question_answer 21)

Explain why (i) the dipole moment of chlorobenzene is lower than that ofcyclohexyl chloride? (ii) alkyl halides, though polar, are immiscible with water? (iii) Grignard reagents should be prepared under anhydrousconditions? (i) Find the hybridization of C-atoms in chlorobenzene and cyclohexylchloride and electronegativity to find the bond polarity' and thus,dipole moment. (ii) Strong intermolecular-H bonding among water molecules, (iii) Consider reactive nature of Grignard reagents with water.  

Answer:

(i)                                                                            (ii)           Due to  hybridization of C-atom in chlorobenzene, C-atom ismore electronegative (greater s-character) whereas in cyclohexylchloride, C-atom is hybridized, i.e., less electronegative (lessers-character). So polarity of  bond in chlorobenzene is lessthan the  bond in cyclohexyl chloride. Therefore, the dipolemoment of chlorobenzene is lower than that of cyclohexylchloride. (ii) Water molecules have enough strong H-bonding (intermolecular)which is difficult to be broken by alkyl halides, though they arealso polar in nature. Therefore, alkyl halides do not dissolve inwater and form separate layers. (iii) Grignard reagents (R?Mg?X) are readily decomposed by waterto produce alkanes. That is why they should be prepared underanhydrous conditions. Instead, ether is used as a solvent duringthe preparation of Grignard reagent.