Chemistry MCQs for NEET — Practice Questions with Answers

Zn dust is used for dehalogenation,

${\mathrm{CH}}_2\mathrm{X}·{\mathrm{CH}}_2\mathrm{X}\underset{\mathrm{alc}.}{\overset{\mathrm{Zn} \mathrm{dust}}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2$

To prepare 3-chloroaniline from benzene, the best sequence is: nitration (to introduce -NO2 group), chlorination (to introduce -Cl group at 3-position), and finally reduction (to convert -NO2 to -NH2). Nitration is carried out first as the nitro group is a strong deactivating group.

(a) A white precipitate with am. AgNO₃ confirms the presence of terminal alkyne.

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.

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

(d) Na/C₂H₅OH, LiAlH₄ or NaBH₄ are used for this purpose.

(b) CH₃CH=CH₂ $\stackrel{{\mathrm{B}}_2{\mathrm{H}}_6}{\to }$(CH₃CH₂CH₂)₃B 

                        $\stackrel{{\mathrm{H}}_2{\mathrm{O}}_2}{\to }$CH₃CH₂CH₂OH + H₃BO₃;

The process is called hydroboration.

(b) A characteristic reaction of aldehyde with Tollen's reagent. Rest all also attacks C=C.

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.