Why can Miso turn a whole molecule but not freely twist its carbon–carbon double bond?
A C=C bond has sigma and pi bonding. Internal rotation disrupts pi overlap, so free rotation is restricted under ordinary conditions. A rigid rotation of the whole molecule preserves its E/Z arrangement.
E/Z is possible only when each double-bond carbon has two different groups. In but-2-ene, CH3 has higher priority than H on each carbon: same-side higher-priority groups give Z; opposite-side higher-priority groups give E. This priority statement is specific to the example.
The separate 2-methylpropene model, CH2=C(CH3)2, has H/H on one double-bond carbon and CH3/CH3 on the other. It has no E/Z pair. Both connectivity patterns have molecular formula C4H8. Original schematic artwork only.
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