochloric acid ($\mathrm{HCl}$) to the mixture to generate $\mathrm{HNO}{}_2$ in situ.
2. Heat gently: The primary amine (A) decomposes to form propan-1-ol, which is distilled off due to its lower boiling point (~97°C).
3. The secondary amine (B) remains as a yellow nitrosoamine, which can be separated by filtration or extraction with an organic solvent (e.g., ether).

4NaOHCH₃CH₂CH₂–NH₂ (A)+H₂OCold, alkaline (NaOH)NH₃↑NaBr+H₂O

Step 4: Confirmation of Products

• The distillate (propan-1-ol) can be confirmed by iodoform test (positive for alcohols with $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OH}){}^{-}$ group).
• The residual yellow nitrosoamine confirms the presence of the secondary amine (B).

---

Nitrous Acid Test to Distinguish A and B

---

HNO₂CH₃CH₂CH₂–NH₂ (A)+HClCold, acidic (HCl)Observation: Bubbles of N₂ gas, clear solutionH₂OCH₃CH₂CH₂–OH+HCl

HNO₂CH₃CH₂–NH–CH₃ (B)+HClCold, acidic (HCl)Observation: Yellow oily layer, no effervescenceH₂O+HCl