Cis and Trans Isomerisation


So what's all this cis and trans stuff about that was on the previous pages?

While the atoms at opposite ends of a single bond can rotate with respect to each other will relative freedom, the atoms at opposite ends of a double bond are fixed in place. This means that most substituted alkenes will have two different isomers, with different physical properties.

1-pentene has only one isomer- one end of the double bond only has hydrogen substituents, so swapping the chains around at the other end of the bond will make no difference to the structure- we can just turn the molecule over and get back to where we started.


With 2-pentene, the situation is different. Each end of the double bond has one substituent that is not hydrogen; these can be placed on the same side of the double bond as it is drawn on the page (cis) or opposite sides (trans).

cis-2-pentene trans-2-pentene

An easy way to remember this is to remember the Cisco Kid and Pancho were on the same side...

Sometimes, cis and trans are not sufficient to describe the compound (for example in the last two hexenes, a few pages back). In these cases a more sophisticated nomenclature is required that assigns precedence to the different substiuents. The descriptors (E) and (Z) are then used. (E) comes from the German entgegen meaning opposite and (Z) comes from the German zusammen meaning together.

(My first year chemistry teacher always said 'Z' isomers had their two largest substituents on "Ze Zame Zide" [insert your own bad german accent]).