Bond order benzene

Run benzene using HMO. Write out the full bond order matr ix, enter ing zero for any element off the tridiagonal. What is the bond order of benzene Is there any Kekule-type alternation in this model ... 

Experimentally, all the C-C bonds in benzene are of equal length, and this is mirrored by the C-C bond orders. 

The aromatic hydrocarbons are characterized by molecules containing six-membered rings of carbon atoms with each carbon atom attached to a maximum of one hydrogen atom. The simplest member of the series is benzene, ChH6. Using the total bond order rules discussed above, the structural formula of benzene can be written as follows ... 

Borazine, 21, is sometimes called inorganic benzene. Like benzene, it achieves a high-symmetry D6h equilibrium structure with equivalent NRT bond orders (1.380)... 

Tables 22.1 and 22.2 show how the general principles sketched above are manifested in real systems. The C-C and C-H bond order indices and the C and H valence indices were calculated for ethane, ethene, ethyne, and benzene at the HF/6-31G geometry with various basis sets. The bond order of the C-H bond is close to unity in all cases. The carbon-carbon bonds have bond orders close to one, two and three in ethane, ethene, and ethyne, respectively. In benzene, all C-C bonds have the same bond order, which is close to 1.5. Note that definition (Equation 22.1) yields nonzero bond orders between nonbonded atoms also, and in certain cases,...

Orientation effects in benzene derivatives operate in two ways. If the substituent is inductive there are large first order charge displacements at the ortho and para positions, and these can be estimated approximately using the atom polarizabilities (which is very small at the meta position). The changes of bond order, however, and consequently of free valence, vanish in first order and hence depend on Sa. The charge g g at position s therefore increases or decreases from the value unity in the... 

Each carbon-carbon double bond is constructed from four electrons. In benzene, the electrons that create the apparent double bonds fall into two classes. Two of the electrons are localized between two carbon atoms, just as we have come to expect. The other two electrons that contribute to the apparent double bonds are, in contrast, delocalized over the entire molecule. Since there are three apparent double bonds, we have a total of six electrons that are delocalized over the six carbon atoms. Think of these as free-range electrons. Basically, each of the carbon-carbon bonds in benzene is a 1.5 bond (technically, we say that the bond order in benzene carbon-carbon bonds is 1.5). Hence, the two models for benzene employed above, though universally used in chemistry, leave something to be desired. Benzene is better thought of as a hybrid of the two. Chemists have struggled with ways to depict the reality of benzene better than the stractures A and B. The struggle has not been notably successful. 

Js.JJa.s in these systems is 0.5, indicating appreciable bond length alterations. 74.5 is very similar to 7o of benzene (9 Hz), while 75.6 is only 4.0 Hz. In the pyrazolo[l,5-fl]pyrazine ring system, H-6 and H-7 appeared at low fields (> 8.1 ppm), and Jej is only 5 Hz, indicating that bond order in these systems is less than in benzene (7o = 9 Hz) (74CPB1814). 

The factor ijhb is the bond order between the interacting hydrogen atoms and is unity for H2. Proton-proton bond orders in substituted benzenes as derived from coupling constants and Eq. (21) are given in Table III. The same quantitative significance should not be attached to the proton-proton bond orders of Table III that one gives to the carbon-carbon... 

Photon-Proton Bond Orders in Benzenes as Derived from Coupling Constants... 

If we think of a hybrid of these two structures, then the C—C bonds in benzene are neither single bonds nor double bonds. They should have a bond order between a single (1.47 A) and a double bond (1.33 A). It has actually been proven that benzene is a planar molecule, and ah of its C—C bonds are of equal length (1.39 A). The bond order (1.39 A) is indeed in between a single and a double bond Thus, instead of drawing the benzene structure using alternative single and double bonds, a hybrid structure can be drawn as follows. 

Exercise 3.4. Show that the bond order for benzene is 0.67 from the data in Figure 5.3. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

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