Cyclic hydrocarbons, resonances

Conformational effects on 15N shifts in substituted cyclohexanes make an axial NH2 more shielded than an equatorial one. Also, 15N resonances are deshielded by ft substitution more extensively than are 13C resonances of cyclic hydrocarbons, but the magnitude of the effect depends on the degree of nitrogen substitution. Carbons in the y position shield the nitrogen in a manner analogous to 13C, but to a smaller extent in methanol than in cyclohexane solutions, and less for tertiary amines than for primary and secondary amines. These differences have been attributed in part to possible conformational influences on the stereoelectronic relationships between the lone pair and the C—C bonds. 

Benzene peroxide may be more stable in solution since it is protected by the surrounding benzene molecules. The stability of peroxides in the series benzene—naphthalene—anthracene should increase with increasing resonance energy of the aromatic compound. The resonance energy of benzene is 36 kcal mole-1, that of naphthalene is 61 kcal mole-1 and that of anthracene is 83 kcal mole-1. Thus pure anthracene peroxide can be separated and stored for a long time [Refs. 91, 93, 96, 102, 241], whereas benzene peroxide exists only in situ and decomposes in all attempts at separation. The characteristic absorption band of anthracene peroxide also occurs at 2780 A and it may be assumed that the band is characteristic of the peroxides of cyclic hydrocarbons. 

To produce more reliable predictions of aromaticity, Hess and Schaad (following a suggestion of Dewar) calculated delocalization (resonance) energies of cyclic hydrocarbons by comparing the compounds Htickel-theory with a value calculated for a hypothetical acyclic conjugated polyene with the same number and kinds of bonds as in a localized structure of the cyclic hydrocarbon. [B. A. Hess and L. J. Schaad, J. Am. Chem. Soc., 93, 305, 2413 (1971) 94, 3068 (1972) 95, 3907 (1973) B. A. Hess, L. J. Schaad, and C. W. Holyoke, Tetrahedron, 28, 3657, 5299 (1972) Schaad and Hess,... 

Fig. 13 Alternative resonance forms for benzene (aromatic), cyclobutadiene and cyclo-octatetraene (antiaromatic). Other aromatic cyclic hydrocarbons and ferrocene are illustrated at the bottom...

Benzene is a unique cyclic hydrocarbon that is more stable than expected due to resonance delocalization. 

The best known example of resonance in organic chemistry concerns the cyclic hydrocarbon benzene, CgHg, which can be described by two major resonance structures called Kekul structures (Figure 14.52). 

The delocalization of tt bonds represented by resonance in benzene results in a gain in stability according to the principle stated above. It should be noticed, however, that by this description benzene owes its special position as the most stable of the cyclic hydrocarbons, G H , particularly to the fact that its six-membered ring exactly accommodates the 120 angle, resulting in maximum overlap in the a bonds. In any carbon ring other than the six-membered this is not the case23. ... 

Baird, N. Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3.pi-pi. state of cyclic hydrocarbons. J. Am. Chem. Soc. 1972, 94, 4941. 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

The aromatic hydrocarbons contain at least one unsaturated ring system with the general structure C6R6, where R is any functional group (see Chap. 1). The parent hydrocarbon of this class of compounds is benzene (C6H6), which exhibits the resonance, or delocalization of electrons, typical of unsaturated cyclic structures. Owing to its resonance energy, benzene is remarkably inert. 

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

A member of the class of hydrocarbons consisting of assemblages of cyclic conjugated carbon atoms and characterized by large resonance energies... 

Sheppard, N. Turner, J. J. High-resolution nuclear magnetic resonance (nmr) spectra of hydrocarbon groupings. II. Internal rotation in substituted ethanes and cyclic ethers, Proc. Roy. Soc. (London) 1959, A252, 506-519. 

A special class of cyclic unsaturated hydrocarbons is known as the aromatic hydrocarbons. The simplest of these is benzene (C6H6), which has a planar ring structure, as shown in Fig. 22.11(a). In the localized electron model of the bonding in benzene, resonance structures of the type shown in Fig. 22.11(b) are used to account for the known equivalence of all the carbon-carbon bonds. But as we discussed in Section 14.5, the best description of the benzene molecule assumes that sp2 hybrid orbitals on each carbon are used to form the C—C and C—H a bonds, while the remaining 2p orbital on each carbon is used to form 77 molecular orbitals. The delocalization of these 1r electrons is usually indicated by a circle inside the ring [Fig. 22.11(c)]. 

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