1,2-dichlorocyclopropane and a spiran

As I am reading a book called "Symmetry in Chemistry" (H. H. Jaffé and M. Orchin) I thought I would try out a couple of examples that they use. One is 1,2-dichlorocylopropane :

which is, apparently, dissymmetric because it has a symmetry element (a C2 axis) but is optically active. Incidentally, wedges can look horrible in small structures - this is why:

The box around the hydrogen is shaded in grey, to show the effect of overlap. A possible fix might be to shorten the wedge, but sadly this would require working out the bounds of the text when calculating the wedge, which has to be done at render time. Oh well.

Another interesting example is this 'spiran', which I can't find on ChEBI or ChemSpider:

Image again courtesy of JChempaint. I guess the problem marker (the red line) on the N suggests that it is not a real compound? In any case, some simple code to determine potential chiral centres (using signatures) finds 2 in the cyclopropane structure, and 4 in the spiran. Since the code is not using a 3D structure, only a connected graph, it can't work out the spiran's S4 axis.


Kirill said…

there is a number of problems with this 'spiran' structure. I suppose the nitrogen should have a charge on it. The upper left corner both H and methyl go UP while the upper right corner both H and methyl go DOWN. What does that mean?

Wedges with terminal symbols look horrible unless they go strictly vertical or horizontal. A hint: in tetrahedral stereogenic centre which belongs to a ring, if one wedge goes UP, another must go down. Therefore it is NOT necessary to show both wedges. One solid wedge and one plain bond should be enough. In this way one can avoid ugly combination of a wedge and a symbol, coz there is usually a way to orient one wedge either vertically or horizontally.
gilleain said…

Yeah, the nitrogen doesn't seem to be very useful, as it could be changed to a carbon without altering the molecule's key symmetries.

The wedges are drawn wrong, but JCP doesn't have the thick bonds that indicate chirality in a ring. The best I can do is this which shows the chirality of the central carbon as well as the four methyls.