Question

Suggest a simple test to distinguish \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) from \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\).

Short answer

The bromine water test can be used to distinguish between the compounds. The solution will de-colorize when added to \( \mathrm{CH}_{2} = \mathrm{CHCH}_{2}\mathrm{CH}_{3} \) due to the presence of a double bond, while it will remain yellow or orange with \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \).

Step-by-step solution

Identify the compounds

The first compound given is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \), which is butane, an alkane. The second compound is \( \mathrm{CH}_{2} = \mathrm{CHCH}_{2}\mathrm{CH}_{3} \), which is butene, an alkene. Alkanes are hydrocarbons with single bonds only whilst alkenes contain one or more carbon-carbon double bonds.

Understand the bromine water test

The bromine water test involves adding bromine water to each substance. Bromine water is initially yellow or orange. If the substance is an alkene, the solution will de-colorize, going from yellow or orange to colorless.

Apply the test specificity to the compounds

Add bromine water to each of the two compounds in separate test tubes. The bromine water will remain yellow or orange when added to the butane, i.e., \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \). This is because butane is an alkane and does not have a double bond to react with the bromine. On the other hand, the bromine water will de-colorize when added to the butene, i.e., \( \mathrm{CH}_{2} = \mathrm{CHCH}_{2}\mathrm{CH}_{3} \) as alkenes decolorize bromine water due to the presence of a carbon-carbon double bond.

Key concepts

Alkane vs Alkene

Understanding the difference between alkanes and alkenes is fundamental in organic chemistry. Alkanes are saturated hydrocarbons, meaning they only contain single bonds between carbon atoms. Examples include methane, ethane, and butane. These compounds are generally less reactive due to the stability of their single bonds.

On the other hand, alkenes are unsaturated hydrocarbons that contain at least one double bond between carbon atoms. This double bond is the key feature that distinguishes alkenes from alkanes and contributes to their distinctive chemical properties. Alkenes, such as ethene, propene, and butene, are more reactive because the double bond provides an area of higher electron density which is more accessible to reactants. This fundamental difference is the basis for many chemical tests and reactions that can distinguish alkanes from alkenes.

Organic Compound Identification

Identifying organic compounds is a crucial skill in chemistry, and there are several methods available to do this, depending on the complexity of the compound. The initial step is to analyze the molecular structure to determine the types of bonds and functional groups present. Simple visual inspection of the molecular formula can indicate whether the compound is likely an alkane or an alkene, as noted by the presence of double bonds.

Chemists use a variety of tests to identify organic compounds further. These can include infrared spectroscopy, which identifies functional groups by their unique vibrations, and nuclear magnetic resonance (NMR), which provides detailed information about the carbon and hydrogen framework of a molecule. For simpler tasks or preliminary examinations, chemical tests like the bromine water test are effective because they provide immediate visual cues regarding the type of bond present in the compound.

Chemical Test for Double Bond

The bromine water test is a straightforward and commonly used chemical test that signifies the presence of a carbon-carbon double bond—an identifying feature of alkenes. Bromine water has a characteristic yellow or orange color due to the dissolved bromine. When an alkene is added to bromine water, it undergoes a reaction known as a halogen addition reaction. The double bond is opened up, and bromine atoms add across it, leading to a color change from yellow or orange to colorless.

This reaction does not occur with alkanes as they lack the reactive double bond, and thus the bromine water retains its color. This test is not only simple but highly illustrative, providing clear results that distinguish between alkanes and alkenes. This powerful qualitative analysis is often used in educational settings and professional laboratories to rapidly identify unsaturation in organic compounds.