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14-September-2008 18:38:40 - Bent bond This article is about chemistry. For other uses, see Bent bond disambiguation. One of the first bent bond theories for cyclopropane was the so-called Coulson-Moffitt model 1947. One of the first bent bond theories for cyclopropane was the so-called Coulson-Moffitt model 1947. Bent bond, also known as banana bond, is a term in organic chemistry that refers to a type of covalent chemical bond with a geometry somewhat reminiscent of a banana. The term itself is a general representation of electron density or configuration resembling a similar bent structure within small ring molecules, such as cyclopropane C3H6, three-center two-electron bonds found in diborane B2H6, or as a representation of double or triple bonds within a compound that is an alternative to the sigma and pi bond model. Contents 1 Small cyclic molecules 1.1 Other models 2 Double and triple bonds 3 Other applications 4 References 5 External links Small cyclic molecules Bent bonds 1 2 3 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 derivatives: 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. 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. Other models For cyclopropane, the bent 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 cyclopropaneverification needed. 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. Double and triple bonds Two different explanations for the nature of double and triple covalent bonds in organic molecules were proposed in the 1930s. Linus Pauling proposed that the double bond results from two equivalent bonds,4 which later came to be called banana bonds. Erich Hückel proposed a representation of the double bond as a combination of a sigma bond plus a pi bond.5 The Hückel representation is the better-known one, and it is the one found in most textbooks since the late-20th century. There is still some debate as to which of the two representations is better,6 although some theoretical chemists consider both models to be practically equivalent. In a 1996 review, Kenneth B. Wiberg concluded that although a conclusive statement cannot be made on the basis of the currently available information, it seems likely that we can continue to consider the σ/π and bent-bond descriptions of ethylene to be equivalent.2 Other applications The bent bond theory can also explain other phenomena in organic molecules. In fluoromethane CH3F for instance the experimental F-C-H bond angle is 109° but should be less. This is because 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.2 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.2 References ^ Burnelle, L.; Kaufmann, J.J. J.Chem. Phys. 1965, 43 , 3540;Klessinger, M. J. Chem. Phys.1967, 46 , 3261 ^ a b c d Bent Bonds in Organic Compounds Kenneth B. Wiberg Acc. Chem. Res.; 1996; 295 pp 229 - 234; Article doi:10.1021/ar950207a ^ Advanced Organic Chemistry F.A. Carey R.J. Sundberg ISBN 0-306-41198-9 ^ Linus Pauling. The nature of the chemical bond. Application of results obtained from the quantum mechanics and from a theory of paramagnetic susceptibility to the structure of molecules. J. Am. Chem. Soc. 1931, 53 1367-1400. ^ Hückel, E. Z. Phys 1930, 60, 423.; Penney, W. G. Proc. R. Soc. London 1934, A144, 166; A146, 223. ^ William E. Palke. Double bonds are bent equivalent hybrid banana bonds. J. Am. Chem. Soc. 1986 108, 6543-6544. External links NMR experiment v d e Chemical bonds Strong Covalent bonds Antibonding Sigma bonds: 3c-2e bent bond · 3c-4e Hydrogen bond, Dihydrogen bond, Agostic interaction · 4c-2e Pi bonds: π backbonding · Conjugation · Hyperconjugation · Aromaticity · Metal aromaticity Delta bond: Quadruple bond · Quintuple bond · Sextuple bond Coordinate covalent bond · Hapticity Ionic bonds Cation-pi interaction · Salt bridge Metallic bonds Metal aromaticity Weak Hydrogen bond Dihydrogen bond · Dihydrogen complex · Low-barrier hydrogen bond · Symmetric hydrogen bond · Hydrophile Other noncovalent van der Waals force · Mechanical bond · Halogen bond · Aurophilicity · Intercalation · Stacking · Entropic force · Chemical polarity other Disulfide bond · Peptide bond · Phosphodiester bond Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds Retrieved from http://en..org/wiki/Bent_bond Categories: Chemical bondingHidden categories: All pages needing cleanup | articles needing factual verification since July 2007 | All pages needing factual verification Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch 中文 This page was last modified on 22 July 2008, at 05:47
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