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Bromine Water Test: Unsaturation & Phenols

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

Bromine Water ($Br_2(aq)$) is an intense orange-red solution of bromine dissolved in water. It is a versatile reagent used to detect Unsaturation (alkenes/alkynes) via decolorization and to identify highly activated aromatic compounds like Phenols and Anilines via precipitation.

1. Test for Unsaturation (Alkenes & Alkynes)

When bromine water is added to an alkene or alkyne, the orange color disappears rapidly. This indicates the presence of a carbon-carbon double or triple bond.

$$ >C=C< + \underbrace{Br_2}_{\text{Orange}} + H_2O \rightarrow \underbrace{>C(OH)-C(Br)<}_{\text{Colorless Bromohydrin}} + HBr $$

Mechanism: Electrophilic Addition

1. Electrophilic Attack: The pi-electrons attack $Br_2$ to form a Cyclic Bromonium Ion intermediate.
2. Nucleophilic Attack: Since water is the solvent (present in large excess), it acts as the nucleophile and opens the ring from the backside (Anti-addition). This forms a Bromohydrin (halo-alcohol).
   Note: If done in $CCl_4$, vicinal dibromide is formed. In water, bromohydrin is the major product.

2. Test for Phenols

Phenols are highly reactive towards electrophilic substitution due to the strong $+M$ (mesomeric) effect of the $-OH$ group. In aqueous medium, phenol ionizes to the even more reactive Phenoxide ion.

$$ \text{Phenol} + 3Br_2(aq) \rightarrow \underbrace{\text{2,4,6-Tribromophenol}}_{\text{White Precipitate}} + 3HBr $$

Observation: The orange color of bromine water disappears, and a White Precipitate forms immediately.

Why Polysubstitution?

In water (a polar solvent), the phenoxide ion is formed, which strongly activates the ortho and para positions. The electrophile $Br^+$ is also generated more effectively. This leads to the substitution of all available ortho and para hydrogens.

Contrast with Non-polar Solvent

If the reaction is carried out in a non-polar solvent like $CS_2$ or $CCl_4$ at low temperature, monobromination occurs (mostly p-Bromophenol) because the phenol does not ionize.

3. Test for Aniline

Aniline ($Ph-NH_2$) behaves similarly to phenol. The $-NH_2$ group is a strong activator.

$$ \text{Aniline} + 3Br_2(aq) \rightarrow \underbrace{\text{2,4,6-Tribromoaniline}}_{\text{White Precipitate}} + 3HBr $$

Observation: Formation of a White Precipitate.

4. Distinguishing Compounds

Compound Type	Reaction with Bromine Water	Observation
Alkene / Alkyne	Addition	Decolorization (No Ppt)
Phenol / Aniline	Substitution (Polysubstitution)	Decolorization + White Ppt
Benzene	No Reaction	No change (Orange color persists)
Alkane	No Reaction (in dark)	No change

5. Comparison with Baeyer's Test

• Baeyer's Test ($KMnO_4$): Gives Brown precipitate ($MnO_2$). Tests for oxidation susceptibility (alkenes, aldehydes).
• Bromine Water: Gives Colorless solution (alkenes) or White precipitate (phenols). Specific for high electron density pi-systems or activated aromatic rings.

Bromine Water Quiz

Test your concepts on Unsaturation and Aromatic Reactivity. 10 MCQs with explanations.

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