Chemistry Class 12 NCERT Solutions: Chapter 10 Haloalkanes and Haloarenes Part 8

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Q: 12. 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:

(i)

Q 12 i Chlorobenzene and Cyclohexyl Chloride

Q 12 I Chlorobenzene and Cyclohexyl Chloride

In chlorobenzene, the is linked to a hybridized carbon atom. In cyclohexyl chloride, the is linked to a hybridized carbon atom. Now, hybridized carbon has more s-character than hybridized carbon atom. Therefore, the former is more electronegative than the latter. Therefore, the density of electrons of bond near the is less in chlorobenzene than in cyclohexyl chloride.

Moreover, the effect of the benzene ring of chlorobenzene decreases the electron density of the bond near the . 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.

Q: 13. Give the uses of Freon , carbon tetrachloride and iodoform.

Answer:

Uses of Freon -12

Freon-12 (dichlorodifluoromethane, ) is commonly known as . It is used as a refrigerant in refrigerators and air conditioners. It is also used in aerosol spray propellants such as body sprays, hair sprays, etc. However, it damages the ozone layer.

Hence, its manufacture was banned in the United States and many other countries in 1994.

Uses of DDT

DDT (p, p-dichlorodiphenyltrichloroethane) is one of the best known insecticides. It is very effective against mosquitoes and lice. But due its harmful effects, it was banned in the United States in 1973.

Uses of carbontetrachloride

(i) It is used for manufacturing refrigerants and propellants for aerosol cans.

(ii) It is used as feedstock in the synthesis of chlorofluorocarbons and other chemicals.

(iii) It is used as a solvent in the manufacture of pharmaceutical products.

(iv) Until the mid-1960’s, carbon tetrachloride was widely used as a cleaning fluid, a degreasing agent in industries, a spot reamer in homes, and a fire extinguisher.

Uses of iodoform

Iodoform was used earlier as an antiseptic, but now it has been replaced by other formulations-containing iodine-due to its objectionable smell. The antiseptic property of iodoform is only due to the liberation of free iodine when it comes in contact with the skin.

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