Rotation around Single bonds in Conjugated Systems

When you have conjugated systems in a carbon chain, there are other factors that need to be considered compared to when they are cyclic.

One thing we need to consider is bond rotation. In cyclic aromatic rings, the bonds are rigid and there can't be rotation around the single bond. In chain conjugated molecules, however, there can be.

Take trans-hexatriene for example.

Figure 1: Planar conformations of trans-hexatriene

Structure C is the general representation of trans-hexatriene and structures A and B are other planar conformations of this structure where the conjugated system is still intact.

However, there are conformations where there is rotation around the single bond where the p-orbital alignment is disrupted. This is why geometry is important when it comes to conjugation.