12 Chemistry Impq CH00 Organic Chemistry

by : Directorate of Education, Delhi

SOME IMPORTANT REASONING BASED
QUESTIONS OF ORGANIC CHEMISTRY
1.

Chlorobenzene is less reactive than chloromethane.

Ans. In chlorobenzene, each carbon atom of benzenering is sp2 hyridised and is electron withdrawing. Chlorine atom donates a lone pair of electron and acquire positive charge. The negative charge is delocalised on ortho and para position by resonance.

C-Cl bond acquires partial double bond character and is 169 pm as compared to 17.0 pm in chloromethane where C atom is sp3 hybridised.
So larger amount of energy is required to break C-Cl bond in chlorobenzene and therefore is less reactive than haloalkane towards nucleophilic substitution reaction.
2.

Haloalkanes react with KCN to form alkyl cyanide as main product while
AgCN form isocyonide.


Ans. KCN is, predominantly ionic compound and provide cyanide ion : C  N in solution. The attack mainly takes place through lone pair of electron on carbon atom due to high electron density and not through nitrogen atom since C-C bond is more stable than C-N bond. AgCN: is mainly a covalent compound therefore, nitrogen donates electron pair forming isocyanide as the main product.
3.

Grignard Reagent is prepared under anhydrous conditions.

Ans. In Grignard Reagent, the carbon-magnesium covalent bond is highly polar with carbon atom of alkyl or aryl group withdrawing electrons from electropositive magnesium and magnesium halogen bond is also ionic
–





R – Mg – X . So Grignard reagent is highly reactive, and react with any source of proton to give hydrocarbons.
176

Twinklegraphics*chemistry-XII 2013 # Question Organic Chemistry *3rd

[XII – Chemistry]

by : Directorate of Education, Delhi

R – Mg – X + H2O  RH + Mg(OH)X
4.

Wurtz Reaction is carried in dry ether.

Ans. CH3 Br  2Na  Br dry
 other

 CH3CH3  2NaBr
Sodium metal is sensitive towards air and moisture forming :
4Na  O2  2Na2 O
2Na  2H2O  2NaOH  H2

So reaction is carried in dry