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O: + + aromatic antiaromatic 16.18 The resonance structure of cyclopropenone is aromatic (see solution 16.17). Therefore cyclopropenone is less reactive than cyclopropanone despite its high angle strain. 16.19 The compound on the left (8 TT electrons) and the one in the middle (12 TT electrons) are antiaromatic and are unstable. The compound on the right, known as azulene, has 10 TT electrons and is very stable because it is aromatic. 16.20 In the resonance structure of coumarin shown below, the oxygen containing ring has 6 TT electrons, giving it aromatic character. O: : + H H + 16.21 A resonance structure of the conjugate acid of this ketone is aromatic. + 16.22 Benzocyclobutadiene is very unstable because it has an antiaromatic ring in addition to a considerable amount of angle strain. 242