Bond order naphthalene

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Using a similar approach, Evans and co-workers obtained values of hsi = -1.50, kcsi = 0.55, 8 = 0.15 for a series of trimethylsilyl-substituted naphthalene radical anions based on a Q value of -26.1 49). For phenyltrimethylsilylacetylene radical anion, the values of the heteroatom parameters that gave the best fit of the Huckel calculated spin densities with experimental values, using a Q value of 28, were hs, = -1.3, and kCSi = 0.65 when a 8 value of 0.1 was assumed. A resulting C—Si 7r-bond order of about 0.3 is obtained 43). 

Table III compares the bond orders calculated both by the HMO and by the semiempirical methods. The geometry of a molecule may be crucial to quantum chemical calculations it is not known for 4. Whereas Dewar et used an iterative procedure (correlation between bond lengths and bond orders), other workers preferred standard geometry or a combination of naphthalene and furan values. ...

Work on the Mills-Nixon effect has been reviewed by Badger [2]. Most of the experimental work done in this field has involved a study of relative chemical reactivities and hence is subject to the severe limitations mentioned earlier [ lc]. From a study of the ease with which the benzoates of isomeric hydroxy-5,6,7,8-tetrahydroacetonaphthones underwent the Baker-Venkataraman transformation O Farrell et ah [3] recently concluded that the 1,2-bond of 5,6,7,8-tetrahydro-naphthalene has a higher bond order than the 2,3-bond. Similar work also led these authors to conclude that the 4,5-bond in indan is of higher order than the 5,6-bond. These workers did not attempt to assess the extent of the difference in bond order presumably exhibited by the bonds in question. 

There are four nonequivalent types of C—C bonds in naphthalene, these being represented by C,—C2, C2—C3, C4—C10, and Q—C,(). By using the MO expressions in Table 7.2, the n bond order of each type may be computed. As before, in treating tetramethylenecyclobutane, the bond order, pmtn of the bond between atoms m and n is defined as the sum of contributions from each occupied MO, each contribution being given by twice (for two electrons) the product of the coefficients of 0OT and 0 in that MO. For p,2 in naphthalene we have... 

Figure 4 - Pauling bond orders and free valences for benzene, naphthalene, and trimethylene-methyl.

Activation across the 2,3-bond is much higher than across the 3,4-bond, because of the differences in bond order [cf. the effects of a 2-methyl substituent in naphthalene [68JCS(B)1112]]. 

While thus the energy depends only on the class, the bond order is also determined by the classes of the adjacent bonds35. Here a table can be made for all possibilities. In this the symbol 3333 denotes a bond such as the central bond in naphthalene which is bounded by four bonds of class 3 the bond itself is thus of the 4th class. From the relation between this V.B. bond order and bond length (compare M.O. bond order and G—C distance Fig. 19) the calculated length can also be given for each bond. 

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

As an example of the effect of aromaticity, consider the carbon-carbon bonds in benzene. Assuming a bond order of 1.5 for each of the six bonds (or equivalently three single and three double bonds) we would calculate a refraction of 2.71 cm per bond, while the observed value is 2.73 cnp per bond (see Table 3). Naphthalene bonds are still more polarizable, with a refraction of 2.78 cm per bond. 

The length of each C-C bond in a polycyclic aromatic hydrocarbon is near that expected from the various resonance hybrids that can be drawn, and the bond orders that can be calculated.The main nonion-ized contributors for benzene are available from neutron studies of crystalline deuterobenzene at 15 K (-258° C) and for crystalline naphthalene at 92 K (-181 C) and are shown in Figure 11.18. The delocalization of electrons in benzene is shown by its equal bond lengths while, in naphthalene there is also evidence of some localization of double bonds. 

For the three-bond coupling constants, J C,OW)tram > - (C,OH) j and a plot of 7(C,OH)c vs. 50H shows a good correlation for o-hydroxybenzoyl derivatives. The corresponding correlation hne for olefinic derivatives is parallel. Data for naphthalene derivatives fall mainly in between . Bond order is clearly an important parameter. 

C-C) and 3/(C-C) values in the naphthalene (63) and anthracene (64) derivatives have been plotted against the sum of the 7r-bond orders. Reasonable correlations are obtained if one assumes that 2/(C-C) is negative in these systems. 

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