4.5 Isomerism of Double Bonds

Single bonds between atoms are generally free to rotate. But double bonds cannot rotate; they remain planar (flat) and rigid. This causes some compounds to have isomers differing in the arrangement of parts of the molecule around a double bond.

The examples of butane CH₃CH₂CH₂CH₃ and 2-butene CH₃CH=CHCH₃ demonstrate this difference. The butane molecule has all single bonds, so it can freely change shape to form a zigzag, a parallelogram, or anything in between. 2-Butene can also have either shape, but it is stuck in the same shape all the time. The Latin prefixes cis-, meaning "on this side", and trans-, meaning "across", distinguish the two isomers of 2-butene.

[table of shapes of butane and isomers of butyfylene.]

butane

butane

trans-but-2-ene

cis-but-2-ene

We can indicate cis/trans isomerism in condensed formulas with slashes, e.g. CH₃/CH=CH/CH₃ for trans-2-butene and CH₃/CH=CH\CH₃ for cis-2-butene. For some molecules it may be difficult to figure out which isomer is "cis" and which one is "trans". Take the following molecule for example:

Is it trans because the two chlorines are on opposite sides, or cis because the bulkier of each pair of groups (bromide > chloride and chloride > methyl) are on the same side? For better accuracy, IUPAC recommends the prefixes (E)- and (Z)-. They're derived from some long German words I don't have memorized.

Whichever atom on each side has the higher atomic number determines the group's "priority". In this case, bromine (element number 35) takes priority over chlorine (element 17), while on the other end of the bond, chlorine takes priority over carbon (element 6).

The higher priority groups are on the same side, so this is the (Z)- isomer, which I like to remember as "zey are on ze zame zide." If they were on opposite sides, then it would be the (E)- isomer, as if they were enemies.

When two nitrogen atoms share a double bond, the bond is planar and rigid much like in an alkene. These are called azo compounds, and the simplest example is azobenzene C6H5N=NC6H5. Unlike double bonds between carbon atoms, azo groups are able to occasionally sort of flip back and forth between cis and trans configurations. In the case of azobenzene, this change is activated by light: the molecules are most stable in the yellow trans configuration, but ultraviolet light in the UV-B region changes the molecules to the red cis form. In the dark the material slowly reverts back to trans, however visible blue light changes cis azobenzene to trans more quickly.

Next Lesson | Table of Contents | Main