Alkenes and haloalkanes undergo characteristic test-tube reactions that allow rapid and reliable qualitative identification. This subsubtopic focuses on the bromine test for unsaturation and the use of silver nitrate with ethanol to identify haloalkanes through their rates of hydrolysis.

Understanding the Purpose of Test-Tube Reactions

Test-tube reactions provide quick, functional-group-level identification without the need for advanced instrumentation. They are particularly important in organic qualitative analysis where structural clues must be obtained from simple chemical observations such as decolourisation, precipitate formation, and reaction rate.

Alkenes: The Bromine Test

Alkenes contain a C=C double bond, which is a region of high electron density and reacts readily with electrophiles. The OCR specification emphasises that alkenes can be detected using bromine.

The bromine test exploits the electrophilic addition reaction between bromine and the C=C bond. Students must be able to recognise the resulting loss of colour, an essential visual indicator.

Why Bromine Water Works

Bromine water is orange because of dissolved Br₂. When an alkene is present, bromine adds across the double bond, forming a colourless dibromoalkane. The absence of any other strong competing reactions makes this test highly selective for unsaturation.

Procedure for Detecting Alkenes

The decolourisation of bromine water occurs rapidly at room temperature and does not require catalysts or UV light. Key procedural points include:

• Add a small volume of bromine water to the sample.

• Shake gently and observe the colour change.

• Rapid decolourisation indicates the presence of an alkene.

• If no alkene is present, the orange colour persists.

A positive test for an alkene is the rapid decolourisation of bromine water from orange/brown to colourless, showing a C=C bond is present.

Bromine water remains orange with an alkane but is rapidly decolourised by an alkene as bromine adds across the C=C bond, removing Br₂ from solution. Source

The test does not provide structural details, only confirmation of a C=C bond.

Haloalkanes: Hydrolysis and Precipitate Formation

Haloalkanes undergo nucleophilic substitution, yielding alcohols and halide ions. The specification states that haloalkanes should be identified using aqueous silver nitrate in ethanol. This method relies on precipitation of silver halides, each with a distinct colour.

A sentence is required here to ensure appropriate spacing. Hydrolysis frees halide ions, which then react with silver ions to form insoluble salts.

Why Ethanol Is Used

Ethanol acts as a co-solvent, allowing the haloalkane (organic) and aqueous reagents (inorganic) to mix uniformly. Without ethanol, the reactants would form two layers and hydrolysis would be significantly slower.

Procedure for Identifying Haloalkanes

Students must know the essential steps and the meaning of the observed precipitates. The following points outline the process:

• Warm the sample with aqueous silver nitrate in ethanol.

• Hydrolysis releases halide ions: Cl⁻, Br⁻, or I⁻.

• These ions react with Ag⁺ to form precipitates:

  • AgCl — white

  • AgBr — cream

  • AgI — yellow

• The precipitate forms more rapidly for iodoalkanes than for bromo- or chloroalkanes due to weakening of the C–X bond down the group.

Haloalkanes can be identified by warming with aqueous silver nitrate in ethanol; hydrolysis releases halide ions that form a silver halide precipitate (AgCl white, AgBr cream, AgI yellow).

Silver halide precipitates form when halide ions react with aqueous silver nitrate, producing visible cloudiness or solids that confirm the presence of a haloalkane after hydrolysis. Source

Bond Strength and Its Influence on Test Outcomes

The C–X bond strength decreases from C–Cl to C–I. Because hydrolysis requires bond cleavage, the reaction rate follows:

• Fastest hydrolysis: iodoalkanes

• Intermediate: bromoalkanes

• Slowest: chloroalkanes

These differences allow comparison of relative reaction rates, which can help confirm the identity of an unknown haloalkane.

Silver Halide Precipitate Characteristics

Although the specification focuses on identification, understanding the nature of these precipitates supports clearer interpretation:

• AgCl dissolves in dilute ammonia, confirming chloride ions.

• AgBr dissolves in concentrated ammonia, confirming bromide ions.

• AgI does not dissolve in ammonia, confirming iodide ions.

These solubility patterns help validate qualitative observations when colours appear ambiguous.

Key Chemical Principles Underpinning the Tests

Both reactions hinge on interaction between electron-rich and electron-poor centres:

• In alkenes, the π-bond provides electron density for electrophilic attack.

• In haloalkanes, the polar C–X bond allows nucleophilic substitution during hydrolysis.

A sentence after the equation is required here. This general reaction represents the essential chemical step allowing identification of the halide ion.

Practical Considerations and Good Analytical Technique

Reliable qualitative analysis requires careful attention to experimental conditions:

Control of Temperature

Warming accelerates hydrolysis without causing side reactions. Excessive heating, however, may degrade the haloalkane or alter precipitate appearance.

Avoiding False Positives

• Samples must be free from inorganic halide contamination.

• Glassware should be rinsed thoroughly.

• Ethanol volume should be sufficient to ensure homogeneous mixing.

Observation Skills

Students should be familiar with terms such as:

• Decolourisation — loss of bromine colour in alkenes.

• Precipitate formation — appearance of solid silver halide.

• Reaction rate comparison — evaluating the time taken for cloudiness to appear.

These interpretative skills are essential for correct qualitative analysis in accordance with the specification.

Bringing Together the Two Qualitative Tests

Although they address different functional groups, both tests require students to observe visible changes that correlate directly with underlying chemical behaviour. The bromine test detects unsaturation, while the silver nitrate test identifies the halogen present in a haloalkane and provides further insight through reaction rate differences.