patino_chm2041_chapter10

patino_chm2041_chapter10 - Chapter 10 Chapter Bonding and...

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Unformatted text preview: Chapter 10 Chapter Bonding and Molecular Structure: Orbital Hybridization and Orbital Molecular Orbitals Molecular Goals Goals • Understand the differences between Understand valence bond theory and molecular orbital theory. theory. • Identify the hybridization of an atom in a Identify molecule or ion. molecule • Understand the differences between Understand bonding and antibonding molecular orbitals. orbitals. • Write the molecular orbital configuration Write for simple diatomic molecules. for Orbitals and Bonding Theories Orbitals VSEPR Theory only explains molecular shapes. It says nothing about bonding in molecules It In Valence Bond (VB) Theory (Linus Pauling) Valence atoms share electron pairs by allowing their atoms atomic orbitals to overlap. atomic Another approach to rationalize chemical Another bonding is the Molecular Orbital (MO) Theory (Robert Mulliken): molecular orbitals are spread out or “delocalized” over the molecule. out Valence Bond (VB) Theory Valence Covalent bonds are formed by the overlap of overlap atomic orbitals. atomic Atomic orbitals on the central atom can mix and Atomic exchange their character with other atoms in a molecule. molecule. Process is called hybridization. hybridization Hybrids are common: Pink flowers Pink Mules Mules Hybrid Orbitals have the same shapes as Hybrid predicted by VSEPR. predicted 1s 1s + H H σ bond bond 1s 1s + H H E σ bond bond 1s H + H H E H 1s H σ bond bond 2p + 2p F F σ bond bond F2 2p E 2s 1s 1s F F 2p E 2s 1s 1s F Methane Methane CH4 2p E 2s 1s C Methane Methane CH4 H H 2p E 2s 1s C Methane Methane CH4 H H+ E 2p 2s 1s H C Methane Methane CH4 H H+ E 2p 2s 1s H H– C Methane Methane CH4 H H+ E Z Y H H– X 2p H 2s H 1s C C 90° H 90° H The approach is not The correct, because… Methane Methane CH4 H 109.5° C H H H Tetrahedral Geometry 4 Identical Bonds Identical Problem and Solution Problem C must have 4 identical orbitals in must valence shell for bonding valence solution: hybridization (theoretical theoretical mixing of the four atomic orbitals of carbon atom, the 2s and the three 2p) 2s 2p Methane Methane CH4 2p E 2s 1s Methane Methane CH4 2s 2p E 2s 1s E 1s 2p Methane Methane CH4 2s 2p E 2s 1s E 1s 2p Methane Methane CH4 2s 2p E 2s 1s E 1s 2p Methane Methane CH4 2p E 2s 1s four sp3 orbitals E 1s + – 2p + + 2s – + + three 2p + 2s = = four sp3 hybrid orbitals 4 identical sp3 hybrid orbitals: they are four identical because there was the combination of one s and three p atomic orbitals (25% s, 75% p) and tetrahedral geometry tetrahedral Methane Methane CH4 H 2p E 2s 1s H H sp3 E 1s H Valence Bond (VB) Theory Valence Regions of High Regions Electron Density (BP+LP) (BP+LP) Electronic Electronic Geometry Geometry Hybridization, Angles(°) 2 3 Linear Trigonal Trigonal planar planar sp, 180 sp, 180 sp2 120 4 Tetrahedral 5 Trigonal Trigonal bipyramidal bipyramidal sp3 109.5 sp3d 120, 90, 180 6 Octahedral sp3d2 90, 180 Predict the Hybridization of the Central Atom Predict in aluminum bromide in •• • Br• •• Al • Br• • Br• ••• • ••• • Electron-pair shape 3 regions trigonal planar Hybridization: sp2 Trigonal Planar Electronic Geometry, sp2 Trigonal Electronic Structures: BF3 1s ↑↓ B 1s B ↑↓ F [He] ↑ 2s 2p ↑↓ ↑ 2s 2p 2p ↑↑ 2s 2p ↑↓ ↑↓ ↑↓ ↑ 1s ⇒ ↑↓ sp2 hybrid ↑↑ ↑ Trigonal Planar Electronic Geometry, sp2 Trigonal BF3 Predict the Hybridization of the Central Atom Predict in carbon dioxide in CO2 •• •• OCO •• •• 2 regions Electron-pair shape, linear Hybridization: sp (50% s, 50% Hybridization: p) p) Linear Electronic Geometry, sp Linear Electronic Structures: BeCl2 Be 1s ↑↓ 2s 2p ↑↓ Cl [Ne] 3s ↑↓ 3p ↑↓ ↑↓ ↑ 1s ⇒↑ ↓ sp hybrid ↑↑ Predict the Hybridization of the Central Atom in Beryllium Chloride Central Two regions: electron-pair shape sp hybridization Predict the Hybridization of the Central Atom in PF5 in Five regions: Trigonal Bipyramidal Electronic Five Geometry sp3d hybridization, five sp3d hybrid orbitals sp Predict the Hybridization of the Central Atom Predict in xenon tetrafluoride in Predict the Hybridization of the Central Atom Predict in xenon tetrafluoride