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11/30/2011 Chapter 10 Molecular Geometry VSEPR Model: Geometries Valence Shell Electron Pair Repulsion Theory • Electron pairs repel and get as far apart as possible • Example: Water – Four electron pairs – Farthest apart is a tetrahedron 1 11/30/2011 VSEPR Model: Geometries • Experimentally, we only observe the atoms and bonds between them. • Water has a bent geometry VSEPR Model: Geometries • Start with the Lewis structure • Count number of repelling groups around an atom – Bonds (single, double, triple) – Lone pairs • # repelling groups determines their arrangement 2 11/30/2011 VSEPR Model: Geometries # Repelling Groups Arrangement of Groups Approx. Bond Angles 2 Linear 180o 3 Trigonal 120o 4 Tetrahedral 109o 5 Trigonal Bipyramidal 120o, 90o 6 Octahedral 90o You should know the arrangement and the approximate bond angles. VSEPR Model: Geometries • What is the arrangement of groups and the geometry of each molecule below? 3 11/30/2011 VSEPR Model: Geometries • XeF4 • Arrangement: octahedral • Geometry: square planar VSEPR Model: Geometries • Acetone 4 11/30/2011 VSEPR Model: Geometries • Ethanol VSEPR Model: Bond Angles • We can make better estimations of bond angles – Some groups repel better than others • Repelling power: – Lone pair > triple bond > double bond > single bond • Compare CH4, NH3, and H2O – Arrangement of groups is tetrahedral in all cases – Bond angles differ slightly 5 11/30/2011 VSEPR Model: Bond Angles Double bonds have more repelling power than single bonds. Lone pairs are even better “repellers” VSEPR Model: Bond Angles • Draw the Lewis structure of each molecule below. • Draw a picture of each molecule, showing its geometry and predicting approximate bond angles • CH2CH2 • O3 • IF3 6 11/30/2011 Valence Bond Theory • Quantum mechanical theory of bond formation • Extension of Lewis theory – Lewis: Atoms share a pair of electrons to form bond – VB: Orbitals of atoms overlap to form bonds • Only orbitals containing unpaired valence electrons • Example – HF – Unpaired valence electrons: H = 1s F = 2p Valence Bond Theory • Advantage of VB Theory over Lewis – Explains why covalent bond is formed – Increased electron density between positive nuclei – Nuclei are attracted to electrons between them 7 11/30/2011 Sigma Bonds • These bonds are classified as sigma bonds • Rotational symmetry about internuclear axis Hybrid Orbitals • Our model of bond generation – – – – 2 electrons per bond Only unpaired electrons involved All atoms get full octet of electrons Bonds formed by overlap of atomic orbitals • Seems to conflict with carbon – 1s22s22p2 – If we use only unpaired e-, how many bonds predicted? – How many bonds do we know C always forms? 8 11/30/2011 Hybrid Orbitals • Carbon always forms 4 bonds • Explanation – Atom forms new orbitals in process of forming bonds 1s 2s 2p sp3 1s – New orbitals are called hybrid orbitals – Orientation of hybrid orbitals is same as VSEPR group arrangements Hybrid Orbitals • Assume that hybrid orbitals always are formed before bonding • Can predict hybridization of central atom from arrangement of e- pairs Arrangement of e- pairs Hybridization 2 sp 3 sp2 4 sp3 5 sp3d 6 sp3d2 9 11/30/2011 Hybrid Orbitals • Example: CH4 (methane) • Lewis structure: • Four repelling groups – Tetrahedral geometry – sp3 hybridization – Four hybrid orbitals • Four sigma bonds Hybrid Orbitals • Predict hybridization of central atoms in following molecules: • O in H2O • Two C’s in • S CH3CHO in SF6 10 11/30/2011 Multiple Bonds • Example: Ethylene • Each C has 3 repelling groups – sp2 hybrid 1s sp2 2p • sp2 hybrids form σ-bonds with H atoms and other C atom Multiple Bonds • What about the remaining unpaired e- in the 2p orbital on each C atom? • Parallel p-orbitals overlap and form a π-bond 11 11/30/2011 Multiple Bonds • Single bond: • Double bond: • Triple bond: one σ one σ and one π one σ and two π • How many σ and π bonds in CH2=C=CH2? • Check blog. I will add a question about this molecule. Sigma and Pi Bonds • Electron density in σ-bonds is concentrated between nuclei • Electron density in π-bonds is above and below nuclei • Electrons in a π-bond are less tightly held than those in a σ-bond 12 11/30/2011 Sigma and Pi Bonds • A π-bond is weaker than a σ-bond – A double bond is stronger than a single bond, but not twice as strong • A π-bond is more reactive than a σ-bond – Example: Bromination of 2-butene Sigma and Pi Bonds • Alkanes (CnH2n+2) – Example: n-Butane: C4H10 – All bonds are sigma: strong and not readily broken – Alkanes are not very reactive – Used as solvents and fuels • Reactivity of organic compounds mainly dependent on multiple bonds and functional groups CH3CH2-OH CH3-NH2 13 11/30/2011 Dipole Moments • Polar bonds have unequal sharing of electrons • The dipole moment is a measure of the polarity – High dipole moment = high polarity Dipole Moments • Not all bonds are polar • Identical atoms: F – F – Completely nonpolar • Atoms with similar electronegativities: C – H – ΔEN ≤ 0.4 – Dipole moment is very small Bond is essentially nonpolar 14 11/30/2011 Molecular Dipole Moment • Molecules have dipole moments – Resultant of dipole moments of bonds • Example: water – O–H bond: EN difference = 3.5 – 2.1 = 1.4 Polar – Two polar bonds point in same direction • Reinforce each other – Water molecule is polar (has a significant dipole moment) Molecular Dipole Moments • For a molecule to be polar – It must have polar bonds – Its geometry must not balance • If all bonds are nonpolar or essentially nonpolar, then the molecule is nonpolar. – CH3CH2CH3 • C-C bond: Completely nonpolar • C-H bond: ΔEN = 2.5 – 2.1 = 0.4 Essentially nonpolar • The molecule is nonpolar 15 11/30/2011 Molecular Dipole Moment • Polar bonds do not necessarily mean a molecule is polar – Polar bonds can balance each other • Example: CO2 • The dipole moment of CO2 is zero Molecular Dipole Moment • If all bonds in an arrangement (linear, trigonal, tetrahedral, etc) are identical, then the molecule will be nonpolar • Example: CF4 • C-F electronegativity difference – ΔEN = 4.0 – 2.5 = 1.5 Polar bond • Identical bonds in all positions – Dipole moments of bonds will balance – Molecule has zero dipole moment 16 11/30/2011 Molecular Dipole Moment • Rank these isomers in order of increasing dipole moment Molecular Dipole Moment • Polar bonds are local properties of a molecule • Overall polarity of molecule depends on relative significance of polar and nonpolar groups • Which of these molecules would be more polar? – CH3CH2CH2CH2NH2 – CH3CH2NH2 17 11/30/2011 End of material for Exam 4 Molecular Orbital Theory • Molecular orbital theory – Looks at molecule as a whole – All orbitals of atoms in molecule combine to form molecular orbitals • MO theory problems from text – 52, 54, 55, 56, 58, 63, 64, 65, 68, 107 18 11/30/2011 Interference of Waves • Constructive – In phase • Destructive – Out of phase • Noninteracting – Physically separated Molecular Orbital Theory • Atomic orbitals combine to form molecular orbitals • Three ways they can combine – Antibonding: destructive interference • Destabilizes molecule – Nonbonding: non-interacting • Neither stabilizes or destabilizes molecules – Bonding: constructive interference • Stabilizes molecule 19 11/30/2011 Molecular Orbital Theory • Atomic orbitals combine to form molecular orbitals • # molecular orbitals = # atomic orbitals • 2 atomic orbitals => 2 molecular orbitals – One bonding, one antibonding • 3 atomic orbitals => 3 molecular orbitals – One bonding, one nonbonding, one antibonding MO Theory: H2 Molecule • H2 molecule – Combine two 1s atomic orbitals (one from each H atom) – Get one bonding and one antibonding molecular orbital • Bonding = σ; antibonding = σ* 20 11/30/2011 MO Theory: H2 Molecule • Feed electrons from atomic orbitals into mo’s – Each H atom: 1 e– H2 molecule: 2 e- MO Theory: Bond Order • Bond order: indicator of bond strength in molecule • Bond order = ½(# bonding e- - # antibonding e-) – – – – BO = 0 BO = 1 BO = 2 Etc. Molecule not stable Single bond Double bond • What is bond order for H2 molecule? 21 11/30/2011 MO Theory: He2 • Does MO theory predict that He2 will be stable? • What about He2+? MO Theory: Homonuclear Diatomics • There are higher MO energy levels • Li2, B2, C2, N2 O2, F2 22 11/30/2011 MO Theory: Homonuclear Diatomics • What is the bond order of the carbide ion, C22- ? – Use Li2, B2, C2, N2 energy level diagram – Ten electrons • Four from each carbon, two extra from -2 charge MO Theory: Homonuclear Diatomics • Predict the magnetism of N2 and O2. – Dia- or paramagnetic? • http://www.youtube.com/watch?v=yJs5ENtilIo 23 11/30/2011 MO Theory: Homonuclear Diatomics • Make predictions about O2 molecule and ions • Which has the strongest bond: O2,O2+, or O2-? Why Use MO Theory? • MO theory is used most of the time to calculate properties of large molecules – Predicts energy levels, so can be compared with electronic spectroscopy experiments – Better at predicting magnetic properties – Better method of describing molecules for which electrons are shared by more than two atoms 24 11/30/2011 MO Theory: Ozone • O3: two resonance hybrids • Each oxygen is sp2 hybridized – Leaves one extra p-orbital on each oxygen • Focus on these p-orbitals MO Theory: Ozone • Three p-orbitals • Combine to form three molecular orbitals – Antibonding – Nonbonding – Bonding 25 11/30/2011 MO Theory: Ozone • Bonding molecular orbital is most important • This is a delocalized molecular orbital – Spread over three atoms – Contains 2 electrons Delocalized Molecular Orbitals • Compounds that must be described by resonance • Example: carbonate ion • The lowest bonding molecular orbital usually is delocalized 26 11/30/2011 Delocalized Molecular Orbitals • Benzene, C6H6, is most important example • Lowest energy bonding molecular orbital is doughnut-shaped, above and below ring Most common symbol for Benzene is below. Molecules with delocalized molecular orbitals generally are more stable than molecules with localized double bonds 27