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Coulson and Moffitt’s Modification, Sachse Mohr’s Theory (Theory of Strainless Rings)

Known as banana bonds or bent bonds, bent bonds are covalent chemical bonds with a shape reminiscent of a banana.

Coulson and Moffit’s Modification

Known as banana bonds or bent bonds, bent bonds are covalent chemical bonds with a shape reminiscent of a banana. The term itself represents an alternative to the sigma and pi bond model, which depicts a density of electrons or configuration similar to a "bent" structure within certain rings, such as in cyclopropane (C3H6).

It is important to note that a sigma bond (strong bond) overlaps on both ends while a pi bond overlaps both sides. This makes bent bonds an intermediate bond between these two. There is neither end-to-end nor side-to-side overlap. Thus, cyclopropane C-C bonds are weaker than normal C-C sigma bonds and the overlap is less efficient than sigma overlap. As described by Baeyer's strain theory, cyclopropane has a triangle configuration with 600 carbon valency angles. As carbon valence angles can never be any less than 900 (in pure p-orbital systems), the value is impossible. When the hybridization occurs, the angles are more than 90°. Suppose we assume a carbon valency angle of 104°. According to Coulson-Moffit's, this is the smallest angle one should reasonably expect to have. Thus, in cyclopropane only a partial overlap of the sp3 hybrid orbitals occurs, so the bond angle between C and C is decreased from 109.5° to 104° and the bond angle between H and H is increased to 1200 compared to the normal tetrahedral angle. In the absence of overlap, the bond weakens, which partially explains cyclopropane's instabilities. The bent bonds are also present in cyclobutane, although the overlap decrease is less than that in cyclopropane. In this regard, it is more reactive (it doesn't participate in ring-opening reactions) and more stable than cyclopropane.

Sachse Mohr’s Theory (Theory of Strainless Rings)

A carbon ring is said to be a plane where all its atoms are. According to this assumption, cyclic rings should be planer than noncyclic rings. As the size of the ring increases, the strain increases from the minimum for cyclopentane. Hermann Sachse soon pointed out that rings larger than cyclopentane do not necessarily need to be strained since carbons do not necessarily have to plane. If all ring carbons are not forced into one plane, Sachse Mohr's theory states that higher member rings are free from strain. In two non-planar conformations, they are folded or puckered, without any strain whatsoever. Since the carbon atoms are arranged differently on the atoms, the valency angle remains constant (109.5°). Because of their shapes, these form types are called the 'Chair' Form and the 'Boat' Form.

Cyclohexane is in its most stable conformation in the chair. Axially and equatorially are the two types of chair cyclohexane positions. Perpendicular to the circle's plane are the axial positions, while the equatorial positions are around the circle's center. Further refinement of this theory was made by Mohr (1918), which applied to compounds that contained a fused pair of rings.

Coulson and Moffitt’s Modification

A modification of Coulson-Moffitt's strain theory demonstrates that C-C bonds in cyclopropane are situated in the actual placement of their carbon bonds, thereby reducing strain on C-C bonds. The banana bond/bent bond were introduced to satisfy the tetrahedral geometry and equilateral triangle angle (600) for the overlapping geometry. The banana bond/bent bond are intermediate between sigma and pi bonds for overlapping geometry.

What is bend bond - There is head-to-head and end-to-end overlapping in sigma bonds. On both sides of the molecular axis, the hybrid orbitals as well as the intersection bonds are present. However, in the case of pi-bonds, the p-orbitals overlap on both sides of the bonding intersection, which places the bonding intersection area outside of the molecule's axis.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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