Hydrocarbons MCQs for NEET — Chemistry Questions with Answers

O – P directors except halogen activates benzene ring while m-director deactivates.

The correct matching is: A (s) Benzene undergoes electrophilic substitution reactions. B (r) Ethene can be obtained by the dehydration of ethanol. C (q) Ethyne evolves hydrogen when heated with sodium metal. D (p) Butane can be prepared by the Wurtz reaction of ethyl chloride.

The reagent $\text{cold, dilute, alkaline KMnO}_4$ (potassium permanganate) is used for the detection of unsaturation in alkenes. This reagent oxidizes the double bond in alkenes, resulting in the formation of diols (glycols). The reaction is accompanied by a color change from purple to brown due to the reduction of KMnO$_4$ to MnO$_2$, which helps in identifying the presence of unsaturation.

(c) stability order of carbon family hydride silane$<$German$>$stannane$>$Plumbane

(a) In the nitration of benzene in the presence of conc. H₂SO₄ and HNO₃, nitrobenzene is formed.

HNO₃ + H₂SO₄ => NO₂⁺+ HSO₄ ^-+ H₂O

                                  Electrophilc  Nucleophile

If large amount of KHSO₄ is added to this mixture, more HSO₄ ion furnishes and hence the concentration of NO₂, i.e. electrophile decreases.

As concentration of electrophile decreases, rate of electrophilic aromatic reaction slows down.

 Friedel-Craft's alkylation : When benzene reacts with alkyl halide in presence of anhy. AlCl₃, toluene is obtained. It is called Friedel-Craft's alkylation. Friedel-Craft's reaction is type of electrophilic substitution.

  C₆H₆ + CH₃Cl $\stackrel{\mathrm{Anhy}.{\mathrm{AlCl}}_3}{\to }$C₆H₅CH₃ + HCl

                                      Toluene

$3{\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2\stackrel{{\mathrm{B}}_2{\mathrm{H}}_6}{\to }{\left({\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2\right)}_3\mathrm{B}\stackrel{{\mathrm{H}}_2{\mathrm{O}}_2/{\mathrm{OH}}^{-}}{\to }\underset{\mathrm{Propan}-1-\mathrm{ol}}{{\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{OH}}$

Here, Half mole of (B₂H₆) diborane reacts with propane by Markownikoffs addition it gives tripropyl borane called hydroboration. In presence of H₂O₂ in basic medium tripropyl borane gives alcohol. Remember that product is Anti-Markownikoff's rule that is 1-propanol. Reaction is called hydroboration oxidation.