whereas fumaric acid does not yield an anhydride. Optical isomerism is characteristic of organic molecules that do not have a plane of symmetry (a plane dividing the molecule into two mirror images) and are not coincident with its mirror image (that is, with a molecule corresponding to the mirror image).hereas fumaric acid does not yield an anhydride. Optical isomerism is characteristic of organic molecules that do not have a plane of symmetry (a plane dividing the molecule into two mirror images) and are not coincident with its mirror image (that is, with a molecule corresponding to the mirror image).