ARENES AND ARO 257 energy orbitals. There are 12 T electrons, and so the lowest 5 orbitals are each doubly occu- ied, whereas each of the next 2 orbitals--orbitals of equal energy-is singly occupied Antibonding orbitals (5) Bonding orbitals (5) 11 14 One way to evaluate the relationship between heats of combustion and structure for compounds that are not isomers is to divide the heat of combustion by the number of carbons so that heats of com- bustion are compared on a " per carbon"basis Cyclooctatetraene [161-Annulene [181-Annulene 3265Jm 4543 k/mol 9121 k/mol 9806 k/mol (2182 kcal/mol) (2346 kcal/mol) 544 k/mol 568 kJ/mol 570 k/mol (130 kcal/mol) (136 kcal/mol) (130 kcal/mol) As the data indicate(within experimental error), the heats of combustion per carbon of the two aro- matic hydrocarbons, benzene and [18]-annulene, are equal. Similarly, the heats of combustion per carbon of the two nonaromatic hydrocarbons, cyclooctatetraene and [16]-annulene, are equal. The two aromatic hydrocarbons have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons. On a per carbon basis, the aromatic hydrocarbons have lower potential energy(are more stable) than the nonaromatic hydrocarbons 11.15 The seven resonance forms for tropylium cation(cycloheptatrienyl cation) may be generated by moving T electrons in pairs toward the positive charge. The resonance forms are simply a succes- sion of allylic carbocations H H Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Websiteenergy orbitals. There are 12 electrons, and so the lowest 5 orbitals are each doubly occu￾pied, whereas each of the next 2 orbitals—orbitals of equal energy—is singly occupied. 11.14 One way to evaluate the relationship between heats of combustion and structure for compounds that are not isomers is to divide the heat of combustion by the number of carbons so that heats of com￾bustion are compared on a “per carbon” basis. As the data indicate (within experimental error), the heats of combustion per carbon of the two aro￾matic hydrocarbons, benzene and [18]-annulene, are equal. Similarly, the heats of combustion per carbon of the two nonaromatic hydrocarbons, cyclooctatetraene and [16]-annulene, are equal. The two aromatic hydrocarbons have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons. On a per carbon basis, the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons. 11.15 The seven resonance forms for tropylium cation (cycloheptatrienyl cation) may be generated by moving electrons in pairs toward the positive charge. The resonance forms are simply a succes￾sion of allylic carbocations. H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H Benzene Heats of combustion: 3265 kJ/mol (781 kcal/mol) Heats of combustion per carbon: 544 kJ/mol (130 kcal/mol) Cyclooctatetraene 4543 kJ/mol (1086 kcal/mol) 568 kJ/mol (136 kcal/mol) [16]-Annulene 9121 kJ/mol (2182 kcal/mol) 570 kJ/mol (136 kcal/mol) [18]-Annulene 9806 kJ/mol (2346 kcal/mol) 545 kJ/mol (130 kcal/mol) Antibonding orbitals (5) Nonbonding orbitals (2) Bonding orbitals (5) ARENES AND AROMATICITY 257 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website