Question

A reagent that can help us to distinguish between alkenes and alkynes.

• A. Bromine water
• B. Carbon tetrachloride
• C. Alkaline ${KMnO}_4$
• D. Ammoniacal Cuprous chloride

Answer 1

The correct option is D

Ammoniacal Cuprous chloride

The explanation for the correct option:

D: Ammoniacal Cuprous chloride

1. Alkene and Alkyne both are unsaturated hydrocarbons.
2. Alkene contains a double bond between two Carbon atoms whereas in Alkyne two Carbon atoms are bonded together by a triple bond.
3. For distinguishing Alkene and Alkyne we use a reagent called Ammoniacal Cuprous chloride.
4. Ammoniacal Cuprous chloride is formed when we react or dissolve Cuprous chloride in the solution of water and Ammonia.
5. The reaction between Ammoniacal Cuprous chloride with Ethyne is shown below.
   $$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_2\left(\mathrm{g}\right)+2\mathrm{CuCl}\left(\mathrm{l}\right)+2{\mathrm{NH}}_3\left(\mathrm{l}\right)\to {\mathrm{Cu}}_2{\mathrm{C}}_2\left(\mathrm{s}\right)+2{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right) \\ \mathrm{Ethyne}\mathrm{Cuprous}\mathrm{Ammonia}\mathrm{Cuprous}\mathrm{Ammonium} \\ \mathrm{chloride}\mathrm{acetylide}\mathrm{chloride} \end{gathered}$$
6. Ammoniacal Cuprous chloride on reacting with Ethyne gives Cuprous or Copper acetylide and Ammonium chloride.
7. Alkene does not show any reaction with Ammoniacal cuprous chloride.
8. Therefore, option D is correct. Ammoniacal cuprous chloride is used as a distinguishing reagent between alkene and alkyne.