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16.34 The benzylic cation rearranges to form cycloheptatrienyl cation, which is even +CH₂ + more stable. Note that the cycloheptatrienyl carbocation is aromatic (6 TT electrons) and has the positive charge delocalized around the entire ring. 16.35 The two atoms of the double bond and the four atoms attached to them must lie in the same plane in order for the p-orbitals of the pi bond to be parallel. Examine the model to see for which adjacent carbons this is true. (When viewed down a C-C bond, the H's are eclipsed as are the C's.) These are the carbons that are connected by double bonds. When viewed down a C-C single bond, the H's are not eclipsed, nor are the C's. 16.36 Only the electron pair on the nitrogen bonded to the H hydrogen is part of the pi system. The electrons on the N N nitrogens that are part of double bonds are perpendicular to the pi system. Both heterocyclic rings : N have six pi electrons and are aromatic, so purine is N aromatic. purine 16.37 For oxazole and isoxazole, one electron pair on the is part of the pi system. The other electron pair on the O and the pair on the N is perpendicular to the pi system. Both compounds have six electron in their pi systems and are aromatic. For pyrazole, the pair of electrons on the N that is doubly bonded to the C is not part of the pi system. The pair of electrons on the N that is bonded to the H is part of the pi system. There are six electrons in the pi system, so pyrazole is aromatic. Like pyridine, the electrons on the N's of pyrimidine are not part of the pi system. There are six pi electrons, so pyrimidine is aromatic. : N : N N : N N: : N H oxazole isoxazole pyrazole pyrimidine 245