Liebermann's Nitroso Test: Identification of Secondary Amines | Chemca

Organic Chemistry

Liebermann's Nitroso Test

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

Liebermann's Nitroso Test is a classic qualitative test used primarily for the identification of Secondary Amines and Phenols. The test involves the formation of a characteristic N-nitrosoamine followed by a specific color change sequence upon treatment with phenol and sulfuric acid.

1. Identification of Secondary Amines

Secondary aliphatic or aromatic amines react with Nitrous Acid ($HNO_2$) to form an N-Nitrosoamine, which separates as a yellow oily layer.

$$ R_2NH + \underbrace{HNO_2}_{\text{from } NaNO_2/HCl} \rightarrow \underbrace{R_2N-N=O}_{\text{N-Nitrosoamine (Yellow Oil)}} + H_2O $$

Since N-nitrosoamines are non-basic and insoluble in dilute acid, they separate as a distinct yellow oily layer. To confirm this layer is a nitrosoamine, the Liebermann reaction is performed.

2. The Liebermann Reaction (Confirmation Step)

The yellow oil is warmed with a crystal of Phenol and a few drops of concentrated Sulfuric Acid ($H_2SO_4$).

Color Sequence Observation

Initial: A Deep Green or Blue solution forms.
Dilution: On pouring into water, the color turns Red.
Alkaline: On adding excess Sodium Hydroxide ($NaOH$), the color turns Deep Blue or Green again.

3. Detailed Mechanism

The chemistry involves the formation of an Indophenol dye.

Step 1: Generation of Electrophile

Concentrated $H_2SO_4$ hydrolyzes the N-Nitrosoamine (or reacts with $NaNO_2$) to generate the Nitrosonium ion ($NO^+$).

Step 2: Nitrosation of Phenol

The $NO^+$ attacks phenol (mostly at the para position) to form p-Nitrosophenol, which exists in equilibrium with its tautomer, Quinone Monoxime.

$$ Ph-OH + NO^+ \rightarrow \underbrace{HO-C_6H_4-NO}_{\text{p-Nitrosophenol}} \rightleftharpoons \underbrace{O=C_6H_4=N-OH}_{\text{Quinone Monoxime}} $$

Step 3: Indophenol Formation

The Quinone Monoxime condenses with another molecule of phenol in the presence of acid to form Indophenol (which is red in acid).

$$ \text{Quinone Monoxime} + \text{Phenol} \xrightarrow{H^+} \text{Indophenol Hydrogen Sulfate (Red)} $$

Step 4: Effect of Alkali

Adding NaOH deprotonates the indophenol to form the Indophenol Anion, which is blue in color due to extended conjugation.

4. Distinguishing Amines

Amine Type	Reaction with $HNO_2$	Liebermann Test
Primary Aliphatic ($1^\circ$)	Evolves $N_2$ Gas	Negative
Primary Aromatic ($1^\circ$)	Forms Diazonium Salt	Negative (gives Azo dye instead)
Secondary ($2^\circ$)	Forms Yellow Oil ($N$-Nitroso)	Positive
Tertiary Aliphatic ($3^\circ$)	Forms soluble Nitrite salt	Negative

5. Reaction of Phenols

This test is also directly used to identify Phenols. When phenol is treated with sodium nitrite and concentrated sulfuric acid, it undergoes the same mechanism described above, resulting in the deep green/blue $\to$ red $\to$ blue color changes.

Liebermann Quiz

Test your concepts on Nitroso reactions. 10 MCQs with explanations.

Search This Blog

Translate