Bent bond

From Wikipedia, the free encyclopedia

Bent bond, also known as banana bond, is a term in 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 orconfiguration resembling a similar "bent" structure within small ringmolecules, 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 External links
• 5 References

[edit] 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 angleswith standard sp³ hybridization. Increasing the p character makes itpossible to reduce the bond angles to 60°. At the same time the carbonto hydrogen bonds gain more s-character and shorten. In cyclopropanethe maximum electron density between two carbon atoms does notcorrespond 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.

Cyclobutaneis a larger ring but still has bent bonds. In this molecule the carbonbond angles are 96° for the planar conformation and 88° for thepuckered one. Contrary to cyclopropane the CC bond lengths actuallyincrease and not decrease and this is mainly due to 1,3-nonbondedsteric repulsions. In terms of reactivity cyclobutane is relativelyinert and behaves like ordinary alkanes.

[edit] 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 theoryin light of spectroscopic evidence and group symmetry arguments.Critics of the Walsh orbital theory argue that this model does notrepresent the ground state of cyclopropane^{[verification needed]}.There have been attempts to "repair" the walsh orbital theory but thishas largely met with resistance; Walsh orbitals may still explainbonding in other molecules of interest.

[edit] 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 onefound in most textbooks since the late-20th century. There is stillsome debate as to which of the two representations is better,^[6]although some theoretical chemists consider both models to bepractically equivalent. In a 1996 review, Kenneth B. Wiberg concludedthat "although a conclusive statement cannot be made on the basis ofthe currently available information, it seems likely that we cancontinue to consider the σ/π and bent-bond descriptions of ethylene tobe equivalent."^[2]

[edit] 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 due to the fact that according to Bent‘s rule,the C-F bond gains p-orbital character leading to high s-character inthe 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]

[edit] External links

• NMR experiment

[edit] References

1. ^ Burnelle, L.; Kaufmann, J.J. J.Chem. Phys. 1965, 43 , 3540;Klessinger, M. J. Chem. Phys.1967, 46 , 3261
2. ^ ^{a b c d} Bent Bonds in Organic Compounds Kenneth B. Wiberg Acc. Chem. Res.; 1996; 29(5) pp 229 - 234; (Article) doi:10.1021/ar950207a
3. ^ Advanced Organic Chemistry F.A. Carey R.J. Sundberg ISBN 0-306-41198-9
4. ^ 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.
5. ^ Hückel, E. Z. Phys 1930, 60, 423.; Penney, W. G. Proc. R. Soc. London 1934, A144, 166; A146, 223.
6. ^ William E. Palke. Double bonds are bent equivalent hybrid (banana) bonds. J. Am. Chem. Soc. 1986 108, 6543-6544.