Question

$[P]\stackrel{B{r}_2}{\to }{C}_2{H}_{4}B{r}_2\underset{N{H}_3}{\overset{NaN{H}_2}{\to }}[Q]$
$[Q]\underset{H{g}^{2+},\Delta }{\overset{20\%H_{2}S{O}_4}{\to }}[R]\stackrel{Zn-Hg/HCl}{\to }[S]$
The species P, Q, R and S respectively are

• A. ethene, ethyne, ethanal, ethane
• B. ethane, ethyne, ethanal, ethene
• C. ethene, ethyne, ethanal, ethanol
• D. ethyne, ethane, ethene, ethanal

Answer 1

Answer: (A)

Given reactions are
$[P]\stackrel{B{r}_2}{\to }{C}_2{H}_{4}B{r}_2\underset{N{H}_3}{\overset{NaN{H}_2}{\to }}[Q]$
$[Q]\underset{H{g}^{2+},\Delta }{\overset{20\%/H_{2}S{O}_4}{\to }}[R]\stackrel{Zn-Hg/HCl}{\to }[S]$
$[P]$ is an alkene with two carbon atoms i.e. ethene. When it reacts with $B{r}_2$, addition reaction occurs $1$ and $C_2{H}_{4}B{r}_2$ is formed.

$C_2{H}_{4}B{r}_2(1,2$- dibromoethane) on reaction with $NaN{H}_2/N{H}_3$ gives ethyne $(C_2{H}_2)$.
Ethyne $[Q]$ in presence of $20\%H_{2}S{O}_4,H{g}^{2+}$ at $333K$
gives ethanal.
$CH=CH+H-OH\underset{333K}{\overset{H{g}^{2+}/H^{+}}{\to }}C{H}_2=\underset{\stackrel{|}{O}H}{C}-H$
$\stackrel{\text{Tautomerisation}}{\to }\underset{\text{Ethanal[R]}}{C{H}_{3}CHO}$
Ethanal $[P]$ undergoes reduction in presence of $Zn-Hg/HCl$ to give ethane $[S]$
$\underset{\stackrel{\text{Ethanal}}{[R]}}{C{H}_{3}CHO}\underset{\text{Clemmensen reduction}}{\overset{Zn-Hg/HCl}{\to }}\underset{\stackrel{\text{Ethane}}{[S]}}{C{H}_3-C{H}_3}$