Orbital Overlap in Covalent Bond Formation

Orbitals of different sizes overlap poorly and therefore form weak bonds. In general, the overlap with a carbon-based orbital decreases as one goes down a column in the periodic table. For example, the strengths of a 2p, 3p, 4p, and 5p halogen to carbon bond are, respectively, C-F, 116 kcal/mol (485 kJ/mol) C-Cl, 81 kcal/mol (339 kJ/mol) C-Br, 68 kcal/mol (285 kJ/mol) and C-I, 51 kcal/mol (213 kJ/mol). Also, bonds between atoms that both contain lone pairs of electrons (for example, bromine) are weak because of lone paii lone pair repulsion. 

CIS double bond in small ring no twist in pi bond 

The smallest ring that contains a trans double bond and is still stable enough to be isolated at room temperature is fran -cyclooctene, which is about 11 kcal/mol (46 kJ/mol) more strained than cis-cyclooctene. Cyclobutene, with an untwisted cis double bond, is stable well above room temperature. Pi bonds between orbitals of different sizes are very weak because of poor overlap a good example is the 2p-3p carbon-sulfur double bond (Fig. 2.3). 

The orbital alignment requirements have some slack an error of 10° off the proper angle appears to have little effect. Pi-bond-forming reactions that involve deformations from coplanarity of up to 30° can occur in rare cases. Pi overlap falls off with the cosine of the twist angle. The extreme, a 90° twist, would have no pi bonding at all, a loss of about 63 kcal/mol (264 kJ/mol) bonding energy stabilization because perpendicular orbitals do not interact. 

The remaining p orbitals in the product are parallel to the newly formed bond. It is less clear in the reverse reaction, which follows the same path but in the opposite direction, that the lone pair orbitals of O and L must be aligned parallel to each other and to the breaking bond at the transition state. The orbitals are then lined up so that they can easily become the allylic pi system of the ester. If one of the lone pairs were not lined up, the allylic system could not be established at the transition state, and that transition state would be much higher in energy. Allylic stabilization is about 14 to 25 kcal/mol. 

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Section 2.2 Orbital Overlap In Covalent Bond Formation... 

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