D. A. Evans Bonding generalizations Chem 206 I Bond strengths(Bond dissociation energies)are composed of a Orbital orientation strongly affects the strength of the resulting bond covalent contribution (o Ecow and an ionic contribution( Eionid For g Bonds Bond Energy(BDE)=o Covalent+ 8 Ionic( Fleming, page 27) A○ OBo teoA○B When one compares bond strengths between C-C and C-x, where X is some other element such as O, N, F, Si, or S, keep in mind that covalent and ionic contributions vary independently. Hence, the mapping of trends is not a trivial exerci A-B Useful generalizations on covalent bonding This is a simple notion with very important consequences. It surfaces in OVerlap between orbitals of comparable energy is more effective the delocalized bonding which occurs in the competing anti(favored) than overlap between orbitals of differing energy syn (disfavored)E2 elimination reactions. Review this situation For example, consider elements in Group IV, Carbon and Silicon. We An anti orientation of filled and unfilled orbitals leads to better overlap know that C-C bonds are considerably stronger by Ca 20 kcal mol This is a corrollary to the preceding generalization than c-si bonds There are two common situatio ⊙⊙→(t(⑥( Case-1: Anti Nonbonding electron pair&C-x bond 土 X G'C-X lone pa 00 LUMO HOMO LUMO C-SP CSP. 土 cC-C H3C-CH3 BDE =88 kcal/mol H3C-SiH3 BDE -70 kcal/mol Case-2: Two anti sigma bonds Bond length =1.534 A Bond length= 1.87A This trend is even more dramatic with pi-bonds F"C-X C-C= 65 kcal/mol C-si= 36 kcal/mol Si-si= 23 kcal/mol 你8m 0 LUMO a Weak bonds will have corresponding low-lying antibonds Formation of a weak bond will lead to a corresponding low-lying antibonding orbital. Such structures are reactive as both nucleophiles electrophileA B A A C A C A Y C X A C X X X ·· lone pair HOMO s* C–X LUMO s* C–X LUMO lone pair HOMO C C C C C Si C-SP3 C-SP3 C-SP3 C Si Si-SP3 Y C C X A B A B Y C C B X D. A. Evans Bonding Generalizations Chem 206 ■ Weak bonds will have corresponding low-lying antibonds. p C–C = 65 kcal/mol p C–Si = 36 kcal/mol p Si–Si = 23 kcal/mol This trend is even more dramatic with pi-bonds: s* C–Si s* C–C s C–Si s C–C Bond length = 1.534 Å Bond length = 1.87 Å H3C–CH3 BDE = 88 kcal/mol H3C–SiH3 BDE ~ 70 kcal/mol Useful generalizations on covalent bonding When one compares bond strengths between C–C and C–X, where X is some other element such as O, N, F, Si, or S, keep in mind that covalent and ionic contributions vary independently. Hence, the mapping of trends is not a trivial exercise. Bond Energy (BDE) = Ecovalent + Eionic (Fleming, page 27) ■ Bond strengths (Bond dissociation energies) are composed of a covalent contribution ( Ecov) and an ionic contribution ( Eionic). better than For example, consider elements in Group IV, Carbon and Silicon. We know that C-C bonds are considerably stronger by Ca. 20 kcal mol-1 than C-Si bonds. ■ Overlap between orbitals of comparable energy is more effective than overlap between orbitals of differing energy. Formation of a weak bond will lead to a corresponding low-lying antibonding orbital. Such structures are reactive as both nucleophiles & electrophiles Better than Better than Case-2: Two anti sigma bonds s C–Y HOMO s* C–X LUMO s* C–X LUMO Case-1: Anti Nonbonding electron pair & C–X bond ■ An anti orientation of filled and unfilled orbitals leads to better overlap. This is a corrollary to the preceding generalization. There are two common situations. Better than For p Bonds: For s Bonds: ■ Orbital orientation strongly affects the strength of the resulting bond. Better than This is a simple notion with very important consequences. It surfaces in the delocalized bonding which occurs in the competing anti (favored) syn (disfavored) E2 elimination reactions. Review this situation. s C–Y HOMO