Geometric Isomer

∞ generated and posted on 2016.12.19 ∞

Molecule that differs from another molecule solely on the basis of the non-rotatability of a carbon-to-carbon double bond.

Geometric isomers share molecular formulas and what atoms are specifically bonded to each other. Nevertheless, they differ, perhaps not surprisingly, in terms of the geometric arrangement of those bonds. Specifically, a carbon atoms which is bonded to three other atoms or groups of atoms – where one of those bonds is a double bond to another carbon atom – have those three atoms or groups arranged as if at the corners of an equilateral triangle.

Picture grasping a ceramic dish that is approximately the same shape as the Greek letter Delta (upper case), i.e., Δ. Now grasp two corners, one with each hand, and try to make them change place, i.e., with the corner held by your right hand and therefore to your right now found to your left. Note that to do this you have to flip the dish over. Notice too that the third corner, the one not being held, also had to flip over. The dish itself thus is rotatable, but within in the dish, with regard to itself, no rotation can occur—except by breaking the dish. So too is the case with double bonds, that is, you cannot flip the position of atoms found at the "corners" of a carbon atom without flipping the molecule upside down or, alternatively, smashing the molecule and then "gluing" it back together again (breaking and then making chemical bonds).

Figure legend: The triangle to the left represents a molecule containing a carbon-to-carbon double bond (here presented as carbon to '3' for third 'corner'). You can flip the molecule over (right) but you still have the same molecule. Alternatively, you break the double bond – with the break shown as a dotted line – flip the left and right corners, and then rejoin the double bond. What you then have is the geometric isomer (bottom) of the molecule found on left.

Double bonds are particularly prevalent in lipids and most obviously in fatty acids. In all or nearly all cases the double bonds found in fatty acids as produced by organisms possess what is known as the cis conformation. This means that across the double bond the carbon atoms are arranged on the same side of the double bond. That is, for C-C=C-C, the two Cs on the ends are either both arranged up or both arranged down but not one up and one down. By contrast, with a trans arrangement, which represents the geometric isomer, one is up and one is down. The latter for the most part is both artifactual and not particularly good dietarily for your cardiovascular health.