As we saw first in Chapter 9, 1,3-butadiene is a planar molecule
because its double bonds are delocalized along the entire four-carbon
chain (right).
Although the conventional representation shows double bonds between
the end pairs of carbon atoms, resonance structures can be drawn
in which the central two carbons are double-bonded, and the electron
pair of the other double bond is either given to one of the two
outside carbon atoms or divided between them.
Twisting about the central carbon-carbon bond is as restricted
as twisting about either of the outer carbon-carbon bonds, so all
ten atoms of the 1,3-butadiene molecule are constrained to lie in
one plane.
Just as three electron pairs are delocalized around six carbon
atoms in a benzene ring, so two electron pairs are delocalized along
the four-carbon chain of butadiene.
Delocalization can occur whenever single and double bonds alternate
along a chain of atoms, so that after all single bonds are formed,
each atom along the chain has an unused p orbital and one unused
electron. Such molecules are termed conjugated.s
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