Question

Describe a simple chemical test to tell the difference between \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) and its isomer cyclopentane.

Short answer

Use bromine water. The alkene decolorizes it; cyclopentane does not.

Step-by-step solution

Identify the Functional Group in Alkene

The given compound, \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2} \), contains a carbon-carbon double bond which is characteristic of alkenes. This functional group can react with certain reagents, enabling us to identify its presence.

Determine the Absence of Reactivity in Cyclopentane

Cyclopentane is a saturated cyclic hydrocarbon with only single bonds between its carbon atoms. As a result, it lacks the reactive double bond seen in alkenes and does not react with the same reagents that alkenes do.

Select a Reagent for Detection

A common reagent used to differentiate alkenes from alkanes and cycloalkanes is bromine water. Bromine water is an orange solution of bromine in water. It reacts readily with the double bonds in alkenes, leading to decolorization of the solution.

Perform the Bromine Water Test

Add a few drops of bromine water to separate samples of both \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2} \) and cyclopentane. Observe the changes. The alkene will turn the bromine water colorless due to the reaction with the double bond, while the cyclopentane will not change its color as there is no reaction.

Key concepts

Bromine Water Test

The bromine water test is a simple yet effective chemical reaction used to distinguish between alkenes and saturated hydrocarbons like alkanes or cycloalkanes. Bromine water is an orange solution of bromine dissolved in water. When this solution is added to a compound, it reacts with unsaturated hydrocarbons by breaking their carbon-carbon double bonds. Such a reaction causes the bromine water to lose its characteristic orange color, turning colorless. This distinct color change is used to confirm the presence of a carbon-carbon double bond, a signature of alkenes. In practical terms, no change in color indicates the absence of these double bonds, such as in saturated hydrocarbons.

Alkene Functional Group

Alkenes are a class of hydrocarbons characterized by at least one carbon-carbon double bond in their molecular structure. This double bond is what we refer to as the 'alkene functional group'. It's important because it influences both the chemical reactivity and physical properties of alkenes. As part of the functional group, the double bond acts as a reactive site that can be targeted by various reagents. For educational and practical purposes, identifying an alkene often revolves around spotting and utilizing this functional group to trigger specific reactions, such as with bromine water.

Saturated Hydrocarbons

Saturated hydrocarbons, as the name suggests, have carbon atoms that are "saturated" with hydrogen atoms. This means they only contain carbon-carbon single bonds. Examples include alkanes and cycloalkanes. These hydrocarbons are known for being less reactive compared to their alkene counterparts. This lower reactivity is because single bonds do not act as available reactive sites for many chemical reactions. Thus, saturated hydrocarbons like cyclopentane will not react with certain reagents like bromine water, unlike alkenes, which rapidly engage due to their unsaturated double bonds.

Cycloalkanes

Cycloalkanes are a unique variety of saturated hydrocarbons, forming a ring-like structure. Each carbon in these rings is bound to two other carbons, besides binding to enough hydrogens to satisfy valency. Common examples include cyclopentane and cyclohexane. Despite their cyclic structures, cycloalkanes share similar characteristics to straight-chain alkanes, particularly their absence of reactive double bonds. This makes them unreactive to certain chemical tests like the bromine water test, which are otherwise effective in detecting alkenes due to their reactivity.

Carbon-Carbon Double Bond

Carbon-carbon double bonds are the hallmark of alkenes. These double bonds involve the sharing of two pairs of electrons between the two carbon atoms, resulting in a stronger and shorter bond compared to single bonds. The presence of a carbon-carbon double bond impacts the chemical nature of a molecule, as this bond is often the site for chemical reactions. In the context of distinguishing alkenes from saturated hydrocarbons, these double bonds enable alkenes to participate in the bromine water test. The reaction of alkenes, where the double bond opens to accommodate bromine atoms, confirms the double bond's presence and facilitates clear identification.