Hw Ch.9 #75 (6th Edition) Residual Entropy

[005115864]

How does one one know the orientations of a cis versus trans molecule using MX2Y4?

The solutions manuel says cis has a total of 12 possible orientations and trans a total of 3 possible orientations.

[Yousif Jafar 1G]

You would have to draw it out. If its cis, the molecules can only be in three positions of the octahedral and the top and bottom and the two diagonals. However, if they are trans, they can be in any of the six positions and there are two molecules that are trans for those six positions so twelve possible positions.

[Sean_Rodriguez_1J]

By definition, a trans isomer must have atoms that are on opposite sides of each other (ie the bond angle between them is 180 degrees in a MX2Y4 molecule). With an octahedral molecule, there are only three unique ways to orient the atoms so that they remain opposite to each other (left/right, up/down, and front/back). In a cis isomer, where the atoms are adjacent to each other (there is a 90 degree angle between them in a MX2Y4 molecule), there are less restrictions for the positions of where the same atoms can be. Specifically, you could have left/up, right/up, back left/up, back right/up, left/right, back left/ back right, left/down, right/down, back left/down, back right/down, left/back left, and right/back right, which totals 12 different orientations. Hope that makes sense.