in ••• F ••• ••• F ••• •• F• •• • • Xe •• • •• F • ••• 6 regions electron-pair shape octahedral Predict the Hybridization of the Central Atom Predict in xenon tetrafluoride in ••• F ••• ••• F ••• •• F• •• • • Xe •• • •• F • ••• 6 regions electron-pair shape octahedral sp3d2 hybridization Predict the Hybridization of the Central Atom in SF6 Atom Six regions: Octahedral Electronic Geometry Six - sp3d2 hybridization, sp six sp3d2 hybrid orbitals Consider Ethylene, C2H4 Consider Consider Ethylene, C2H4 Consider H H C C H H Consider Ethylene, C2H4 Consider H C C H 3 regions trigonal planar H H Consider Ethylene, C2H4 Consider H C C H H H 3 regions trigonal planar sp2 hybridization Consider Ethylene, C2H4 Consider H H C C H H 3 regions trigonal planar sp2 hybridization 2p E 2s 1s 1s 2s 2p E 2s 1s 1s E 1s 2p 2p E 2s 1s 1s sp2 E 1s 2p sp2 sp 2p sp2 sp2 2p sp2 sp2 sp sp2 σ bond framework bond π bond bond π bond bond Compounds Containing Double Bonds Compounds Thus a C=C bond looks like this and is made Thus of two parts, one σ and one π bond. Consider Acetylene, C2H2 Consider H C C H Consider Acetylene, C2H2 Consider H C 2 regions linear C H Consider Acetylene, C2H2 Consider H C C H 2 regions linear sp hybridization Consider Acetylene, C2H2 Consider H C C H 2 regions linear sp hybridization 2s 2p E 2s 1s 1s E 1s 2p 2p E 2s 1s 1s sp E 1s 2p 2p sp sp 2p sp σ bond framework bond π bonds bonds Compounds Containing Triple Bonds Compounds A σ bond results from the head-on overlap of head-on two sp hybrid orbitals. two The unhybridized p orbitals form two π bonds The (side-on overlap of atomic orbitals.) Note that a triple bond consists of one σ and two π bonds. π bonds bonds Generally Generally • single bond is a σ bond single • double bond consists of 1 σ and 1 π double bond bond • triple bond consists of 1 σ and 2 π triple bonds bonds Molecular Orbital (MO) Theory Molecular When atoms combine to form molecules, When atomic orbitals overlap and are then combined to form molecular orbitals. combined # of orbitals are conserved. A molecular orbital is an orbital associated molecular with more than 1 nucleus. with Like any other orbital, an MO can hold 2 Like electrons. electrons. Consider 2 hydrogen atoms bonding to form Consider H2 Molecular Orbital Theory Molecular • Combination of atomic orbitals on different atoms forms Combination molecular orbitals (MO’s) so that electrons in MO’s belong to the molecule as a whole. to • Waves that describe atomic orbitals have both positive Waves and negative phases or amplitudes. and • As MO’s are formed the phases can interact As constructively or destructively. constructively destructively Molecular Orbitals Molecular There are two simple types of molecular There orbitals that can be produced by the overlap of atomic orbitals. of Head-on overlap of atomic orbitals Head-on produces σ (sigma) orbitals. Side-on overlap of atomic orbitals Side-on produces π (pi) orbitals. Two 1s atomic orbitals that overlap produce two molecular orbitals designated as: two σ 1s or bonding molecular orbital σ 1s* or antibonding molecular orbital. + H H subtract add add subtract antibonding add add bonding subtract antibonding σ *1s add add bonding σ 1s Molecular Orbital Energy Level Diagram Molecular Now that we have seen what these MO’s look Now like and a little of their energetics, how are the orbitals filled with electrons? the Order of filling of MO’s obeys same rules as Order for atomic orbitals. for Including Aufbau principle: increasing energy Pauli’s Excluion: two unaligned e- per orbital, two with opposite spins (+1/2 and -1/2) with Hund’s Rule: maximum spin; unpaired electrons in degenerate orbitals have same electrons spin (+1/2 or -1/2) spin Thus the following energy level diagram results for the homonuclear diatomic molecules H2 and He2. σ *1s E E 1s 1s 1s σ 1s H H2 H σ *1s E E 1s 1s 1s σ 1s H H2 H σ *1s E E 1s 1s 1s σ 1s H H2 H σ *1s E E 1s 1s 1s σ 1s H H2 H (σ 1s ) 2 1s σ *1s E E 1s 1s 1s σ 1s H H2 H (σ 1s ) 2 1s total spin = 0 σ *1s E E 1s 1s 1s σ 1s H H2 H • Diamagnetic: slightly repelled by a Diamagnetic: magnetic field magnetic – total spin = 0 • paramagnetic: attracted to a magnetic field – total spin not 0 (bonding e– – antibonding e–) (bonding