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16.14 a) b) c) E 0 16.15 a) 8 TT electrons, antiaromatic b) 6 TT electrons, aromatic (One electron pair on the O is part of the cycle of TT electrons. The other pair on the O and the pair on the N are not part of the cycle.) c) The cycle of p orbitals is interrupted by a sp³-hybridized carbon, so the compound is neither aromatic nor antiaromatic. d) 6 TT electrons, aromatic (One electron pair on the O is part of the conjugated TT system.) e) Neither (The cycle of p orbitals is interrupted by the sp³-hybridized nitrogen.) f) 10 TT electrons, aromatic (if it is planar) 16.16 Although cyclooctatetraene itself is neither aromatic nor antiaromatic because it is non-planar, the dianion is planar because it has 10 TT electrons and is aromatic. It is much more stable than other dianions. 16.17 Recall that an important resonance structure for the carbonyl group moves the electrons of the carbon-oxygen double bond onto the oxygen, leaving an empty p-orbital on the carbon. This resonance structure for 2,4,6-cycloheptatrienone is aromatic, whereas the similar resonance structure of 2,4-cyclopentadienone is antiaromatic. Therefore is more stable than 2,4-cyclopentadienone. 241