Cyclic polyenes stabilization

Both thermochemical and MO approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy which is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncydic polyene models are called antiaro-maticf ... 

Benzene conforms to Hiickel s ruie, which predicts that planar cyclic polyenes containing 4 -I- 2 7t electrons show enhanced stability associated with aromaticity (see Section 2.9.3). Pyridine is also aromatic nitrogen contributes one electron in a orbital to the Jt electron system, and its lone pair is located in an sp orbital that is in the plane of the ring and perpendicular to the n electron system. It also conforms to HtickeTs rule, in that we still have an aromatic sextet of Jt electrons. 

Pyridine and benzene conform to Hiickers rule, which predicts that planar cyclic polyenes containing (4n + 2) -electrons ( = 0, or an integer) should show added stability over that anticipated for theoretical polyenes composed of formal alternate single and double bonds. This difference is sometimes called the empirical resonance energy. For example, benzene, where n = 1, is estimated to be 150 kJ moT more stable than the hypothetical molecule cyclohexatriene (Box 1.8) for pyridine, the empirical resonance energy is 107 kJ mol . ... 

Planar cyclic polyenes containing (4rt + 2) rt-electrons obey Huckel s rule for aromaticity and show greater stability than that predicted from their classical structures. 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

Note the signs of the coefficients. We can conclude from what was said above that the higher or lower stability of a cyclic polyene as compared to an acyclic one depends on the combination of signs of the coefficients at the ends of the demethylized compound. If the signs are identical, the even AS is aromatic due to cyclic stabilization if the signs are different, the system is anti-aromatic due to cyclic destabilization. Hence, the Hiickel aromaticity... 

It is quite illuminating to use these results to comment ) on the stability of cyclic polyenes containing rings of various sizes compared to their open-chain analogs. The first moment which will be different for the pn orbitals of an m-ring compared with those of the open chain, or of a ring of large size, will be the m-th. Since in each neutral C H molecule there are m pjt orbitals and m electrons, the fractional orbital occupancy is 0.5. 

Cicek J, Paldus J. Stability conditions for the solutions of the Hartree-Fock equations for atomic and molecular systems. Application to the pi-electron model of cyclic polyenes. J Chem Phys 1967 47 3976-3985. 

On the other hand, the difference in stabilization between acyclic and cyclic polyenes turns out to be a very useful indicator of the extra stabilization associated with cyclic systems. This extra stabilization or aromaticity is well represented by the difference in the DE of the cyclic compound and the polyene having the same number of conjugated double bonds." " Eor 1,3,5-hexatriene and benzene, this difference is 1.012(3. For comparison of molecules of different sizes, the total stabilization energy is divided by the number of tt electrons." We will see in Chapter 9 that this value gives a very useful estimate of the stability of cyclic conjugated systems. 

Unusual Stability of the Cyclic Electron Sextet Benzene, Other Cyclic Polyenes, and Electrophilic Aromatic Substitution... 

I, 3-butadiene delocalization energy as only 4 kcal/mol. Moreover, this method predicts substantial delocalization stabilization for certain cyclic polyenes that experiment reveals to be unstable, with no aromatic character. (A cyclic conjugated polyene is said to be aromatic when it shows substantially more stability than a hypothetical structure in which the double bonds do not interact with one another and when it undergoes substitution, rather than addition, when treated with electrophilic reagents like Br2.)... 

Furthermore, although delocalisation of (4n + 2) TV-electrons provides an important contribution to the overall stability of a conjugated cyclic polyene, it should be borne in mind that other contributing factors may in some circumstances nullify or override its effect as, for example, do steric factors in the case of cyclo-decapentaene, [10]annulene. 

It was later recognized that the characteristic chemical behavior of certain planar cyclic polyenes is the result of increased thermodynamic stability caused by the delocalized 71-electron system. Thus, a thermodynamic criterion for distinguishing between aromatic and non-aromatic compounds was created. 

In this chapter, we will uncover the sources of these differences in stability between cyclic polyenes and begin to see the chemical and physical consequences of this special kind of conjugative interaction of 2p orbitals. We will encounter the molecule benzene (Fig. 13.2), in which the overlap of six carbon 2p orbitals in a ring has great consequences for both structure and reactivity. We will also see a generalization of the properties that make benzene so stable, and will learn how to predict which cyclic polyenes should share benzene s stability and which should not. 

The for benzene is +19.82 kcal/mol, or 3.3 kcal/mol per CH unit (Fig. 13.14). Therefore, benzene is about 5.6 kcal/mol more stable per CH unit than a cyclic polyene with no special stability (8.9 — 3.3 = 5.6 kcal/mol). Because there are six CH units in benzene we can estimate the special stability (resonance or delocalization energy) to be 6 X 5.6 = 33.6 kcal/mol. Note that the two methods, which use the related heats of hydrogenation and heats of formation, are in rather good agreement. 

The material in this chapter is composed almost entirely of concepts. There are few new reactions or synthetic procedures. We concentrate here on the special stability of some planar, cyclic, and fully conjugated polyenes. The special stability called aromaticity is encountered when the cyclic polyene has a molecular orbital system in which all degenerate bonding molecular orbitals are completely fiUed. 

This argument can obviously be extended to concerted pericyclic reactions of all kinds. The transition state for any such reaction will be isoconjugate with a normal Hiickel-type cyclic polyene or an anti-Hiickel analog of one. If the transition state is aromatic, the resulting stabilization will lower its energy and so accelerate the reaction. If it is antiaromatic, the converse will be true. Since, moreover, the rules for aromaticity in Huckel-type and anti-Huckel-type systems are diametrically opposite, in each case one will be aromatic and the other antiaromatic. If, then, a reaction can follow one of two alternative pericyclic paths, one involving a Hiickel-type transition state and the other an anti-Hiickel-type transition, the reaction will prefer to follow the path in which the transition state is aromatic. If, on the other hand, only one of the two alternatives is sterically possible, the reaction will take place relatively easily if the corresponding transition state is aromatic and with relative difficulty if it is antiaromatic. In the latter case, the antiaromatic transition state will, if possible, be bypassed by a two-step mechanism in which the transition state is linear instead of cyclic [e.g., equation (5.291)]. 

J. PaldusandJ. Cisek,/. Chem. Phys., 47,3976 (1967). Stability Conditions for the Solutions of Hartree-Fock Equations for Atomic and Molecular Systems. I. Application to the Jt-Electronic Model of Cyclic Polyenes. 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

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