Question

Which of the following will give red ppt on reacting with ammoniacal cuprous chloride solution?
A: butyne
B: 2-butene
C: 2-butyne
D: butane

Answer 1

Hint: Terminal alkynes react with ammonium cuprous chloride to give red ppt. of a copper salt, which undergoes an additional reaction also.

Complete step by step answer:
In this reaction, an terminal alkyne reacts with ammoniacal cuprous chloride which on subsequent oxidation in air gives diyne. For example, in the presence of ammoniacal cuprous chloride, acetylene couples with itself to give but-1,3-yne (red brown ppt).
This can be understood by the following general equation
$ - R - C \equiv C - H + CuC{l_2} + N{H_3} \to R - C \equiv C - Cu + N{H_4}Cl$
Terminal alkynes (including acetylene) form a red precipitate of copper acetylides when it is passed through ammoniacal cuprous chloride solution.
2-butyne ${H_3}C−C≡C−C{H_3}$ does not precipitate with ammoniacal cuprous chloride. This reaction is given by terminal alkynes having acidic H atoms.
Hence , according to the above information the correct option is A i.e. butyne.

So, the correct answer is Option A .

Additional Information:
Ammoniacal cuprous chloride solution is a solution of cuprous chloride in ammonia. It is prepared by adding cuprous chloride in water and ammonia till it dissolves and forms a blue coloured solution. ... Ammoniacal solutions of CuCl absorb carbon monoxide to form complexes.
In the presence of oxygen, this reaction proceeds further to dimerize the corresponding alkyne. This reaction, the oldest homo-coupling of terminal alkyne reaction.
$2R - C \equiv C - Cu\xrightarrow[{N{H_3}/EtOH}]{{{O_2}}}R - C \equiv C - C \equiv C - R$
Let us consider the reaction of ammoniacal cuprous chloride with acetylene we will get
$HC \equiv CH + CuC{l_2} + 2N{H_4}OH \to Cu - C \equiv C - Cu + 2N{H_4}OH + 2{H_2}O$

Note:
Terminal alkynes are those alkynes in which alkyne triple bonds are present on the terminals of the compound. Terminal alkynes are generally non-polar in nature and show low melting and boiling point. They do not exhibit hydrogen bonding and are generally insoluble in water.