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Banana bonds or bent bonds Burnelle, L.; Kaufmann, J.J. J.Chem. Phys. 1965, 43 , 3540;Klessinger, M. J. Chem. Phys.1967, 46 , 3261 Bent Bonds in Organic Compounds Kenneth B. Wiberg Acc. Chem. Res.; 1996; 29(5) pp 229 - 234; (Article) Advanced Organic Chemistry F.A. Carey R.J. Sundberg ISBN 0306411989 are a special type of chemical bonding in which the ordinary hybridization state of two atoms making up a chemical bond are modified with increased or decreased s-orbital character in order to accommodate a particular molecular geometry. Bent bonds are found in strained organic compounds such as cyclopropane, oxirane and aziridine.
In these compounds it is not possible for the carbon atoms to assume the 109.5° bond angles with standard sp3 hybridization. Increasing the p character makes it possible to reduce the bond angles to 60°. At the same time the carbon to hydrogen bonds gain more s-character and shorten. In cyclopropane the maximum electron density between two carbon atoms does not correspond to the internuclear axis hence the name bent bond. In cyclopropane the interorbital angle is 104°. This bending can be observed experimentally by X-ray diffraction of certain cyclopropane derviatives: the deformation density is outside the line of centers between carbons. The carbon carbon bond lengths are unusually short: 151 pm versus 154 pm for a regular bond.
One of the first bent bond theories for cyclopropane was the so-called Coulson-Moffitt model (1947).
Cyclobutane is a larger ring but still has bent bonds. In this molecule the carbon bond angles are 96° for the planar conformation and 88° for the puckered one. Contrary to cyclopropane the CC bond lengths actually increase and not decrease and this is mainly due to 1,3-nonbonded steric repulsions. In terms of reactivity cyclobutane is relatively inert and behaves like ordinary alkanes.
The bent bond theory can also explain other phenomenon in organic molecules. In fluoromethane (CH3F) for instance the experimental F-C-H bond angle is 109° but should be less. This is due to the fact that according to Bent's rule, the C-F bond gains p-orbital character leading to high s-character in the C-H bonds and H-C-H bond angles approaching that of sp2 orbitals e.g. 120° leaving less for the F-C-H bond angle. The difference is again explained in terms of bent bonds.
Bent bonds also come into play in the gauche effect explaining the preference for gauche conformations in certain substitutend alkanes and the cis effect associated with some unusually stable alkene cis isomers.
Other models
For cyclopropane, the banana bond model continues to have support despite the emergence of other theories such as Walsh orbitals, which aimed to do a better job fitting molecular orbital theory in light of spectroscopic evidence and group symmetry arguments. Critics of the Walsh orbital theory argue that this model does not represent the ground state of cyclopropane. There have been attempts to "repair" the walsh orbital theory but this has largely met with resistance; Walsh orbitals may still explain bonding in other molecules of interest.
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"Banana bond"