A dipole moment, a vector quantity, in a diatomic molecule results from unequal

Question

A dipole moment, a vector quantity, in a diatomic molecule results from unequal sharing of the electron density in a bond (i.e. a polar bond). This results from the difference in the electron affinity between atoms in a bond (i.e. the electro negativity). Dipole moments in molecules will exist unless the individual bond dipoles vectors cancel, as found highly symmetric molecules such carbon dioxide (linear, CO_2), carbon tetrachloride (tetrahedral; CCl_4) or methane tetrahedral, CH_4). Dipole moments are described on pg. 14-16 in J&F.; Dichloromethane with a dipole moment of 1.14 Debye is an important solvent for organic reactions. Does the existence of the dipole moment allow definitive assignment of its structure from the 3 geometric isomers presented to the right? Explain. Assuming equivalent C-H and C-Cl distances in isomers A and B, which has the greater dipole moment? Explain.

Explanation / Answer

(a) No, dipolemoment does not affirm the structural assignment from the isomers at the right. For isomer C, the C-Cl bons are placed perpendicularly opposite to each other, cancelling the effect. Hence, C will have no dipolemoment. However, B is C-Cl bonds at 90 degrees to each other and A has tetrahedral shape, this dipolemoment can be due to either A or B.

(b) in isomer A, bond angle between 2 bonds is 109 degrees. However, in isomer B, it is 90 degrees. So, more bond angle, more dipole moment. Hence, A will have more dipole-moment.

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