16. Conjugated Systems

Why are conjugated molecules like the product of the ninhydrin reaction brightly colored?

(a) Draw the molecular orbitals for the cyclopropenyl case.

(Because there are three p orbitals, there must be three MOs: one all-bonding MO and one degenerate pair of MOs.)

(b) Draw an energy diagram for the cyclopropenyl MOs. (The polygon rule is helpful.) Label each MO as bonding, nonbonding, or antibonding, and add the nonbonding line. Notice that it goes through the approximate average of the MOs.

(c) Add electrons to your energy diagram to show the configuration of the cyclopropenyl cation and the cyclopropenyl anion. Which is aromatic and which is antiaromatic?

Which of the following would you expect to be a more stable molecular orbital? Why?

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a. For conjugated systems with two, three, four, five, six, and seven conjugated p-bonds, construct quick MOs (just draw the lobes at the ends of the conjugated system as they are drawn on pages 1220 and 1221) to show whether the HOMO is symmetric or antisymmetric.

b. Using these drawings, convince yourself that the Woodward–Hoffmann rules in Table 28.1 are valid.