Hydrocarbons 2 Question 21
21. Identify a reagent from the following list which can easily distinguish between 1-butyne and 2-butyne.
(2002, 3M)
(a) bromine, $\mathrm{CCl}_{4}$
(c) dilute $\mathrm{H}{2} \mathrm{SO}{4}$, $\mathrm{HgSO}_{4}$
(b) $\mathrm{H}_{2}$, Lindlar catalyst
(d) ammoniacal $\mathrm{CuCl}_{2}$ solution
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Answer:
Correct Answer: 21. (b)
Solution:
- Ammoniacal $\mathrm{CuCl}_{2}$ forms red precipitate with terminal alkynes, can be used to distinguish terminal alkynes from internal alkynes:
$$ \begin{aligned} \mathrm{CH}{3}-\mathrm{CH}{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+\mathrm{CuCl}{2} \xrightarrow{\mathrm{NH}{3}(a q)} \ \mathrm{CH}{3}-\underset{\text { red ppt. }}{\mathrm{CH}{2}-\mathrm{C} \equiv \mathrm{C}^{-} \mathrm{Cu}^{+} \downarrow} \end{aligned} $$
