Hydrocarbons MCQs for NEET — Chemistry Questions with Answers

Lindlar's catalyst (Pd-CaCO3 poisoned with Pb(OAc)2 and quinoline) is used for the semi-hydrogenation of alkynes to cis-alkenes. It adds hydrogen atoms in a cis fashion across the triple bond, selectively producing the cis-alkene product.

$$\begin{gathered} \left(\mathrm{c}\right)\mathrm{CH}\equiv \mathrm{CH}+\mathrm{Na}\to \mathrm{CH}\equiv \mathrm{CNa}\stackrel{\mathrm{RX}}{\to }\mathrm{CH}\equiv \mathrm{CR} \\ \mathrm{CH}\equiv \mathrm{CH}+\mathrm{RMgX}\to \mathrm{CH}\equiv \mathrm{CMgX}\stackrel{\mathrm{RX}}{\to }\mathrm{CH}\equiv \mathrm{CR} \end{gathered}$$

Propene (CH3CH=CH2) reacts with diazomethane (CH2N2) in the presence of UV radiation via a carbene insertion reaction. The methylene carbene (:CH2) generated from diazomethane inserts into the C=C bond of propene, forming methyl cyclopropane.

$\left(\mathrm{d}\right)\mathrm{CH}\equiv \mathrm{CH}\underset{0°\mathrm{C}}{\overset{\mathrm{NaOCl}}{\to }}\mathrm{CCl}\equiv \mathrm{CCl}$

$\left(\mathrm{d}\right) {\mathrm{CH}}_4+{\mathrm{HNO}}_3\stackrel{∆}{\to }{\mathrm{CH}}_3{\mathrm{NO}}_2; \mathrm{nitration} \mathrm{reaction}$

$\left(\mathrm{b}\right) {\mathrm{CH}}_2{\mathrm{BrCH}}_2\mathrm{Br}\underset{\mathrm{dust}}{\overset{\mathrm{Zn}}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2 + {\mathrm{ZnBr}}_2$

According to the NCERT text, 'However, it may be remembered that rotation around a C-C single bond is not completely free. It is hindered by a small energy barrier of 1-20 kJ mol$^{-1}$ due to weak repulsive interaction between the adjacent bonds.' Therefore, the statement that rotation is completely free is incorrect.