Bent Bond - Small Cyclic Molecules

Small Cyclic Molecules

Bent bonds 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 90° 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.