Molecules containing a diazo group (e.g. azobenzene) can exist as cis- and trans-isomers (Figure 1). Cis/ trans isomerism is a special case of E/Z isomerism (see CHEMISTRY REVIEW, Vol. 23, No. 2, p. 31). This is a form of stereoisomerism caused by restricted rotation about a double bond, when two different groups are attached to each atom at either end of the double bond.
In cis/trans isomerism at least one substituent group on the atoms at each end of the double bond is identical to the other. In the cis-isomer the identical groups are on the same side of the double bond. In the trans-isomer these groups are on opposite sides of the double bond.
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