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【正文】 only two covalent bonds, leaving two lone pairs. ? The H–O–H bond angle is 176。, slightly smaller than the perfect tetrahedral angle () because of electronelectron repulsion between the lone pairs. 35 Molecular Orbital Theory ? A molecular orbital (MO): where electrons are most likely to be found (specific energy and general shape) in a molecule. ? The two (or more) atomic orbitals bine to make two (or more) molecular orbitals. ? Additive bination (bonding) MO is lower in energy. ? Subtractive bination (antibonding) MO is higher. 36 Molecular Orbitals in Ethene ? The ? bonding MO results from bining p orbital lobes with the same algebraic sign. ? The ? antibonding MO es from bining lobes with opposite signs. ? Only the bonding MO is occupied by electrons. 37 Summary ? Organic chemistry – chemistry of carbon pounds ? Atom: positively charged nucleus surrounded by negatively charged electrons ? Electrons occupy orbitals around the nucleus. ? Different orbitals have different energy levels and different shapes ? s orbitals are spherical, p orbitals are dumbbellshaped ? Covalent bonds electron pair is shared between atoms ? Valence bond theory electron sharing occurs by overlap of two atomic orbitals 38 Summary, cont’d ? Hybrid Atomic Orbital Theory electron sharing occurs by overlap of two orbitals formed by bining (hybridizing) two or more atomic orbitals (sp, sp2, sp3) ? Molecular orbital (MO) theory bonds result from bination of atomic orbitals to give molecular orbitals, which belong to the entire molecule ? Sigma (s) bonds Circular in crosssection and are formed by headon interaction ? Pi (?) bonds – “dumbbell” shape, from sideways interaction of p orbitals。 located above and below the s bond framework of the molecule 39 Summary, cont’d. ? Carbon uses hybrid orbitals to form bonds in anic molecules. ? In single bonds with tetrahedral geometry, carbon has four sp3 hybrid orbitals ? In double bonds with planar geometry, carbon uses three equivalent sp2 hybrid orbitals and one unhybridized p orbital ? Carbon uses two equivalent sp hybrid orbitals to form a triple bond with linear geometry, with two unhybridized p orbitals ? Atoms such as nitrogen and oxygen also hybridize to form strong, oriented bonds ? The nitrogen atom in ammonia and the oxygen atom in water are sp3hybridized 40 Quick Review ? Carbon ? One s and three p orbitals hybridize to form four sp3 orbitals ? In methane and ethane, C is tetrahedral, with ~176。 bond angles ? In ethene, One s and two p orbitals hybridize to form three sp2 orbitals. The bonds between the nuclei are the s bonds from the overlapped sp2 orbitals. The remaining p orbitals overlap sidetoside to form a ? bond. CC ? bonds are weaker than CC s bonds.
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