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Chapter 9 Chemical Bonding and Molecular Structure
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Chapter 9 Chemical Bonding and Molecular Structure

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  1. Chapter 9 Chemical Bonding and Molecular Structure http://www.youtube.com/watch?v=BVvvx5HGpLg&feature=related http://www.youtube.com/watch?v=KSasTS-n_gM&feature=related

  2. Chapter 7 Chapter 8 Chapter 9 Chapter 11 Chapter 22

  3. Chapter 9: Chemical Bonding and Molecular Structure Sections 9.1 to 9.6 and part of Section 9.7 (up to p 376) Overall Goals: Chemists gain information about the 3-dimensional shape of molecular structures using experimental data. Chemists then use quantum theory to explain these observed shapes and bonding patterns. With the information from this chapter, you will be able to: describe the shape (geometry) of small molecules, and identify the dipoles in those molecules describe the two fundamental bonding theories (valence bond theory and molecular orbital theory) and use them to interpret the chemical and physical properties of chemical species describe the nature of single, double and triple bonds in simple organic compounds

  4. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY VSEPR Valence Shell Electron Pair Repulsion Model is used to predict the shapes of covalently bonded molecules or polyatomic ions. An atom surrounded by 2 to 6 pairs of electrons -the pairs of electrons repel each other and position themselves as far apart as possible. Five Basic Arrangements

  5. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: In reality an atom of interest is not necessarily surrounded by electron pairs it is surrounded by groups of electrons. Non- bonding domains Bonding domains (BP) The basic arrangements of the VSEPR model still apply BUT in the presence of non-bonding domains -names will change as only atoms define shape -angles will change based on the strength of repulsions

  6. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: Linear Species All atoms lie in a The angle between three atoms is Examples – BeCl2, N3-, CO2, HCN Planar Triangular Species A central atom is surrounded by Bond angle: Examples – BCl3, CO32-resonance considerations Bent Species with a Planar Triangular Geometry A central atom is bonded Bond angle: Examples – SnCl2 , NO2- resonance considerations

  7. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: Tetrahedral Species A central atom surrounded by Bond angles: Examples – CH4, PO43- resonance considerations Trigonal Pyramidal Species with a Tetrahedral Geometry A central atom surrounded by Bond angles: Examples – NH3,IO32- resonance considerations Bent Species with a Tetrahedral Geometry A central atom surrounded by Bond angle: Examples – H2O

  8. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: Trigonal bipyramidal Molecules A central atom surrounded The shape contains two trigonal pyramids. The bond angle between two equatorial bonds is The bond angle between two axial bonds is The bond angle between an axial and equatorial is Example: PCl5, Seesaw or Distorted Tetrahedral with a Trigonal bipyramidal Geometry A central atom surrounded by Bond angles: Examples – SF4

  9. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: T-shaped with a Trigonal bipyramidal Geometry A central atom surrounded by Bond angles: Examples – ClF3 Linear with a Trigonal bipyramidal Geometry A central atom surrounded by Bond angle: Examples – I3- but notice x-ray needed to know “best” Lewis

  10. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY SIMPLE molecules or polyatomic ions: Octahedral Molecules Contains two square pyramids. Can again visualize axial and equatorial. Bond angles between opposite groups are between adjacent groups are Example: SF6, [Co(NH3)6]3+ Square pyramidal with an Octahedral Geometry A central atom surrounded by Bond angles: Example: XeOF4, BrOF4- Square planar with an Octahedral Geometry A central atom surrounded by Bond angles: Example: BrF2O2+

  11. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY MORE COMPLEX molecules: Those with more than one atom surrounded by at least two other atoms The shape around EACH non-central atom is considered.

  12. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY MORE COMPLEX molecules: If an “overall” shape or characteristic of shape is applicable chemists often state that http://mt.seas.upenn.edu/Archive/Graphics/A/gallery.html http://qsad.bu.edu/curriculum/pdbfiles/index.html

  13. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY MORE COMPLEX molecules: Usually chemists zero in on the 3D structure for portion(s) of more complex molecules that is/are “reactive” site(s). http://loschmidt.chemi.muni.cz/enantis/publicity/G2B_230806.htm http://www.pdg.cnb.uam.es/cursos/hola/pages/Farmac_prac/ CMU_course/BuildBlocks/AAViewer/AAPDBs/?C=M;O=A

  14. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY Examples of the importance - helps explain the existence http://web99.arc.nasa.gov/~astrochm/chirality.jpg

  15. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING SHAPE AND GEOMETRY Examples of the importance - often considered to explain the lock and key nature of enzymes and their various substrates

  16. Chapter 9: Chemical Bonding and Molecular Structure Sections 9.1 to 9.6 and part of Section 9.7 (up to p 376) Overall Goals: Chemists gain information about the 3-dimensional shape of molecular structures using experimental data. Chemists then use quantum theory to explain these observed shapes and bonding patterns. With the information from this chapter, you will be able to: describe the shape (geometry) of small molecules, and identify the dipoles in those molecules describe the two fundamental bonding theories (valence bond theory and molecular orbital theory) and use them to interpret the chemical and physical properties of chemical species describe the nature of single, double and triple bonds in simple organic compounds

  17. F H Chapter 9: Chemical Bonding and Molecular Structure DIPOLES The distribution of electron density between atoms (bond dipoles) coupled with the 3D arrangement of atoms allows one visualise electron rich and electron poor regions. From this is predicted the separation of charge within a molecule (molecular dipole).

