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.)

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.

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.

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.

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.

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, 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.