antibonding • Bond Order = Bond ──────────────────── ──────────────────── 2 Bond Order and Bond Stability Bond The larger the bond order, the more stable the The molecule or ion is. molecule Bond order = 0 implies there are equal numbers of Bond electrons in bonding and antibonding orbitals, electrons ~ same stability as separate atoms: no bond formed same no Bond order > 0 implies there are more electrons in Bond bonding than antibonding orbitals. bonding Molecule is more stable than separate atoms. The greater the bond order, the shorter the bond The length and the greater the bond energy. length (σ 1s ) 2 1s total spin = 0 diamagnetic σ *1s E E 1s 1s 1s σ 1s H H2 H BO = 1/2 ( 2 – 0) = 1 σ *1s E E 1s 1s 1s σ 1s H H2 H Consider He2 Consider σ *1s E E 1s 1s 1s σ 1s He He2 He σ *1s E E 1s 1s 1s σ 1s He He2 He (σ 1s ) 2 (σ *1s ) 2 1s 1s σ *1s E E 1s 1s 1s σ 1s He He2 He diamagnetic σ *1s E E 1s 1s 1s σ 1s He He2 He BO = 1/2 ( 2 – 2 ) = 0 He2 does not exist σ *1s E E 1s 1s 1s σ 1s He He2 He Combination of p Atomic Orbitals Orbitals Molecular Orbitals Molecular The head-on overlap of two corresponding p The atomic orbitals on different atoms, say 2p x atomic with 2px produces: with σ 2px bonding orbital σ 2px* antibonding orbital 2p 2p 2p subtract subtract add subtract antibonding MO add bonding MO bonding subtract antibonding MO σ *2p add bonding MO bonding σ 2p 2p Molecular Orbitals Molecular Side-on overlap of two corresponding p Side-on atomic orbitals on different atoms (say 2p y atomic with 2py or 2pz with 2pz) produces: with π 2p y π*2p y π 2p (both are bonding orbitals) or or z * or π 2p z (both are nonbonding orbitals) orbitals) 2p 2p 2p subtract subtract add subtract antibonding MO antibonding add bonding MO subtract π *2p add π 2p subtract π *2p add π 2p Consider Li2 Consider σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s Li σ 2s Li2 Li σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s Li σ 2s Li2 Li σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s Be σ 2s Be2 Be σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s Be σ 2s Be2 Be σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s B σ 2s B2 B σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s B σ 2s B2 B σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s C σ 2s C2 C σ *2p π *2p 2p E σ 2p 2p E π 2p σ *2s 2s 2s 2s N σ 2s N2 N Homonuclear Diatomic Molecules Homonuclear In shorthand notation we represent the In configuration of N2 as configuration N2 σ σ σ σ π 2 1s *2 1s 2 2s *2 2s 2 2 py π 2 2 pz σ 2 2p Bond Order of N2 Bond N2 σ σ σ σ π 2 1s *2 1s 2 2s *2 2s 2 2 py π 2 2 pz σ 2 2p The greater the bond order of a bond the The more stable we predict it to be. more For N2 the bond order is 10 - 4 bo = 2 6 = 2 = 3 corresponding to a triple bond in VB theory σ *2p π *2p 2p E π 2p 2p E σ 2p σ *2s 2s 2s 2s O σ 2s O2 O Homonuclear Diatomic Molecules Homonuclear In shorthand notation we represent the In configuration of O2 as configuration 2 *2 2 *2 2 2 2 * * O 2 σ1s σ1s σ 2s σ 2s π 2 p y π 2 p z σ 2 p x π 21 y π 2 1 z p p 10 - 6 bo = =2 2 We can see that O2 is a paramagnetic molecule (two unpaired electrons). molecule σ *2p π *2p 2p E π 2p 2p E σ 2p σ *2s 2s 2s 2s F σ 2s F2 F σ *2p π *2p 2p E π 2p 2p E σ 2p σ *2s 2s 2s 2s Ne σ 2s Ne2 Ne Bond Order for Ne2 Bond 2× 4 - 2 × 4 BO = ─────── = 0 BO ─────── 2 We can see that Ne2 is not stable. It does not Ne exist. exist. Delocalization and Shapes of Molecular Orbitals Orbitals Molecular orbital theory describes Molecular shapes in terms of delocalization of electrons. electrons Carbonate ion (CO32-) is a good example. VB Theory VB MO Theory Delocalization and Shapes of Molecular Orbitals Orbitals Benzene, C6H6, Resonance structure - VB theory Delocalization and Shapes of Molecular Orbitals Orbitals This is the picture of the valence bond This (VB) theory (VB) Delocalization and Shapes of Molecular Orbitals Orbitals The structure of benzene is described The well by molecular orbital theory. well ...
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This note was uploaded on 11/07/2011 for the course CHM 2045 taught by Professor Geiger during the Fall '08 term at University of Central Florida.

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