Bromine Water Test for Different Substances

The bromine water test is a chemical analysis technique used to identify the presence of unsaturation in organic compounds. It involves the addition of bromine water ( Br₂ + H₂O ) to various substances, with the reaction revealing the presence of carbon-carbon double or triple bonds. This test is particularly useful in POC for distinguishing between saturated and unsaturated hydrocarbons. However this test is also useful to identify some other functional groups such as enols and ketones.

Bromine Water Test for Alkanes

Alkanes C_nH_2n+2 (such as methane, ethane, propane) does not react with the bromine water solution, so there isn’t any color changes observed. This indicates presence of saturation.

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

Bromine Water Test for Alkenes

Alkenes C_nH_2n ( such as ethene , propene ) undergoes an addition reaction with the bromine water solution. For example, ethene reacts with bromine water to give 1,2 dibromo ethane. This reaction takes place at room temperature if the reactants are in the gaseous state .Bromine water change from reddish-brown to a colorless liquid. This indicates presence of unsaturation.

Br₂ + C₂H₄ → Br-CH₂-CH₂-Br

Bromine Water Test for Alkynes

Alkynes C_nH_2n-2 (such as Ethyne, Propyne) undergoes an addition reaction similar to the alkenes .For example, ethyne reacts with bromine water to give 1,1,2,2 tetrabromo ethane. Bromine water change from reddish-brown to a colorless liquid. This indicates presence of unsaturation .

2Br₂ + C₂H₂ → Br₂-CH-CH-Br₂

Bromine Water Test for Phenol

Phenol undergoes substitution reactions with the bromine water solution giving a white precipitate named as 2, 4, 6-tribromophenol .This reaction is also known as bromination of phenol . Bromine water change from reddish-brown to a colorless liquid .This indicates presence of unsaturation.

Bromine Water Test for Aniline

Aniline or phenylamine undergoes substitution reactions with the bromine water solution giving a white precipitate named as 2, 4, 6-tribromoaniline. Bromine water change from reddish-brown to a colorless liquid. This indicates presence of unsaturation.

Bromine Water Test for Benzene

Benzene is an unsaturated compound but it doesn’t react with bromine water . This is because benzene is an aromatic compound and if it undergoes anti addition with bromine, it will lose its aromaticity, which is not possible. Hence we observe no color changes.

C₆H₆ (Benzene) + Br₂ → No reaction

Bromine Water Test for Enols

Enols are characterized by an alkene (C=C) and an adjacent alcohol group (-OH). It undergoes the bromine water test, and brominated ketone is formed. This reaction, which is highly selective for enols, causes the bromine water to change from reddish-brown to colorless indicating the presence of unsaturation.

Bromine Water Test for Glucose and Fructose

Bromine Water test is a simple test to distinguish between Glucose and Fructose. Glucose undergoes an oxidation reaction to give gluconic acid in reaction with the bromine water solution. Since bromine water is a mild oxidizing agent, Fructose fails to undergo an oxidation reaction with bromine water. Hence bromine water change from reddish-brown to a colorless liquid when it reacts with Glucose and there is no color changes observed when it reacts with Fructose.

Bromine Water Test for Ketones and Aldehydes

A reaction between the ketone and bromine water is an electrophilic alpha substitution reaction adjacent to the carbonyl group, and gives the colorless solution of brominated compounds. Similarly when an aldehyde interacts with bromine water, an oxidation effect proceeds, resulting in the formation of a colorless solution. Hence bromine water change from reddish-brown to a colorless liquid for both ketones and aldehydes.

Bromine Water Test is an important test in Practical Organic Chemistry that deals with detecting the presence of unsaturation, such as carbon-carbon (C-C) double or triple bonds in organic compounds, i.e. it is used to identify the presence of alkane or alkene in an organic compound.

In this article, we will discuss this test in detail, including the preparation of bromine water, the principle of the test, and various substances that undergo this test with their reactions.