1°, 2°, 3° Amines with HNO₂

Primary (1°) Amine

(Just for comparison)

• Aliphatic:

$$RNH_2 + HNO_2 \rightarrow ROH + N_2 \uparrow + H_2O$$

Brisk effervescence (N₂ gas)

• Aromatic: forms diazonium salt

2° (Secondary) Amine

Reaction:

$$R_2NH + HNO_2 \rightarrow R_2N–N=O + H_2O$$ Product: N-nitrosamine

Observation:

• Yellow oily liquid
• No gas evolution

✔ Used as a test for 2° amines

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3° (Tertiary) Amine

(A) Aliphatic 3° Amine:

$$R_3N + HNO_2 \rightarrow R_3NH^+ NO_2^-$$​

Forms ammonium nitrite salt

Observation:

• No visible reaction
• No gas

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(B) Aromatic 3° Amine:

Undergoes electrophilic substitution (ring reaction)

Example: aniline derivatives like dimethylaniline$$\text{Ar–N(CH₃)₂} + HNO_2 \rightarrow p\text{-nitroso compound}$$

✔ Green-colored solution (often)

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Summary Table

Amine Type	Product	Observation
1°	Alcohol / Diazonium	N₂ gas
2°	N-nitrosamine	Yellow oily liquid
3° (aliphatic)	Salt	No reaction
3° (aromatic)	p-nitroso compound	Colored solution

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Quick Trick

• 1° → Gas (N₂)
• 2° → Nitroso (yellow oil)
• 3° → No gas

Correct Reagent

We use NaNO₂ + HCl, but the actual reacting species is HNO₂ (nitrous acid)

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Why?

• HNO₂ is unstable, so it is not stored directly
• It is generated in situ:

$$NaNO_2 + HCl \rightarrow HNO_2 + NaCl$$

Conclusion

✔ In reaction/mechanism → write HNO₂
✔ In practical/lab conditions → write NaNO₂ + HCl

emperature is very important in reactions with nitrous acid (HNO₂ / NaNO₂ + HCl).

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Required Temperature

✔ Primary (1°) Aromatic Amines

0–5°C (ice-cold conditions)

• Needed to form stable diazonium salt
• Above 5°C → diazonium salt decomposes

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Primary (1°) Aliphatic Amines

Also done at 0–5°C

• But diazonium salt is unstable → immediately decomposes to alcohol + N₂

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Secondary (2°) Amines

0–5°C preferred

• Forms N-nitrosamine
• Low temperature prevents side reactions

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Tertiary (3°) Amines

• Aliphatic: 0–5°C (no major reaction, just salt formation)
• Aromatic: slightly higher temp possible, but usually cold conditions used

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Summary Table

Amine Type	Temperature
1° Aromatic	0–5°C (very important)
1° Aliphatic	0–5°C
2°	0–5°C
3°	0–5°C (generally)