19. The Simple Compounds
                                   of Carbon
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      Conjugation, Aromaticity, and Colour

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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