Question

The organic compound, which gives a red precipitate with ammoniacal cuprous chloride and undergoes an addition reaction is:
(A) ethane
(B) ethene
(C) ethyne
(D) ethanol

Answer 1

Hint: We will start with additional reactions and move on to molecules which undergo addition reactions. This will enable us to exclude two options and we will continue with the remaining options. The answer is basically the simplest alkyne molecule.

Complete step by step solution:
-Let's start with the concept of additional reactions. It’s a chemical reaction in which a larger single product is formed when two or more reactants come together. But only those chemical compounds which contain multiple bonds usually undergo the additional reaction. So, we could exclude ethanol and ethane from the options since they are having a single bond.
-The remaining options are ethene and ethyne. Ethene has double bonds and ethyne has triple bonds in it. Among these compounds, ethyne or acetylene reacts with ammoniacal cuprous chloride. The reaction between acetylene and ammoniacal cuprous chloride is given below.
$HC\equiv HC+C{{u}_{2}}C{{l}_{2}}+2N{{H}_{4}}OH\to Cu-C\equiv C-Cu+2N{{H}_{4}}Cl+2{{H}_{2}}O$
-As we can see, when acetylene or ethyne is allowed to pass through ammoniacal cuprous chloride solution, copper acetylide which is a red precipitate, ammonium chloride and water are formed. The copper acetylide is shown below
$Cu-C\equiv C-Cu$
-Therefore, the compound which reacts with ammoniacal cuprous chloride which produces a red precipitate is ethyne (acetylene).

Thus, the answer is option (C) Ethyne.

Note: It should be noted that alkynes such as acetylene undergo various additional reactions such as hydrogenation, halogenation, hydrohalogenation, etc. Also, generally, an additional reaction is considered as the opposite reaction of decomposition reactions. In addition, a combination of two or more elements takes place whereas in decomposition a compound is broken down into constituent elements.