Question

Suppose you were given four unlabeled bottles, each of which is known to contain one of the following compounds: n-pen-tane, 1-pentene, 2-pentyne, and 1-pentyne. Explain how you could use- simple chemical tests (preferably test-tube reactions) to identify the contents of each bottle. (Note that all four compounds are low-boiling liquids.)

Short answer

To identify the contents of each unlabeled bottle containing n-pentane, 1-pentene, 1-pentyne, and 2-pentyne, carry out the following tests: 1. Bromine water test: Add a few drops of bromine water to each bottle in separate test tubes. The compound that doesn't react (brownish-orange) is n-pentane, while the compounds that react (colorless) are 1-pentene, 1-pentyne, and 2-pentyne. 2. Silver nitrate test: Add a few drops of silver nitrate in aqueous ammonia to the three remaining test tubes from step 1. The compound that forms a white precipitate is 1-pentyne, and the one that doesn't is 2-pentyne. 3. Potassium permanganate test: Add a few drops of dilute potassium permanganate solution to the remaining test tube with the unreacted compound from step 1. If a clear reaction occurs (precipitate forming or purple color disappearing), it confirms the presence of 1-pentene.

Step-by-step solution

Identify alkenes and alkynes

To differentiate alkenes (1-pentene) and alkynes (1-pentyne and 2-pentyne) from saturated hydrocarbons (n-pentane), we can use bromine water test. Alkenes and alkynes react with bromine water to form colorless dibromo compounds while the bromine water stays brownish-orange in the presence of saturated hydrocarbons. Add a few drops of bromine water to each of the four unlabeled bottles in separate test tubes. The compound that does not react with bromine water (remaining brownish-orange) will be n-pentane, and the three compounds that react (forming colorless solutions) will be 1-pentene, 1-pentyne, and 2-pentyne.

Identify the alkyne location

Next, we will differentiate between 1-pentyne and 2-pentyne by using the silver nitrate test. Terminal alkynes, like 1-pentyne, will react with silver nitrate in aqueous ammonia solution to form a white precipitate of silver acetylide. Non-terminal alkynes, like 2-pentyne, will not react. Add a few drops of silver nitrate solution in aqueous ammonia to each of the remaining three test tubes containing the reacted compounds from step 1. The compound that forms a white precipitate will be 1-pentyne, whereas the compound that doesn't form the precipitate will be 2-pentyne.

Confirming the identity of 1-pentene

Finally, double-check the identity of 1-pentene using the potassium permanganate test. Alkenes readily react with dilute potassium permanganate solution to form colorless or slightly brown manganese dioxide and a diol, whereas alkynes show little to no reaction under the same conditions. Add a few drops of dilute potassium permanganate solution to the remaining test tube containing the compound from step 1 that didn't react with silver nitrate. A clear reaction (precipitate forming or purple color disappearing) will confirm the presence of 1-pentene. By performing these three tests, it is possible to identify the contents of each unlabeled bottle as n-pentane, 1-pentene, 1-pentyne, or 2-pentyne.

Key concepts

Bromine Water Test

The bromine water test is a simple, yet effective way to identify the presence of alkenes or alkynes in a compound. In this test, bromine water, which is a brownish-orange solution, is added to the compound in question. If the compound is an alkene or alkyne, it will react with the bromine to form a colorless dibromo compound, indicating a double or triple carbon-carbon bond.

In the exercise provided, you'd add bromine water to each of the four unlabeled bottles. You are essentially looking for a color change. The solution that remains brownish-orange contains the saturated hydrocarbon, which in this case is n-pentane. The others, which turn colorless, contain either an alkene or an alkyne. This step effectively groups your compounds into 'reacted with bromine' and 'did not react with bromine', setting you up for further tests to pinpoint exact identities.

Silver Nitrate Test

To differentiate between terminal and non-terminal alkynes, we can use the silver nitrate test. When terminal alkynes (which have a triple bond at the end of the carbon chain) are treated with silver nitrate in ammonia, they form a white precipitate of silver acetylide.

Apply a few drops of the silver nitrate solution to the test tubes containing compounds that reacted with bromine water. The appearance of a white precipitate indicates a terminal alkyne, such as 1-pentyne, whereas no precipitate suggests a non-terminal alkyne, like 2-pentyne. This test zeroes in on the structure of the alkyne by pinpointing the location of the triple bond.

Potassium Permanganate Test

The potassium permanganate test is typically used to confirm the presence of alkenes. Alkenes will chemically react with dilute potassium permanganate solution, resulting in a disappearance of the purple color of the permanganate or the formation of a brown manganese dioxide precipitate. This reaction also forms a diol, indicating that a double bond is present in the compound.

This final test, applied to the sample that did not form a precipitate in the silver nitrate test, will help you confirm that the compound is 1-pentene. The formation of a brown precipitate or the loss of the purple coloration of potassium permanganate will clearly indicate an alkene and, hence, the identity of 1-pentene.

Alkene Identification

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond and are unsaturated. In the exercise, the alkene is 1-pentene. To identify an alkene presence in a mixture, we used the bromine water test and potassium permanganate test. While the bromine water test tells us there is a double or triple bond, the potassium permanganate test is more specific as it indicates a double bond due to the typical diol formation.

The judicious use of these tests in sequence is vital for the correct identification of an alkene. Once you have a compound that reacts with bromine water but doesn't produce a precipitate with silver nitrate, you're left with potassium permanganate to affirm the presence of the alkene.

Alkyne Identification

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond and are also unsaturated. In our scenario, the alkyne identification hinges on the silver nitrate test. After finding out which compounds react with bromine water, indicating unsaturation, we apply the silver nitrate test. A terminal alkyne will form a white precipitated due to the reaction with silver ions, which is a clear indication of a terminal triple bond, such as in 1-pentyne.

Since alkynes are less reactive with potassium permanganate under mild conditions, a lack of a reaction in the last step solidifies our alkyne identification. This test effectively differentiates between 1-pentyne and 2-pentyne based on the position of their triple bonds.

Saturated Hydrocarbons

Saturated hydrocarbons, such as n-pentane in our example, have single bonds only and do not react to tests that identify double or triple bonds. They do not decolorize bromine water, differentiating them from unsaturated hydrocarbons (alkenes and alkynes).

Recognizing saturated hydrocarbons is just as crucial as identifying unsaturated ones. In your exercise, the presence of n-pentane was inferred from its non-reaction to bromine water, which is a hallmark characteristic of saturated hydrocarbons. Understanding their lack of reactivity helps in the process of elimination when identifying unknown compounds.