Common Questions About the Bromine Water Test

Bromine water, commonly known as bromide bromate solution, is a chemical compound having the formula Br2. Bromine water has a molecular mass of 159.81 and a density of 1.307 g/mL. 

Bromine water is just a yellow mixed solution with a strong oxidation ability due to the combination of diatomic bromine (Br2) and water (H2O). We can utilise the halogenation process to detect the functional group existing within organic molecules using bromine water. 

Bromine water (saturation) test

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

Alkene groups react to bromine water, mostly in the dark, and conduct an addition reaction, resulting in a decolourised solution. Alkanes, in contrast, don’t interact with bromine water, so the colour of bromine water doesn’t change. 

The most common substances tested in bromine water include enols, anilines, alkenes, phenols, acetyl groups, and glucose. 

This test also determines if the substance has an aldehyde functional group. The colour of the bromine liquid changes from yellow to neutral if an aldehyde functional group is present.

Reactions with various functional groups

Alkene

A type of reaction known as an addition reaction is the reaction involving bromine and alkenes. A colourless dibromo molecule develops, which decolourises the bromine. Example:

ethene + bromine → dibromoethane 

C2H4 + Br2 → C2H4Br2

Other alkene addition processes include hydrogen inclusion to something like a C=C dual bond. It causes the molecule to become ‘saturated’, converting an alkene to an alkane.

C2H4 + H2 → C2H6 

When we introduce steam to an alkene, it forms alcohol.

C2H4 + H2O → C2H5OH 

Alkanes

The alkanes are a group of hydrocarbons that are all the same. It signifies that their chemical characteristics are comparable, and their physical qualities are trending in the same direction. The boiling point of a chain, for example, rises when the length of the chain grows.

The general formula states that an alkane has double the amount of carbon molecules and two hydrogens. Methane, for instance, is CH4, while ethane is C2H6.

Alkanes seem to be hydrocarbons that are saturated. It indicates that the carbon atoms remain linked by single bonds. Except for their interaction with oxygen in the atmosphere, which we name burning or combustion, they are generally relatively inert.

In sunlight, alkanes conduct a substitution reaction involving halogens.

Methane, for example, interacts with halogen atoms like chlorine and bromine when exposed to UV light.

methane + bromine → methyl bromide + hydrogen bromide

CH4 + Br2 → CH3Br + HBr

Anilines

Bromine water interacts with aniline or phenylamine. The reaction results in the formation of a white precipitate and the decolourisation of bromine solution.

NH2 + 3Br2  → 3HBr

Phenols

With the addition of bromine water, phenolic compounds conduct substitution reactions, yielding a brominated product. Bromine liquid is decolourised throughout the procedure, resulting in a white precipitate.

Ketones

The electrophilic alpha substitution effect proceeds between ketone reacting with bromine water close to the carbonyl group. It yields a clear solution containing brominated molecules.

Glucose

Bromine water testing is an effective and easy way to tell the difference between fructose and glucose. 

When glucose reacts to bromine water, it conducts an oxidation process, yielding glucuronic acids. Fructose does not initiate an oxidation process because bromine water is just a moderate oxidising agent.

Aldehydes

When an aldehyde interacts with bromine water, an oxidation effect proceeds, resulting in the formation of a colourless solution.

Bromine water applications

Bromine water has various applications, namely:

• Cooling tower and cooling water

• Poultry processing

• Pools and spas

• Wastewater

• Oil and gas applications

• Breweries

Conclusion

The bromine water test is helpful to determine if hydrocarbons are saturated or unsaturated. Bromine water tests are easy to perform on organic substances, including phenols, acetyl compounds, alkenes and even anilines. 

A colour change in bromine water during the procedure indicates the existence of such an unsaturated group (mostly organic molecules). 

Furthermore, we commonly use bromine water to detect the presence of an alkene with the double covalent link, which interacts with bromine water to alter its colour from a bright yellow to something colourless. 

Bromine water is also widely used to determine if a chemical contains an aldehyde group.