Bond length, in benzene

Measure the carbon-carbon bond length in benzene. Would you describe it as a single bond, a double bond, or somewhere in between Draw whatever resonance contributors are needed to justify your conclusion. 

Support for the above view is provided by the observation that all the carbon-carbon bond lengths in benzene are exactly the same, ... 

With use of Equation 7-5 we calculate 1.420 A for the length of a carbon-carbon bond with bond order 1.5. This may be taken a8 the length that the bonds in benzene would have if there were no stabilizar tion (and consequent bond shortening) by resonance. The actual bond length in benzene is 1.397 A, and we conclude that resonance between the two Kekul6 structures decreases the bond length by about.0.023 A. 

We show the MP2/6-311+G optimized geometry of the cation in Figure 2. Selected bond distances are shown. The molecule has C2v symmetry. The C1-C2 bond lengths (1.395 A) are very similar to the C-C bond lengths in benzene (1.400 A at MP2/6-311+G ), consistent with the delocalized nature of the electron density and sp ... 

Benzene is actually a resonance hybrid of the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 1 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths. 

The cyclooctatetraene dianion has a planar, regular octagonal structure with C — C bond lengths of 1.40 A close to the 1.397 A bond lengths in benzene. Cyclooctatetraene itself has eight pi electrons, so the dianion has ten (41V+2), with N = 2. The cyclooctatetraene dianion is easily prepared because it is aromatic. 

Putting 7 = 1.39 A = 0.139 nm (the bond length in benzene, a molecule with similar bonding) and expressing A in nanometers, we find... 

The most obvious example on which to test the predictions of this curve is benzene. The Coulson rt-bond-order for benzene (which could be calculated from equation (4-6) and the LCAO-coefficients for benzene) is p enzene = 1 and thus the total bond-order for benzene, Pb Azcne, is ca- 1-67. From Fig. 4-5, a bond order of 1-67 corresponds to a bond length of 1-40 A. The observed value for this bond length in benzene is 1-397 A and hence this is very satisfactory. We might now therefore further calibrate this curve by adding the point (1-67, 1-397 A) for benzene to the three which already define the plot reproduced in Fig. 4-5. We could, if we wished, include on this curve a... 

First, geometry of each substructure was fully optimized except some internal coordinates described below. The bond lengths in benzene rings were fixed to 1.399 and 1.101 A for C-C and C-H bonds, respectively. All the bond angles in benzene rings were fixed to 120... 

In fact, none of these structures correctly describes benzene. All the carbon-carbon bond lengths in benzene have been found to be the same, but the structures above do not predict this since double bonds are shorter than single bonds between the same atoms. Also, benzene is relatively unreactive towards addition, which we would not expect in a compound that contains three double bonds. These days the benzene ring is written as ... 

Delocalization of the leftover p electrons is believed to make benzene more stable than would be expected if the Kekule structures were correct, and explains why benzene does not react as if it had three double bonds. The model predicts that all the carbon-carbon bond lengths in benzene would be of the same length, which they are. The length of each bond is between those normally observed for C=C and C-C. 

Experimentally (and consistent with the calculated bond lengths in Model 1), we find that all six C-C bond lengths in benzene have the same bond length, 139 pm. Is this fact more consistent with the bond orders predicted by the Lewis structure or with the calculated bond orders [Hint refer to your answer for CTQ 4.]... 

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