Question

All reagents, ${{\left[ Cu{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, ${{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, $C{{H}_{3}}MgBr$ and $NaN{{H}_{2}}$ react with:
This question has multiple correct options
(A)- cyclooctyne
(B)- pent-1-yne
(C)- pent-2-yne
(D)- ethyne

Answer 1

Hint: Terminal alkynes have carbon-carbon triple bonds ($C\equiv C$) at the end of the carbon chain. They are slightly acidic in character because of the hydrogen attached to triple bonded (hence slightly electronegative) carbon.

Complete answer:
Terminal alkynes ($R-C\equiv C-H$) react with all the given reagents, i.e. ${{\left[ Cu{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, ${{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, $C{{H}_{3}}MgBr$ and $NaN{{H}_{2}}$.
The acidic hydrogen in $R-C\equiv C-H$ can be replaced with metal ions such as $C{{u}^{+}}$, $A{{g}^{+}}$, $N{{a}^{+}}$.
Reaction with Tollen’s reagent
Tollen reagent is an ammoniacal solution of silver nitrate. $R-C\equiv C-H$ reacts with ${{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$ to form white precipitate of silver alkynide.
     \[R-C\equiv C-H+{{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}\to R-C\equiv C-Ag+2N{{H}_{3}}\]
Reaction with Fehling’s solution
$R-C\equiv C-H$ reacts with ammoniacal cuprous ions ($C{{u}^{+}}$) solution to form red precipitates of copper alkynide.
\[R-C\equiv C-H+{{\left[ Cu{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}\to R-C\equiv C-Cu+2N{{H}_{3}}\]
Reaction with Grignard reagent
\[R-C\equiv {{C}^{\delta -}}{{H}^{\delta +}}+{{R}^{\delta -}}MgB{{r}^{\delta +}}\to R-C\equiv CMgBr+R-H\]
Reaction with base sodamide $NaN{{H}_{2}}$
\[R-C\equiv C-H+NaN{{H}_{2}}\xrightarrow{Liq.N{{H}_{3}}}R-C\equiv {{C}^{-}}N{{a}^{+}}+N{{H}_{3}}\]
Now let us look for terminal alkynes in the options given.
Cyclooctyne is a cyclic compound. Therefore it is not a terminal alkyne. It doesn’t react with all the reagents mentioned above.
Pent-1-yne is a terminal alkyne and shows reactions to all the four reagents.
Pent-2-yne has a triple bond at the second position and thus, is not a terminal alkyne.
Ethyne is the simplest alkyne. It reacts with all the reagents given above.
Pent-1-yne ($C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C\equiv CH$) and ethyne ($HC\equiv CH$) are the compounds that react with ${{\left[ Cu{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, ${{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}^{\oplus }}$, $C{{H}_{3}}MgBr$ and $NaN{{H}_{2}}$.

Hence, option (B) and (D) are correct.

Note: Due to the electronegativity of the sp-hybridized carbon, acidic hydrogen in terminal alkyne can be readily displaced. However, in non terminal alkynes acidic hydrogen is not available and hence they do not show reactions with the above mentioned reagents.