Lecture #17
VSEPR geometries and Section 13.3
CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Molecular Geometries

Dipole Moments in Polyatomic Molecules

Rigidity about double bond

Geometrical isomers

Free and restricted rotations about bonds

The simple notion that polarity between two atoms -- the bond dipole moment -- can be vectorally added when estimating the polarity of a complex molecule frequently works. Here, in the two linear molecules, the direction of the resultant dipole is easily argued. The polarities of dichloroethylenes demonstrate next that the molecules must be planar.
In the dichloroethylene shown, each carbon has a trigonal planar geometry about it. This alone does not complete the geometrical description of this molecule because there remains the question of the orientation of one of those planes with respect to the other. Measuring the molecule's dipole moment might resolve this last question. The dipole moment is exactly zero. The molecule is nonpolar.
Three "dichloroethyenes". Given (with an explanation deferred until the theories of the chemical bond are discussed) that all three of these double bonded structures are fairly rigidly planar, we have isomers whose molecular formulas are all C2H2Cl2. The bottom molecule is a structural isomer of each of the other two. Its formula could be written Cl2CCH2 whereas the upper two would be ClHCCClH. .
The upper two, because the chlorines are arranged in distinct geometric locations, are called geometric isomers (of each other). In the top structure, the Cl's are across (trans) the double bond from each other and in the middle structure they are on the same side of the double bond
Dipole moments for each of the three structures based on our simplification that only the C-Cl bond dipole moments need be considered. Be aware that the angle between the pairs of C-Cl bonds is not the same in the bottom two structures.
Here we have the full electrostatic map of the cis-dichloro-ethylene showing the red-colored negative charge region and the blue-colored positive region.
Chlorobenzene, not surprisingly, is a polar molecule. The negative end is at the electronegative chlorine, as expected. The manner in which three dimensional structures are shown, including a map of electrostatic (positive = blue, negative = red, neutral = yellow) potential at the valence surface, is by an illustration like that here. It was generated by a quantum mechanical calculation program.
Demonstrating the additivity of bond dipole moments in complicated molecules. We will consider here only the effect of the carbon-chlorine dipole, ignoring the carbon-hydrogen dipole. This makes the discussion simpler and is partially justified by the greater contribution expected for the C-Cl dipole (large electronegativity difference) vis--vis the C-H dipole. The problem becomes one of finding the resultant dipole "vector".
Some of the vector additions are almost intuitive. Two give non-polar (zero dipole) molecules. The third is polar.
The last is very polar. The vector additions can be accomplished by doing one pair and then, next, using that resultant with the final vector.
Additionally, a question from Exam II from the Spring of 2000 will be gone over to give more examples clarifying the distinction between structural and geometrical isomerism