Planar versus nonplanar bonds

The three-dimensional structure of molecules is important, because it affects both chemical reactivity and biological activity. The number of bonds the carbon atom is involved in is important in this respect, because it affects whether the molecule is planar (i.e. flat) or non-planar. Since the angle between the sp3 orbitals is 109.5°, alkanes (a chain of carbon atoms connected via single bonds) cannot lie in a flat plane. A double bond, however, confines the neighboring single bonds to a plane. Hence, because of the conjugated structure, the benzene ring is planar.

The spatial structure of flavone (2.3) and flavanonol (2.4) is very different. The presence of the double bond in the C-ring of flavone results in the A- and C-rings being planar, whereas the C-ring of flavanonol is not planar.

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