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.

What is Bromine Water?

Bromine Water is a chemical solution comprising diatomic bromine (Br₂) dissolved in water(H₂O). It is also known as bromide bromate solution or bromine solution. The molecular weight of bromine water is 159.808, with a density of 2.91 g/cm³, melting point of -7.2°C, boiling point of 58.8°C at 760 mm Hg, solubility of 35 g/L (20°C), and vapor pressure of 190 mmHg at 25°C with high oxidizing properties. Recognizable by its characteristic reddish-brown color, bromine water serves as a versatile reagent in chemistry, notably in the Bromine Water Test.

Bromine Water Formula

Bromine water, also called as Bromide Bromate solution or Bromine solution. The chemical formula of Bromine Water is Br₂

Bromine Water Color

The color of Bromine water is red-brown due to presence of Bromine in it.

Preparation of Bromine Water

Bromine water can be prepared in the laboratory by direct mixing of fumes of diatomic bromine (Br₂) in water (H₂O) , but this isn’t a safe method and alternatively it is prepared safely by breaking sodium bromide (NaBr) in the presence of bleach (NaClO) and hydrochloric acid (HCl).

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

Principles of Bromine Water Test

Bromine Water Test is based on the principle of bromine’s ability to decolorize in the presence of substances with unsaturated bonds, such as carbon-carbon (C-C) double or triple bonds. The key principles underlying the Bromine Water Test are as follows .

• Unsaturation Detection: Bromine (Br₂) is a reddish-brown liquid, but when it reacts with a substance containing unsaturated bonds (like alkenes or alkynes), it undergoes addition reactions and becomes colorless. This change in color from reddish-brown to colorless is a visual indicator of the presence of unsaturation.

• Electrophilic Addition: Bromine is an electrophilic reagent, meaning it seeks electrons. Unsaturated compounds, such as alkenes or alkynes, readily provide electrons to bromine, leading to the formation of bromonium ions. This electrophilic addition results in the breaking of the bromine molecule and the decolorization of bromine water.

• Selective Reaction: The Bromine Water Test is selective for unsaturated compounds. Saturated hydrocarbons, which lack carbon-carbon double or triple bonds, do not undergo this reaction, and therefore, bromine water remains in its characteristic reddish-brown color when added to saturated compounds.

• Qualitative Analysis: The Bromine Water Test is primarily qualitative, indicating the presence or absence of unsaturation in a given compound. It does not provide quantitative information about the number or nature of unsaturated bonds.

Bromine Water Test Reaction

When Bromine Water is made to react with an unsaturated compound, it undergoes addition reaction and it turns from reddish-brown to a colorless liquid, whereas saturated compounds doesn’t react with it, so there isn’t any color changes observed. Hence it is used for Saturation Test.

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 Phenol

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 Aniline

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 Enols

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 Glucose and 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.

Application of Bromine Water Test

Testing for the presence of bromine in water systems is a popular use for bromine water tests, especially in situations where bromine is used as a biocide or disinfectant. In order to maintain the water’s ability to regulate microbial development while averting possible health problems, monitoring bromine levels is essential. A few uses for bromine water tests are as follows:

• Swimming Pools and Spas: To preserve water quality and guarantee the efficacy of the disinfection procedure, it is important to conduct routine testing of bromine levels in swimming pools and spas. In order to keep the water clean and safe for swimmers, pool managers use bromine water test kits to monitor and regulate bromine concentrations.

• Systems for Cooling Towers: Tests on bromine water in industrial cooling tower systems are carried out to confirm the bromine content used as a biocide. By preventing microbial fouling, corrosion, and scaling, maintaining the proper bromine levels contributes to the cooling system’s effective functioning.

• Plants that Process Poultry: In poultry processing facilities, bromine water tests are utilized to verify the amount of bromine utilized for disinfection. By doing this, the processing environment is kept free of dangerous germs, preserving the safety and quality of chicken products.

• Breweries: Bromine water tests are used by breweries to confirm the level of bromine in their sanitation procedures. In order to keep brewing equipment hygienic and avoid contamination, monitoring bromine levels is essential for retaining the beer’s quality.

• Facilities for Treating Wastewater: Tests using bromine water are used in wastewater treatment to determine how effective bromine is as a disinfectant. In order to assure effective pathogen elimination and minimize any potential negative environmental repercussions, wastewater treatment plants benefit from routine monitoring that helps them adjust bromine dosages.

Limitation of Bromine Water Test

Although bromine water tests are useful for tracking bromine levels in a variety of applications, it’s critical to understand their limitations. The following are some typical restrictions connected to testing using bromine water:

• Sensitivity to Temperature: Temperature fluctuations may affect the results of bromine water tests.

• Shelf Life and Storage: Test kit expiration or incorrect storage can have an impact on the accuracy of bromine water tests.

• Sensitivity to pH: The water’s pH can have an impact on how accurate bromine water testing are.

• Other Chemicals’ Interference: Tests using bromine water may be sensitive to specific compounds or impurities in the water, which could cause interference and provide false findings.

• Restricted Quantitative Accuracy: Tests on bromine-contaminated water are frequently more qualitative than quantitative.

Also, Check

• Chemical Properties of Caron Compounds
• Bromine Formula
• Chemical Reaction of Amine

Sample Questions on Bromine Water Test

Question 1: What is the principle involved in the bromine water test?

Answer:

Principle involved in the bromine water test is based on the reaction between bromine and unsaturated compounds, specifically alkenes or alkynes. Bromine adds across carbon-carbon double or triple bonds, causing the characteristic reddish-brown color of bromine water to disappear. Saturated compounds, lacking double or triple bonds, do not undergo this reaction and do not show a color change.

Question 2: What is the function of sunlight in the bromine water and iodine solution tests?

Answer:

Sunlight is not typically a factor in the bromine water test. In contrast, the iodine solution test, which assesses the presence of starch, can benefit from sunlight.

Question 3: Which hydrocarbon is most easily oxidized by KMnO4 and which is most reactive towards bromine water?

Answer:

Alkenes are readily oxidized by potassium permanganate (KMnO4), leading to the decolorization of the purple KMnO4 solution. This reaction involves oxidative cleavage of the carbon-carbon double bond, forming diols. On the other hand, alkynes are highly reactive towards bromine water, causing the rapid disappearance of its reddish-brown color.

Question 4: What happens when D-glucose is treated with bromine water?

Answer:

When D-glucose is treated with bromine water, D- gluconic acid is produced and bromine water changes from reddish – brown to a colorless solution indicating the presence of unsaturation .

Bromine Water Test – FAQs

What is Bromine Water Analysis?

Bromine water analysis involves testing for the presence of unsaturation in organic compounds by observing decolorization of bromine water.

Why is Bromine Water used to Test for Unsaturation?

Bromine water is used for unsaturation testing because it adds across double bonds, resulting in decolorization and providing a positive indication of unsaturated compounds.

What does a Positive Bromine Test Mean?

A positive bromine test indicates the presence of unsaturation (double or triple bonds) in the tested organic compound, as bromine adds across these bonds, causing decolorization of bromine water.

Which Compounds can Give a Bromine Water Test?

Compounds that can give a bromine water test are those with unsaturation, such as alkenes and alkynes.

Is Bromine Water Dangerous?

Yes, bromine water can be dangerous as bromine is a corrosive and toxic substance.

What are Limitations of Bromine Water Test?

Limitations of the bromine water test include its inability to distinguish between different types of unsaturation and its sensitivity to other factors like impurities and temperature changes.