A FIRST EXAMPLE; HEX-3-ENE The first example of stereoisomerism in the course, and the easiest to comprehend, is cis/trans (Z/E) isomerism about a carbon-carbon double bond. The two molecules below are clearly different, even though they have the same constitution (connectivity).
Z-Hex-3-ene E-Hex-3-ene However, don't forget that molecules are flexible. Although the structure below is different to those above, and has the same constitution, it is not a third stereoisomer. It is another version of E-hex-3-ene, related to the above (right) structure by a rotation about a C-C single bond (a process which is fast at room temperature). The two versions of E-hex-3-ene are conformers of each other - energy minima which are separated by low barriers, and therefore interconvert at room temperature.
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