Suggest two chemical tests that would help you distinguish between these two compounds: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\)

One of the simplest ways to differentiate between an alkene and an alkane is through the bromine water test. Bromine water, which has a characteristic brown color due to the presence of bromine, reacts with alkenes by a process called electrophilic addition. In the presence of an alkene, the bromine molecules add across the carbon-carbon double bond, resulting in a colorless dibromo compound. This loss of color is a definitive indicator that the compound being tested is an alkene.

When the test is conducted, an alkane, lacking the double bond necessary for the reaction, will not cause any color change in the bromine water. It’s a quick, visual cue: brown stays brown for alkanes, and turns colorless for alkenes. Students can remember this reaction by the phrase, 'Bromine loses its brown when alkenes are around.' This test is particularly useful because it is both qualitative and immediate, making it a staple in organic chemistry laboratories.

The combustion test is a practical approach to discern the type of hydrocarbon by observing the flame characteristics. Alkanes generally have a high hydrogen-to-carbon ratio, which allows them to burn more completely in the presence of adequate oxygen. The result is a blue, non-sooty flame indicative of complete combustion, where the hydrocarbon transforms into carbon dioxide and water.

On the contrary, alkenes, with their double bonds, tend to have lower hydrogen content in comparison to alkanes. They undergo incomplete combustion more readily, especially when there is limited oxygen, resulting in a yellowish, smoky flame due to the formation of small particles of carbon, commonly known as soot. Incomplete combustion also leads to products like carbon monoxide and water. For students, the key takeaway is that blue flame signifies alkanes, and a yellow, smoky flame points towards alkenes.

When comparing alkenes to alkanes, several key differences are notable. Firstly, alkenes contain at least one carbon-carbon double bond (C=C), while alkanes are saturated hydrocarbons with single bonds (C-C) only. This double bond gives alkenes their unique chemical reactivity, allowing them to participate in addition reactions where substances, like bromine water, can add across the double bond.

Another point of distinction lies in their physical properties. Alkenes have higher boiling points compared to alkanes with a similar number of carbon atoms due to the increased electron density around the double bond that induces stronger intermolecular forces. However, alkanes usually have a higher melting point than alkenes.

Reactivity and Stability

• Alkenes are generally more reactive than alkanes due to the presence of the double bond, which is electron-rich and thus more susceptible to attack by electrophiles.
• Alkanes are relatively inert and require more energy to react because the C-C and C-H bonds in alkanes are more difficult to break than the pi bond in the double bond of alkenes.

Solubility and Density

Both alkenes and alkanes are non-polar and hydrophobic, making them insoluble in water but soluble in organic solvents. In terms of density, they are both less dense than water, allowing them to float.

Understanding these properties helps students predict the behavior and reactions of alkanes and alkenes, which is crucial in organic chemistry and industrial applications where these hydrocarbons are extensively used.