Question

Ethene and ethyne can be distinguished by: (a) \(\mathrm{Br}_{2}\) water (b) \(\mathrm{KMnO}_{4}\) solution (c) Cuprous chloride solution (d) Any of the above

Short answer

(c) Cuprous chloride solution

Step-by-step solution

Understanding the Compounds

Ethene (\( ext{C}_2 ext{H}_4\)) and ethyne (\( ext{C}_2 ext{H}_2\)) are both hydrocarbons. Ethene is an alkene, which means it contains a carbon-carbon double bond, while ethyne is an alkyne, containing a carbon-carbon triple bond.

Evaluating Option (a): Bromine Water

Bromine water is a test for unsaturation (double or triple bonds). Both ethene and ethyne will decolorize bromine water because both have unsaturated carbon-carbon bonds that react with bromine. Therefore, this test cannot distinguish between them.

Evaluating Option (b): Potassium Permanganate Solution

Potassium permanganate (\( ext{KMnO}_4\)) is another test for unsaturation. Ethene can decolorize \( ext{KMnO}_4\), resulting in a brown precipitate of manganese dioxide, signifying the presence of a double bond. Ethyne also reacts with \( ext{KMnO}_4\), but the reaction produces different products, so this test alone may not clearly distinguish ethene from ethyne without further analysis.

Evaluating Option (c): Cuprous Chloride Solution

Cuprous chloride (\( ext{CuCl}\)) in ammonia forms a complex with alkynes. Ethyne will react to form a red precipitate of the acetylide complex, while ethene will not form such a complex. Hence, this test can distinguish ethyne from ethene.

Selecting the Correct Answer

Since \( ext{CuCl}\) solution allows us to specifically identify ethyne and differentiate it from ethene, the correct option is (c).

Key concepts

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are fascinating chemical compounds characterized by the presence of double or triple carbon-carbon bonds. This category includes both alkenes and alkynes, which significantly react due to their multiple bonds.
In particular, alkenes have at least one double bond, while alkynes are defined by at least one triple bond. These bonds make unsaturated hydrocarbons more reactive compared to their saturated counterparts, alkanes, which only contain single bonds.
When discussing unsaturated hydrocarbons, it's important to highlight that their unique chemical structures allow them to participate in a variety of reactions, such as addition reactions. It is during these reactions that the unsaturation (extra bonds) is key, enabling the molecules to form new bonds with other reactants and undergo changes.

Chemical Tests for Alkenes and Alkynes

To identify unsaturated hydrocarbons such as alkenes and alkynes, chemists employ specific chemical tests based on their reactivity. Here are some common tests:

• Bromine Water Test: Both alkenes and alkynes turn bromine water from its original reddish-brown color to colorless by adding to the carbon-carbon double or triple bond, signaling the presence of unsaturation.
• Potassium Permanganate Test: This test involves the use of a purple potassium permanganate solution, which turns colorless or produces a brown precipitate, manganese dioxide. Although both alkenes and alkynes will react, the reaction paths and products differ.
• Cuprous Chloride Test: A more specific test for alkynes is the cuprous chloride test. When in the presence of ammonia, alkynes form a red precipitate with cuprous chloride, creating a distinct acetylide complex not formed by alkenes.

Each test leverages the reactivity of the unsaturated bonds in differing ways, enabling us to both detect and differentiate between these two types of hydrocarbons.

Distinguishing Alkenes and Alkynes

Though both alkenes and alkynes are unsaturated hydrocarbons, there are clear methods to distinguish between them using their chemical properties. The cuprous chloride solution test exemplifies this distinction.
Alkynes, which contain triple bonds, will react with this solution when ammonia is present to form a distinct red precipitate due to the formation of an acetylide complex. This is specific to alkynes and does not happen with alkenes.
On the other hand, simpler tests like bromine water and potassium permanganate, while signaling unsaturation, cannot effectively differentiate between the two due to their similar reactions. Therefore, the use of cuprous chloride is a precise method to identify the presence of an alkyne.

Chemical Properties of Hydrocarbons

Hydrocarbons, which include a variety of molecules ranging from alkanes, alkenes, to alkynes, each exhibit unique chemical properties based on their structures.

• Alkanes: Known for their lack of reactivity due to single bonds, alkanes exhibit relatively lower reactivity compared to other types of hydrocarbons.
• Alkenes and Alkynes: These unsaturated hydrocarbons are more reactive because of their double or triple bonds. These bonds facilitate a range of chemical reactions, particularly addition reactions, allowing them to combine with many different types of molecules.
• Solubility and Boiling Points: The structure of hydrocarbons, including branching and the type of bonding, affects their solubility and boiling points. For example, alkanes generally have lower boiling points than alkenes and alkynes of similar carbon content due to the lack of additional bonding interactions.

By understanding these chemical properties, we can predict and manipulate reactions to achieve desired chemical processes and products, harnessing the versatility of hydrocarbons.