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?
Answers
Explanation:
(i) In cyclohexyl chloride, the C atom of the C-Cl bond is sp3 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. ... Hence, chlorobenzene has lower dipole moment than cyclohexyl chloride.
(ii) Alkyl halides are polar and immiscible with water. ... Alkyl halides though polar are immissible with H2O because energy is required to overcome the attractions between the haloalkane molecule and to break the hydrogen bonds between the water molecules. Thus, alkyl halides though polar are immisicible with water.
(iii)Grignard reagent should be prepared under anhydrous conditions, because it is very reactive. It reacts very quickly with any source of proton to give hydrocarbon. It reacts with water very quickly. Therefore, it is necessary to avoid moisture from the Grignard reagents.
Explanation:
In chlorobenzene, the Cl-atom is linked to a sp2 hybridized carbon atom. In cyclohexyl chloride, the Cl-atom is linked to a sp3 hybridized carbon atom. Now, sp2 hybridized carbon has more s-character than sp3 hybridized carbon atom. Therefore, the former is more electronegative than the latter. Therefore, the density of electrons of C - Cl bond near the Cl-atom is less in chlorobenzene than in cydohexyl chloride.
Moreover, the - R effect of the benzene ring of chlorobenzene decreases the electron density of the C - Cl bond near the Cl-atom. As a result, the polarity of the C - Cl bond in chlorobenzene decreases. Hence, the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(ii) To be miscible with water, the solute-water force of attraction must be stronger than the solute-solute and water-water forces of attraction. Alkyl halides are polar molecules and so held together by dipole-dipole interactions. Similarly, strong H-bonds exist between the water molecules. The new force of attraction between the alkyl halides and water molecules is weaker than the alkyl halide-alkyl halide and water-water forces of attraction. Hence, alkyl halides (though polar) are immiscible with water.
(iii) Grignard reagents are very reactive. In the presence of moisture, they react to give alkanes.
Therefore, Grignard reagents should be prepared under anhydrous conditions.