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Explain why (i) the dipole moment of c...

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?

Text Solution

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(i) `sp^(2)`-hybrid carbon in chlorobezene is more electronegative than a `sp^(3)`-hybrid carbon in cyclohexylchoride , due to greater s-character. Thus, C atom of chlorobenzene has less tendency to release electrons to Cl than carbon atom of cyclohexylchloride.
As a result, C - Cl bond in chlorobenzene is less polar than in cyclohexylchloride. Further, due to delocalization of lone pairs of electrons of the Cl atom over the benzene ring, C-Cl bond in chlorobenzene acquires some double bond character while the C - Cl in cyclohexy! chloride is a pure single bond. ln other words, C-Cl bond in chlorobenzene is shorter than in cyclohexyl chloride.
Since dipole moment is a product of charge and distance, therefore, chlorobenzene has lowar dipole moment than cyclohexylchloride due to lower magnitude of negative charge on the Cl atom and shorter C-Cl distance.
(ii) Alkyl halides are polar molecules, therefore, their molecules are held together by dipole-dipole attraction. The molecules of `H_(2)O` are hold together by H-bonds . Since 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, therefore, alkyl halides are immiscible (not soluble) in water. Alkyl halide are neither able to form H-bonds with water nor are able to break the H-bounding network of water.
(iii) Grignard reagents are very reactive. They react with moisture present in the apparatus to form alkanes
`{:(R-Mg-X+H-OHrarr),(" "R-H+Mg(OH)X):}`
Thus, Grignard reagents must be prepared under anhydrous conditions.
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