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?

Explanation

(a) In chlorobenzene, the C of C-Cl bond is `sp^(2)`-hybridised while the C of C-Cl bond in cyclohexyl chloride is `sp^(3)`-hybridised.

Therefore, the `sp^(2)`-hybridised C of chlorobenzene has more s-character and hence more electronegative than the `sp^(3)` -hybrid C of cyclohexyl chloride. As a result, the sp hybrid of C-Cl bond in chlorobenzene has less tendency to release electrons to Cl than the `sp^(3)`-hybrid carbon of cyclohexyl chloride. As a result, the C-Cl bond in chlorobenzene is less polar than in cyclohexyl chloride. Thus, chlorobenzene is less polar than cyclohexyl chloride.
In other words, the magnitude of negative charge (`delta-`) is less on Cl atom of chlorobenzene than in cyclohexyl chloride. Further, due to delocalisation of lone pair of electrons of the Cl atom over the benzene ring due to resonance, C-CL bond in chlorobenzene acquires some double bond character. On the other hand, C-Cl bond in cyelohexyl chloride is a pure single bond.

Since dipole moment is a product of charge and distance, therefore, chlorobenzert has lower dipole moment than cyclohexyl chloride due to lower magnitude of charge (`delta-`) on Cl atom and small C-C1 distance.
(b) Alkyl halides are polar molecules and therefore, their molecules are held together by dipole-dipole forces. On the other hand, the molecules of `H_(2)O` are held together by hydrogen bonds. When alkyl halides are added to water, the new forces of attraction between water and alkyl halide molecules are weaker than the forces of attraction already existing between alkyl halide-alkyl halide molecules and water- water molecules. Hence, alkyl halides are in miscible (not soluble) in water.
(c) Grignard reagents are very reactive. They react with the moisture present in the apparatus or the starting materials (RX or Mg).
`RMgX + HOH to R-H + Mg(OH)X`
Therefore, Grignard reagents must be prepared in anhydrous conditions.