Planar chirality

In most organic chemistry textbooks, double bond cis/trans isomerism is exemplified by alkenes. It is also observed in cycloalkenes such as cyclooctene that can exist as either cis (a) or trans (b) isomer:

(a)	(b)

(Z)-cyclooctene (PIN) cis-cyclooctene (E)-cyclooctene (PIN) trans-cyclooctene

To the trans isomer, there is a twist — and the pun is fully intended. Have a look at the structures (c) and (d) (or at their 3-D models here, Fig. 2 and Fig. 3, respectively).

(c)	(d)

(1E,1P)-cyclooct-1-ene (PIN) (E,P)-cyclooctene (E,Rp)-cyclooctene (1E,1M)-cyclooct-1-ene (PIN) (E,M)-cyclooctene (E,Sp)-cyclooctene