Question

Explain why

(i) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride?
(ii) Alkyl halides, though polar, are immiscible with water?
(iii) Grignard reagents should be prepared under anhydrous conditions?

Answer 1

(i) In chlorobenzene, the C atom of the C-Cl bond is $s{p}^2$ hybridized and has more s character and electronegativity. In cyclohexyl chloride, the C atom of the C-Cl bond is $s{p}^3$ hybridized, has less s character and less electronegative. Hence, the C-Cl bond in chlorobenzene is less polar than the C-Cl bond in cyclohexyl chloride.

Also, in chlorobenzene, the lone pair of electron on the Cl atom is in resonance with a benzene ring. This gives partial double bond character to C-Cl bond and there is a decrease in the magnitude of partial negative charge on Cl atom. However, in cyclohexyl chloride, there is a C-Cl single bond. The dipole moment is a product of charge and distance. Also C-Cl bond with partial double bond character has lower bond length than with C-Cl single bond. Hence, chlorobenzene has lower dipole moment than cyclohexyl chloride.
(ii) Inter-molecular attractive forces present in alkyl halides are dipole dipole interactions and in case of water molecules, intermolecular hydrogen bonds are present. When alkyl halides are added to water, the intermolecular attractive force present between an alkyl halide and water molecules are weaker than the forces that are already present between two alkyl halide molecules and two water molecules. Hence, alkyl halides are water-immiscible.
(iii) Grignard reagents are highly reactive with moisture that is present in apparatus or starting materials. Hence, we should prepare Grignard reagents in the anhydrous condition.

$RMgX+HOH\to RH+MgOHX$