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Explain the following oreder of dipole moments of chloromethanes : CH_(3)Cl > CH_(2)Cl_(2) > CHCl_(3) > C Cl_(4). |
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Answer» Solution :The `C Cl_(4)` molecule has a highly symmetrical tetrahedral configuration. So, the resultant of THREE C-Cl bond moments cancels the fourth C-Cl bond moment. Hence, the molecule possesses no net dipole moment `(mu-0D)`. The dipole moment of `CH_(2)Cl_(2)` in which two C-Cl bonds make and angle of neraly `109^(@)28'` is expected to be larger than that of `CH_(3)Cl` because the resultant of two C-Cl bond moments must be greater than one `C-Cl` bond moment. also, the dipole moment of `CH_(3)Cl` is expected to be equal to that of `CHCl_(3)` because the moment of a -`C Cl_3)` group is equal to that of a `C-Cl` bond and the moment of a - `CH_(3)` group is equal to that of a C-H bond. Thus, the expected order of dipole moment is: `CH_(2)Cl_(2) > CH_(3)Cl ~~ CHCl_(3) > C Cl_(4)`. This order, however , does not agree with the experimental dipole moment value. This anomaly may be explained by considering the moment acitng in the opposite direction induced by each C-Cl dipole in the order. Since there is only one C-Cl bond in `CH_(3)Cl`, the opposing induced moment is absent in it. So, it possesses the largest dipole moment. In `CH_(2)Cl_(2)`, there are two C-Cl bond. thus, there ISAN opposing induced moment that partly cancels the resultant moment of the two C-Cl bonds. So, its dipole moment is smaller than `CH_(3)Cl`. In `CHCl_(3)`, there are three C-Cl bonds. So, the magnitude of the opposing induced moment is relatively large which considerably reduces the resultant of the three C-Cl bond moments. THEREFORE, its dipole moment is smaller than that of `CH_(2)Cl_(2)` and MUCH smaller than `CH_(3)Cl`. So, the actual order of dipole moments is `CH_(3)Cl > CH_(2)Cl_(2) > CHCl_(3) > C Cl_(4)`. |
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