Bromine Water Test

Bromine water testing is an essential method for determining the presence of any alkene/alkyne functional groups in a chemical.

Enols, anilines, alkenes, phenols, acetyl groups, and glucose are the most common substances tested in bromine water. 

1.0What Is Bromine Water?

Bromine water, or bromide bromate solution, is a chemical mixture containing diatomic bromine (Br₂). This vivid yellow-to-red liquid solution is characterized by its strong oxidizing properties.

• With a molecular formula of Br₂, bromine water has a molecular mass of 159.81 and a density of 1.307 g/mL.
• Functioning as a yellow mixed solution, bromine water owes its potent oxidation capabilities to the presence of diatomic bromine (Br₂) in combination with water (H₂O).
• The red-brown color of bromine water is attributed to the presence of bromine molecules (Br₂) dissolved within it.
• Utilizing the halogenation process, bromine water serves as a tool to identify specific functional groups within organic molecules.

2.0Preparation of Bromine Water

The traditional method of preparing bromine water solution in a chemistry lab typically involves mixing bromine fumes with water. However, due to safety concerns associated with handling pure bromine, a more preferred method involves the decomposition of sodium bromide (NaBr) in bleach and hydrochloric acid (HCl).

•          2NaBr + NaClO + 2HCl → Br₂ + 3NaCl + H₂O

3.0Features of Bromine Water Test

The Bromine Water Test relies on bromine's capacity to lose its color when exposed to compounds containing unsaturated bonds, such as carbon-carbon (C-C) double or triple bonds. The Bromine Water Test operates on several key features:

Unsaturation Detection: When bromine (Br₂), a reddish-brown liquid, reacts with substances containing unsaturated bonds (such as alkenes or alkynes), it undergoes addition reactions, resulting in the decolorization of the bromine solution. This change from reddish-brown to colorless serves as a visible indication of unsaturation presence.

Electrophilic Addition: Bromine acts as an electrophilic reagent, seeking electrons. Unsaturated compounds, like alkenes or alkynes, readily provide bromine electrons, forming bromonium ions. This electrophilic addition causes the bromine molecule to break, resulting in the decolorization of bromine water.

Selective Reaction: The Bromine Water Test selectively targets unsaturated compounds. Saturated hydrocarbons, lacking carbon-carbon double or triple bonds, do not participate in this reaction. Consequently, when added to saturated compounds, bromine water retains its characteristic reddish-brown color.

Qualitative Analysis: Primarily qualitative, the Bromine Water Test indicates the presence or absence of unsaturation in a given compound. However, it does not furnish quantitative data regarding the number or type of unsaturated bonds present.

4.0Bromine water (unsaturation) test

The bromine water (saturation) test is reliable for identifying a compound's alkene/alkane functional groups.

Alkenes react with bromine water, typically in the absence of light, undergoing an addition reaction that results in a decolorized solution. Conversely, alkanes do not interact with bromine water, thus causing no change in its color.

Common substances subjected to bromine water testing include enols, anilines, alkenes, phenols, acetyl groups, and glucose.

Additionally, this test detects the presence of an aldehyde functional group. If an aldehyde functional group is present, the color of the bromine liquid shifts from yellow to neutral.

5.0Bromine Water Test For Various Functional Groups:

For Alkanes

Alkanes do not undergo a reaction with bromine water, leaving the dark yellow colour of the bromine solution unaffected.

• CH₄ (Methane) + Br₂ → No reaction
• C₂H₆ (Ethane) + Br₂ → No reaction
• C₃H₈( Propane ) + Br₂  → No reaction

For Alkenes

Alkenes, such as ethene, undergo an addition reaction with bromine water, forming 1,2-dibromoethane.

              ${\mathrm{Br}}_2+{\mathrm{C}}_2{\mathrm{H}}_4\to \mathrm{Br}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Br}$

If the reactants are gaseous (e.g., ethene), this reaction can take place at room temperature. The bromine water solution becomes decolorized as it reacts with ethene. However, for liquid alkenes like cyclohexene, the reaction with bromine water occurs in the presence of tetrachloromethane.

For Phenols                 

When phenolic compounds react with bromine water, they undergo substitution reactions, forming a brominated product. Throughout this process, bromine liquid becomes decolorized, resulting in the formation of a white precipitate.

For Aniline 

When bromine water interacts with aniline (phenylamine), a white precipitate forms, and the bromine solution decolorizes

Aniline reacts with bromine to form tribromoaniline (a white precipitate) and hydrobromic acid (HBr).

For ketones                

When a ketone reacts with bromine water, an electrophilic alpha substitution reaction occurs adjacent to the carbonyl group. This reaction forms brominated compounds and produces a colorless solution.

For Aldehydes     

Aldehydes react with bromine water through an oxidation reaction, where the aldehyde is converted to a carboxylic acid, and bromine is reduced to hydrogen bromide.The disappearance of the reddish-brown color of bromine water, resulting in a colorless solution, is a visual indicator of the reaction's completion.

                          CH_3​CHO + Br ​+ H_2​O → CH_3​COOH + 2HBr

For Enols

The bromine water test can detect the presence of enols due to their unsaturation. Enols react with bromine in bromine water, leading to the formation of a brominated ketone. This reaction causes the bromine water to change from reddish-brown to colorless, indicating the presence of the enol.

For Glucose and Fructose

The bromine water test effectively distinguishes between glucose and fructose. Glucose, with its aldehyde group, undergoes an oxidation reaction with bromine water, resulting in a colorless solution due to the formation of glucuronic acid. Fructose, lacking an aldehyde group, does not react with bromine water, and the solution remains reddish-brown.

For Benzene 

Despite being an unsaturated compound, Benzene does not react with bromine water. This is because benzene is aromatic, and undergoing anti-addition with bromine would disrupt its aromaticity, which is highly unfavourable. Consequently, no color change is observed with bromine water.

                   C₆H₆ (Benzene) + Br₂ → No reaction