Consider the following statements. Choose the correct statements
(A) p-dichlorobenzene has zero dipole moment.
(B) Amongst the polychlorocompounds of methane, chloroform has the lowest dipole moment.
(C) Alkyl halides are highly soluble in water.
(D) p-dichlorobenzene has a higher melting point than ortho and meta isomers.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (D) only
The C–Cl bond is polar because chlorine is more electronegative than carbon. The dipole moment of a molecule is the vector sum of its bond dipoles, so in a symmetrical molecule the bond dipoles cancel. Symmetry also affects the melting point, because symmetrical molecules pack better in the crystal lattice.
(A) p-dichlorobenzene has zero dipole moment. Correct. The two C–Cl bonds point in opposite directions, so their dipoles cancel.
(B) Amongst the polychlorocompounds of methane, chloroform has the lowest dipole moment. Incorrect. Tetrachloromethane (\( \ce{CCl4} \)) is a symmetrical tetrahedral molecule in which the four C–Cl dipoles cancel, so its dipole moment is zero, lower than that of chloroform (\( \ce{CHCl3} \)).
(C) Alkyl halides are highly soluble in water. Incorrect. Alkyl halides are only very slightly soluble in water. Energy is needed to break the hydrogen bonds between water molecules, and the weaker attractions formed between the halide and water do not release enough energy to make up for it.
(D) p-dichlorobenzene has a higher melting point than ortho and meta isomers. Correct. The para isomer is more symmetrical and fits closely in the crystal lattice, so it melts at a higher temperature (about 323 K) than the ortho (256 K) and meta (249 K) isomers.