Question

Propyne and propene can be distinguished by: (a) Conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{Br}_{2}\) in \(\mathrm{CCl}_{4}\) (c) Diluted \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{AgNO}_{3}\) in ammonia

Short answer

Propyne and propene can be distinguished by using AgNO3 in ammonia (Option d).

Step-by-step solution

Understanding the Compounds

Propyne is an alkyne with a triple bond and propene is an alkene with a double bond. These compounds have different reactivities due to their different types of carbon-carbon bonds. Identifying the correct distinguishing reagent involves understanding how these react with different chemicals.

Analyzing Option (a) Conc. H2SO4

Both propyne and propene can react with concentrated H2SO4 , but the reactivity patterns are not distinct enough to easily differentiate them using this test. Generally, alkenes and alkynes can undergo addition reactions with H2SO4 .

Analyzing Option (b) Br2 in CCl4

Br2 in CCl4 can react with alkenes like propene to form dibromo compounds, resulting in a decolorization reaction. Alkynes can also react, but this test is not selective enough as both compounds will react.

Analyzing Option (c) Diluted H2SO4

Diluted H2SO4 will not significantly react with either propyne or propene under normal conditions. This option is not suitable for differentiation.

Analyzing Option (d) AgNO3 in Ammonia

AgNO3 in ammonia reacts with terminal alkynes like propyne to form a precipitate, known as a silver acetylide complex. Propene will not react with AgNO3 in this way, as it does not have the requisite triple bond. This makes AgNO3 in ammonia a specific and effective reagent for distinguishing propyne from propene.

Key concepts

Propene and Propyne Reactivity

When distinguishing between organic compounds, understanding the reactivity of propene and propyne is vital. Propene is an alkene, which means it contains a double bond between two carbon atoms. This double bond is where reactions typically occur, making alkenes capable of undergoing addition reactions. These reactions involve the double bond opening up and creating a new single-bonded compound. In simple terms, the double bond allows for various molecules to 'add onto' the alkene. Propyne, on the other hand, is an alkyne. It features a triple bond, that provides similar but more pronounced reactivity compared to double bonds. Because of greater energy and overlap of the orbital structures involved, alkynes can partake in not only addition reactions but more uniquely, they can react to form stable complexes under specific conditions, thanks to this triple bond. The reactivity of each compound is mainly defined by the presence of these multiple bonds, which are more active and reactive, making both alkenes and alkynes intriguing candidates for various chemical identification processes.

Chemical Reagents for Alkene and Alkyne Identification

Chemical reagents play a crucial role in identifying alkenes like propene and alkynes like propyne, due to their distinct reactivity. When considering various reagents, understanding their interaction with organic compounds is essential. - Concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) applies to both alkenes and alkynes as both can undergo addition reactions. This, however, does not distinctly differentiate them.- Bromine in \(\mathrm{CCl}_{4}\) is usually used because alkenes decolorize bromine by forming dibromo compounds. This reaction is also true for alkynes, but since both will react, it lacks specificity.- Diluted sulfuric acid doesn’t react significantly with propyne or propene, making it an unsuitable choice for distinguishing between the two. By specifically choosing reagents suited to their unique functional groups, particularly terminal groups in alkynes, chemists can effectively distinguish between these compounds.

AgNO3 Test for Terminal Alkynes

The \(\mathrm{AgNO}_{3}\) test in ammonia is a pivotal tool in distinguishing terminal alkynes such as propyne from alkenes like propene. It introduces a more precise method for identification.Terminal alkynes have an end carbon in the triple bond, known as a terminal carbon. This position is reactive with \(\mathrm{AgNO}_{3}\), as it forms a silver acetylide precipitate when it reacts with the ammoniacal silver nitrate solution. This distinctive precipitate formation—often tellingly gray or white—serves as a reliable indicator of terminal alkynes.Propene, by comparison, lacks this elusive terminal triple bond and thus does not result in any significant reaction with \(\mathrm{AgNO}_{3}\) in ammonia. Therefore, this test effectively differentiates propyne from propene, being specific to the unique reactive nature of terminal alkynes.