Question

You have an unknown liquid hydrocarbon that may be hexane, 1 -hexene or cyclohexene. What simple chemical tests could you use to distinguish which it is?

Short answer

To identify the unknown liquid hydrocarbon, perform the following chemical tests: 1. Bromine test: Mix the liquid with bromine water. If the color changes to colorless, it is an alkene. If not, it is hexane. 2. Mild oxidation test using KMnO4: If the test in step 1 shows an alkene, heat the liquid with dilute KMnO4 solution. If a brown precipitate forms, it is 1-hexene. If no color change occurs, it is cyclohexene. These simple tests can distinguish between hexane, 1-hexene, and cyclohexene based on the presence of double bonds and their linear or cyclic structure.

Step-by-step solution

Check for unsaturation

The presence of a double bond in a hydrocarbon can be tested using the bromine test. In this test, bromine water (a solution of bromine in water) is mixed with the test sample. If the sample changes from reddish-brown to colourless, it means there is a double bond present in the hydrocarbon, indicating it's an alkene. Perform the bromine test: Mix a small amount of the unknown liquid with bromine water in a test tube and observe the colour change. If the colour changes to colourless, proceed to Step 2. If not, then the unknown hydrocarbon is hexane.

Check for linearity or cyclic structure

To distinguish between 1-hexene (linear alkene) and cyclohexene (cyclic alkene), perform the mild oxidation test using potassium permanganate (KMnO4) solution. Mild oxidation test: Gently heat a small amount of the unknown liquid with dilute KMnO4 solution. Observe the reaction and formation of products. If the solution turns brown and forms a brown precipitate, it means that the hydrocarbon has undergone oxidation, indicating the presence of a terminal double bond (like in 1-hexene). If the mild oxidation test does not produce any change in colour, it means the hydrocarbon is a cyclic alkene (cyclohexene). In summary, based on the results of these two simple chemical tests (bromine test and mild oxidation test), we can distinguish between hexane, 1-hexene, and cyclohexene.

Key concepts

Bromine Test

The bromine test is a fundamental chemical test used to identify the presence of unsaturation in hydrocarbons. Unsaturation refers to chemical compounds that contain double or triple bonds, which are typically found in alkenes and alkynes.

When performing the bromine test, you mix the hydrocarbon sample with bromine water, which is a reddish-brown solution of bromine. The principle is simple; if the compound contains a double bond, the bromine will react with it, breaking the pi bond and adding across the double bond. This reaction decolorizes the solution, turning it from reddish-brown to colorless.

Steps for performing a bromine test:

• Prepare a test tube containing the unknown hydrocarbon.
• Add a few drops of bromine water.
• Observe the color change in the solution.

If the color change to colorless occurs, you have an alkene, confirming unsaturation. If no color change occurs, the hydrocarbon is saturated, such as an alkane like hexane.

KMnO4 Oxidation

Potassium permanganate (KMnO4) is a potent oxidizing agent commonly used to detect unsaturation in hydrocarbons. The test specifically caters to differentiating between linear alkenes and cyclic alkenes.

KMnO4 comes into play once the bromine test confirms the presence of a double bond. This test consists of adding a diluted KMnO4 solution to the hydrocarbon sample and gently heating. The presence of double bonds in a hydrocarbon allows them to oxidize, resulting in visible changes.

Key observations during KMnO4 oxidation:

• If a brown precipitate forms or the solution turns brown, it suggests oxidization of a terminal double bond, like in 1-hexene.
• No change, particularly no color shift, suggests a cyclic structure, as cyclic alkenes like cyclohexene do not easily oxidize under these conditions.

This test helps in distinguishing between linear and cyclic alkenes when initially established there is a double bond.

Double Bond Detection

Detecting double bonds in hydrocarbons is critical because it determines the hydrocarbon structure's nature, such as differentiating between saturated and unsaturated compounds.

The procedures we often employ, like the bromine test and KMnO4 oxidation, both capitalize on the reactivity of double bonds:

• Bromine Test: Uses the highly reactive nature of double bonds to break the pi bond and react with bromine, resulting in decolorization.
• KMnO4 Oxidation: Works on the principle that double bonds can be oxidized by strong oxidizing agents like KMnO4.

These tests provide visual cues—like color changes or precipitate formation—enabling easy identification without requiring complex instrumentation. By identifying the presence or absence of double bonds, we gain insights into the chemical structure and potential reactivity of the hydrocarbon.

Hexane vs Hexene

Hexane and hexene, though similar in carbon atom count, differ significantly in chemical structure and reaction properties.

Hexane is a saturated hydrocarbon containing only single bonds. It belongs to the alkane family, typically unreactive in tests targeting double bonds and remains unchanged during such reactions.

On the other hand, hexene is unsaturated and contains a double bond. It falls under the category of alkenes known for readily reacting with reagents like bromine and potassium permanganate that specifically target unsaturated compounds:

• In the bromine test, hexene would cause a rapid color change to colorless, indicated by the reaction of the double bond with bromine.
• Hexane will not show any reaction, as it has no double bond to interact with bromine.

Understanding this distinction is vital for correctly identifying and classifying unknown hydrocarbons as either saturated or unsaturated.

Cyclic vs Linear Alkenes

One interesting aspect of alkenes is their structural difference: they can be cyclic or linear. This structure plays a significant role in their chemical behavior, particularly during/tests involving organic compounds.

Cyclic alkenes, like cyclohexene, form a closed ring structure and are less reactive to certain oxidation types due to their internal bonds' spatial arrangement. Linear alkenes, such as 1-hexene, have open chain structures that allow for more straightforward interaction with reagents like KMnO4.

Key differences include:

• Cyclic Alkenes: Show minimal reaction in oxidative conditions. They maintain their structure, thus lacking significant color change.
• Linear Alkenes: Generally undergo reactions that lead to the formation of brown precipitates or browning of the solution.

Understanding these differences helps chemists determine the type of alkene and predict potential reactions, based on its structural composition.