  18. Chapter 9: Chemical Bonding and Molecular Structure DIPOLES There are many molecules that have no molecular dipole even though they contain polar bonds. In such cases the molecules are electronically “balanced” or symmetric and usually all of the domains (i.e. groups) attached to them are identical.

  19. Chapter 9: Chemical Bonding and Molecular Structure DIPOLES When all atoms attached to a central atom are not the same the molecule may be electronically “unbalanced” or dissymmetric then

  20. Chapter 9: Chemical Bonding and Molecular Structure DIPOLES If the basic shapes (i.e. linear, trigonal planar, tetrahedral) can be found in more complicated molecules (e.g. trigonal bipyramidal or octahedral) even if all the atoms in these molecules are not identical, such molecules may be non-polar as long as the atoms that comprise the basic shapes lead to an electronically “balanced” structure.

  21. Chapter 9: Chemical Bonding and Molecular Structure DIPOLES When non-bonding domains (lone pairs) are present and bond dipoles do not cancel polar molecules may result When non-bonding domains (lone pairs) are present and bond dipoles cancel non-polar molecules may result Lone pairs will always influence the magnitude of a molecular dipole but this will not be explored in any depth in 201.

  22. Chapter 9: Chemical Bonding and Molecular Structure DIPOLES Examples of the importance - helps explain what happens to molecules when they are in a magnetic field MRI - helps rationalize interactions between molecules (Chapter 11) and hence reactivity/mechanisms

  23. Chapter 9: Chemical Bonding and Molecular Structure Sections 9.1 to 9.6 and part of Section 9.7 (up to p 376) Overall Goals: Chemists gain information about the 3-dimensional shape of molecular structures using experimental data. Chemists then use quantum theory to explain these observed shapes and bonding patterns. With the information from this chapter, you will be able to: describe the shape (geometry) of small molecules, and identify the dipoles in those molecules describe the two fundamental bonding theories (valence bond theory and molecular orbital theory) and use them to interpret the chemical and physical properties of chemical species describe the nature of single, double and triple bonds in simple organic compounds

  24. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES • All theories require orbitals that allow spin paired electrons to be shared between atoms. • Differ in how the model is constructed Valence Bond (VB) Theory • Imagines individual atoms each with own orbitals and electrons coming together to form the bonds of a molecule. Molecular Orbital (MO) Theory • Not concerned with how molecule is formed but looks at the result • a collection of positive nuclei surrounded by electrons in molecular orbitals

  25. Chapter 9: Chemical Bonding and Molecular Structure Valence bond (VB) theory BONDING THEORIES A given atomic orbital can only overlap with one other orbital on a different atom i.e. a given atomic orbital can only form one bond with an orbital on one other atom. H2 molecule formation HF molecule formation

  26. Chapter 9: Chemical Bonding and Molecular Structure Hybrid orbitals are often required to explain molecular geometries and so are assumed for the central atom BONDING THEORIES Valence bond (VB) theory Hybridizationis the mixing of two or more atomic orbitals to form a new set of orbitals. • Mix at least 2 non-equivalent atomic orbitals. • Hybrid orbitals have very different shape from original atomic orbitals. • All hybrid orbitals are equivalent in every respect except in relative orientation. • Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. • Hybrid orbitals are named based on the atomic orbitals that were mixed together • Adopt VSEPR geometries with respect to one another

  27. Chapter 9: Chemical Bonding and Molecular Structure Hybrid orbitals BONDING THEORIES Valence bond (VB) theory sp hybrid orbitals

  28. Chapter 9: Chemical Bonding and Molecular Structure Hybrid orbitals BONDING THEORIES Valence bond (VB) theory sp2 hybrid orbitals sp3 hybrid orbitals

  29. Chapter 9: Chemical Bonding and Molecular Structure Valence bond (VB) theory BONDING THEORIES • Covalent bonds will form when • hybrid orbitals overlap with atomic orbitals • hybrid orbitals overlap with other hybrid orbitals In the molecule BeCl2each Be-Cl bond is formed by the overlap of a Be orbital and a Cl In the molecule BF3each B-F bond is formed by the overlap of a B orbital and a F

