Question

What tests could you perform to distinguish between the following pairs of compounds? a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}, \quad \mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}, \quad \mathrm{CH}_{3} \mathrm{OCH}_{3}\)

Short answer

To distinguish between the compounds in pair a, perform the Bromine water test. The compound that reacts and turns the bromine water colorless is an alkene, while the one that shows no color change is an alkane. For pair d, perform Hinsberg's test using Hinsberg's reagent. A solid insoluble precipitate indicates a primary amine, while a clear solution indicates an ether.

Step-by-step solution

Identify the functional groups

Here, the first compound is an alkane (butane) and the second compound is an alkene (1-butene).

Choose an appropriate test to differentiate between compounds

A suitable test for distinguishing between alkanes and alkenes is the Bromine water test. When bromine water is added to an alkene, the bromine reacts with the carbon-carbon double bond, resulting in a color change, while alkanes don't react to result in a color change.

Perform the test

Add a few drops of bromine water to each compound separately.

Analyze the results

If the bromine water loses its brownish color and becomes colorless after the addition of the compound, the compound is an alkene. The compound that shows no color change is an alkane. Pair d: $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}, \quad \mathrm{CH}_{3} \mathrm{OCH}_{3}$

Identify the functional groups

The first compound is a primary amine (ethylamine), and the second compound is an ether (dimethyl ether).

Choose an appropriate test to differentiate between compounds

A suitable test for distinguishing between amines and ethers is the Hinsberg's test. In this test, we react the compound with Hinsberg's reagent, which is a mixture of benzenesulfonyl chloride and sodium hydroxide.

Perform the test

Add Hinsberg's reagent to each compound separately. Shake the test tubes well and observe the results.

Analyze the results

If a solid insoluble precipitate is formed, then the compound is a primary amine. If no precipitate is formed, and the mixture remains as a clear solution, then the compound is an ether.

Key concepts

Alkanes and Alkenes

Alkanes and alkenes are two fundamental types of hydrocarbons, which are organic compounds made solely of carbon and hydrogen atoms. Alkanes are known as saturated hydrocarbons because they have single bonds, making them less reactive due to the stability of their carbon-carbon single bonds. On the other hand, alkenes contain at least one carbon-carbon double bond and are termed unsaturated hydrocarbons. This double bond makes them more reactive than alkanes, as it can easily participate in chemical reactions such as addition reactions. In summary, the key difference between alkanes and alkenes is the presence of a carbon-carbon double bond in alkenes, which contributes to their reactivity. Understanding these differences is crucial when performing chemical tests, as it allows chemists to choose appropriate reagents that will react differently depending on the saturation of the bonds.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. These groups have particular properties that largely determine the behavior of the molecule they belong to. Common examples of functional groups include:

• Hydroxyl group (-OH), found in alcohols
• Carbonyl group (C=O), found in ketones and aldehydes
• Amino group (-NH₂), found in amines
• Ether linkage (C-O-C), found in ethers

Recognizing functional groups is essential for chemists to predict how a molecule will react under certain conditions. For instance, the presence of a double bond in alkenes is a functional group that makes them target specific reactions like the bromine water test.

Bromine Water Test

The bromine water test is an essential chemical test for differentiating between alkanes and alkenes. This test takes advantage of the fact that alkenes contain a carbon-carbon double bond, which readily reacts with bromine, a halogen. When bromine water, which is a brownish-red solution, is added to an alkene, the double bonds react with bromine, causing the solution to lose its color and become clear. This indicates the presence of an alkene — thus signaling a positive test result. In contrast, alkanes do not react with bromine under normal conditions. Therefore, the bromine water remains its original color when added to an alkane, resulting in a negative test. This straightforward test helps to quickly and effectively distinguish between saturated and unsaturated hydrocarbons, showcasing the reactivity that stems from the presence of a carbon-carbon double bond.

Hinsberg Test

The Hinsberg test is a classic chemical test used to identify primary amines in the presence of other functional groups, such as ethers. The test uses Hinsberg's reagent, which consists of benzenesulfonyl chloride and sodium hydroxide. Primary amines react with benzenesulfonyl chloride to form a sulfonamide, which usually precipitates out of the solution as a solid. This indicates a positive test result for primary amines, as the formation of the solid demonstrates the presence of the amino group. On the contrary, ethers do not react with Hinsberg's reagent. As a result, no precipitate forms, and the solution remains clear. This absence of any visible change indicates a negative result for primary amines, confirming the presence of an ether instead. By distinguishing these functional groups, the Hinsberg test allows chemists to accurately classify and differentiate compounds that may otherwise appear similar.