Question

Consider the following reactions:
‘A’ is:

(A)- $CH\equiv CH$
(B)- $C{{H}_{2}}=C{{H}_{2}}$
(C)- $C{{H}_{3}}-C\equiv CH$
(D)- $C{{H}_{3}}-C\equiv C-C{{H}_{3}}$

Answer 1

. To give a positive test with tollen’s reagent, the compound must have terminal hydrogen which will react with tollen’s reagent forming a precipitate. Secondly, compound A must be a compound which on hydration gives compound B. The compound B on further reduction by sodium tetrahydridoborate or sodium borohydride gives secondary alcohol which will give turbidity within 5 min when reacted with $ZnC{{l}_{2}}+conc.HCl$.

Complete step by step answer:
-Tollen’s reagent is used basically for the presence of a carbonyl group, which can be of an aldehyde or ketone.
-Tollen’s reagent is also used to test for terminal alkynes $(R{{C}_{2}}H)$ . Compound with terminal alkynes confirms with tollen’s reagent by forming a white precipitate of acetylide $(Ag{{C}_{2}}R)$.
-So let us now consider the options given to us and the compound which fulfils all these conditions will be the compound A.
-Ethane in option B has terminal hydrogen but the hydration of alkyne gives alcohols and alcohols cannot be reduced by sodium borohydride.
-Ethyne in option A has terminal hydrogen to react with tollen’s reagent but on hydration, it gives aldehydes which on reduction gives primary alcohol. For turbidity to appear within 5 minutes, the compound must be a secondary carbon which will react with $ZnC{{l}_{2}}/conc.HCl$.
-Propyne in option C have terminal hydrogen to react with tollen’s reagent and on hydration is converted to alcohol, which is compound B. Compound B on reduction with sodium borohydride gives secondary alcohol, which is compound C. This compound C on further reaction with $ZnC{{l}_{2}}/conc.HCl$ gives turbidity.
-Butylene in option D does not have terminal hydrogen.
So, the correct answer is “Option C”.

Note: A solution of zinc chloride in concentrated hydrochloric acid is known as Lucas reagent. This solution is used to distinguish alcohols of low molecular weight. This reaction is a substitution reaction in which chloride ion of zinc chloride replaces a hydroxyl group.