  30. Chapter 9: Chemical Bonding and Molecular Structure Hybridization and molecules with an Expanded Octet BONDING THEORIES Valence bond (VB) theory sp2d hybrid orbitals sp3d2 hybrid orbitals d orbitals are needed to provide room for the extra electrons One d orbital is added for each pair of electrons in excess of the standard octet Hybrid orbitals formed again adopt VSEPR geometries with respect to one another

  31. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Valence bond (VB) theory Hybridization and electron configurations 31

  32. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Valence bond (VB) theory Two types of bonds • sigma bonds • pi bonds

  33. Chapter 9: Chemical Bonding and Molecular Structure Valence bond (VB) theory BONDING THEORIES Let’s look at more complex molecules Each C bonded to two atoms - sp hybridization sigma framework made up of the overlap of the unhybridized H 1s orbitals with sp hybridized C orbitals and the overlap between sp hybridized C orbitals pi framework made up of the overlap between the unhybridized C 2p orbitals

  34. Chapter 9: Chemical Bonding and Molecular Structure Valence bond (VB) theory BONDING THEORIES Let’s look at more complex molecules Recall: Compare:

  35. Chapter 9: Chemical Bonding and Molecular Structure Valence bond (VB) theory BONDING THEORIES Let’s look at more complex molecules Each C bonded to three atoms - sp2 hybridization sigma framework made up of the overlap of the unhybridized H 1s orbitals with sp2 hybridized C orbitals and the overlap between sp2 hybridized C orbitals pi framework made up of the overlap between the unhybridized C 2p orbitals

  36. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Valence bond (VB) theory Where it fails – e.g. O2

  37. Chapter 9: Chemical Bonding and Molecular Structure Sections 9.1 to 9.6 and part of Section 9.7 (up to p 376) Overall Goals: Chemists gain information about the 3-dimensional shape of molecular structures using experimental data. Chemists then use quantum theory to explain these observed shapes and bonding patterns. With the information from this chapter, you will be able to: describe the shape (geometry) of small molecules, and identify the dipoles in those molecules describe the two fundamental bonding theories (valence bond theory and molecular orbital theory) and use them to interpret the chemical and physical properties of chemical species describe the nature of single, double and triple bonds in simple organic compounds

  38. Chapter 9: Chemical Bonding and Molecular Structure Molecular orbital theory explains bonding as constructive interference of atomic orbitals BONDING THEORIES Molecular Orbital (MO) theory • Treats a molecule like an atom with more than one positive center • Like an atom a molecule has orbitals, they are called molecular orbitals • Molecular orbitals are considered to be formed by the constructive and destructive interference of overlapping electron waves representing the VALENCE atomic orbitals of the atoms in the molecule.

  39. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory Types of Orbital Overlaps and Molecular Orbitals YOU need to know

  40. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams – H2 and He2

  41. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams - Li2 through N2

  42. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams - O2 and F2

  43. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams – comparing all 2nd row homonuclear diatomics Li2 through N2 O2 and F2

  44. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams - filling in electrons for 2nd row homonuclear diatomics •  Electrons fill the lowest-energy orbitals that are available. • No more than two electrons, with spins paired, can occupy any orbital. • Electrons spread out as much as possible, with spins unpaired, over orbitals that have the same energy.

  45. bond order = Number of electrons in bonding MOs Number of electrons in antibonding MOs ) ( - 1 2 Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory MO Diagrams – important info

  46. Chapter 9: Chemical Bonding and Molecular Structure BONDING THEORIES Molecular Orbital (MO) theory Application of Bond Order Explaining the relative stability of species H2+ H2 He2+ He2 bond order 46

  47. Chapter 9: Chemical Bonding and Molecular Structure Sections 9.1 to 9.6 and part of Section 9.7 (up to p 376) Overall Goals: Chemists gain information about the 3-dimensional shape of molecular structures using experimental data. Chemists then use quantum theory to explain these observed shapes and bonding patterns. With the information from this chapter, you will be able to: describe the shape (geometry) of small molecules, and identify the dipoles in those molecules describe the two fundamental bonding theories (valence bond theory and molecular orbital theory) and use them to interpret the chemical and physical properties of chemical species describe the nature of single, double and triple bonds in simple organic compounds

  48. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING THE NATURE OF BONDS In 201: For simple organic molecules use VB Theory For simple homonuclear diatomics use MO Theory H2

  49. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING THE NATURE OF BONDS In 201: For simple organic molecules use VB Theory For simple homonuclear diatomics use MO Theory Formaldehyde

  50. Chapter 9: Chemical Bonding and Molecular Structure DESCRIBING THE NATURE OF BONDS In 201: For simple organic molecules use VB Theory For simple homonuclear diatomics use MO Theory