Chapter 1 Chemical Bonding and Chemical Structure

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1 Organic Chemistry, 5th ed. Marc Loudon Chapter 1 Chemical Bonding and Chemical Structure Eric J. Kantorowski California Polytechnic State University San Luis Obispo, CA

2 1.1 Introduc,on Chapter 1 Overview 1.2 Classical Theories of Chemical Bonding 1.3 Structure of Covalent Compounds 1.4 Resonance Structures 1.5 Wave Nature of the Electron 1.6 Electronic Structure of the Hydrogen Atom 1.7 Electronic Structure of More Complex Atoms 1.8 Another Look at the Covalent Bond: Molecular Orbitals 1.9 Hybrid Orbitals 2

3 Organic chemistry The branch of science that deals with compounds that contain carbon Vitalism: Only biological systems (e.g., plants, animals) could produce organic compounds Wohler s synthesis of urea (1828), began to undermine vitalism 1.1 Introduc6on 3

4 Why Study Organic Chemistry? Organic chemistry lies at the heart of the modern chemical industry Central to medicine and pharmacy Interface of physical and biological sciences Everyday applica,ons: Plas,cs, tex,les, communica,ons, transporta,on, food, clothing, cosme,cs, etc. 1.1 Introduc6on 4

5 Electrons in Atoms Valence shell: The outermost shell of electrons surrounding an atom Atoms gain or lose valence electrons to achieve noble gas configura,on Octet rule: Elements want eight valence electrons Chemistry happens because of the behavior of valence electrons! 1.2 Classical Theories of Chemical Bonding 5

6 Ionic Compounds Chemical compounds with ions present Ionic bonds are omni direc6onal Can dissociate into free ions 1.2 Classical Theories of Chemical Bonding 6

7 Covalent Bonds When two atoms share electrons Covalent bonds are direc6onal Lewis Structures use : and to represent bonds 1.2 Classical Theories of Chemical Bonding 7

8 Lewis Structures Nota,on for electron pair bonds Shared electrons (bonding) Unshared electrons (non bonding = lone pairs) Strive to sa,sfy the octet rule (and 2 for H) 1.2 Classical Theories of Chemical Bonding 8

9 Formal Charge The charge assigned to an atom Find valence # (= Group # of atom) Subtract # of unshared electrons Subtract half of the bonding electrons 1.2 Classical Theories of Chemical Bonding 9

10 The Polar Covalent Bond Uneven sharing of electrons in a bond ElectronegaDvity determines direc,on 1.2 Classical Theories of Chemical Bonding 10

11 ElectrostaDc PotenDal Maps A graphical depic,on of electron distribu,on 1.2 Classical Theories of Chemical Bonding 11

12 Dipole Moment (µ) Depends on charge separa,on and distance µ = qr (a vector quan,ty) 1.2 Classical Theories of Chemical Bonding 12

13 Bond Polarity and Formal Charge Formal charge is only a bookkeeping tool Formal charge may or may not correspond to actual charge! 1.2 Classical Theories of Chemical Bonding 13

14 Molecular Structure Atomic connecdvity: How atoms in a molecule are connected Molecular geometry: How far apart atoms are and how they are arranged in space Knowing atomic connec,vity and molecular geometry reveals the overall structure 1.3 Structures of Covalent Compounds 14

15 Methods for Determining Molecular Geometry X ray crystallography: Atomic arrangement of solids Electron diffracdon: Atomic arrangement of gaseous substances Microwave spectroscopy: Structural info of gaseous substances No comparable methods available for analysis of a liquid 1.3 Structures of Covalent Compounds 15

16 Molecular Geometry Molecular geometry influences chemical reac,vity Bond length: distance between nuclei Bond angle: angle between each pair of bonds 1.3 Structures of Covalent Compounds 16

17 Bond Length Increases with atoms in higher rows Decreases with increasing bond order Decreases toward higher atomic number along a row 1.3 Structures of Covalent Compounds 17

18 Bond Angle Bond angles determine shape Valence shell electron pair repulsion (VSEPR) Use molecular models! Line and wedge structures 1.3 Structures of Covalent Compounds 18

19 Tetrahedron Common Shapes to Know Trigonal Planar 1.3 Structures of Covalent Compounds 19

20 Common Shapes to Know Linear Trigonal pyramidal 1.3 Structures of Covalent Compounds 20

21 Dihedral Angle Also known as the torsional angle Rota,on can occur along single bonds 1.3 Structures of Covalent Compounds 21

22 Resonance Structures A single Lewis structure may not be enough More Lewis structures = more info on electron distribu,on (these are not in equilibrium!) The true electron distribu,on is a weighted average of the individual contributors Such structures are resonance stabilized 1.4 Resonance Structures 22

23 Quantum View of the Electron Covalent chemical bonds: The sharing of one or more electron pairs between two atoms This model is highly useful but has its limita,ons Quantum mechanics describes an alternate, more detailed view 1.5 Wave Nature of the Electron 23

24 Heisenberg Uncertainty Principle Impossible to know the posi,on and velocity of a par,cle Not important for large objects (cars, baseballs, and even molecules) Highly important for very small objects (electrons) The loca,on of an electron is fuzzy 1.5 Wave Nature of the Electron 24

25 Orbitals, Quantum Numbers, and Energy Atomic orbital: Describes the wave proper,es of an electron in an atom Each orbital is described by 3 quantum numbers (n, l, m l ) 1.6 Electronic Structure of the Hydrogen Atom 25

26 SpaDal CharacterisDcs of Orbitals 3 D orbital picture = 90% probability level Node: A defined region of no electron density 1s and 2s orbitals: 1.6 Electronic Structure of the Hydrogen Atom 26

27 SpaDal CharacterisDcs of Orbitals Three degenerate p orbitals (p x, p y, p z ) Orthogonal to one another 2p orbital: 1.6 Electronic Structure of the Hydrogen Atom 27

28 Electron ConfiguraDon AuXau principle: Buildup principle Electron spin (m s ): +1/2 (up) or 1/2 (down) 1.7 Electronic Structures of More Complex Atoms 28

29 Electron ConfiguraDon Hund s rule: place electrons into degenerate orbitals before pairing them Pauli exclusion principle: No two electrons can have the same four quantum numbers 1.7 Electronic Structures of More Complex Atoms 29

30 Molecular Orbital Theory Combining atomic orbitals (AOs) generates molecular orbitals (MOs) n AOs n MOs 1.8 Another Look at the Covalent Bond: Molecular Orbitals 30

31 Molecular Orbital Theory Adding AOs results in construc6ve interac,on Remember: Orbitals are described by mathema6cal func6ons they can be added and subtracted 1.8 Another Look at the Covalent Bond: Molecular Orbitals 31

32 Molecular Orbital Theory Subtrac,ng AOs results in destruc6ve interac,on Subtrac,ng an AO is the same as adding an AO of opposite phase 1.8 Another Look at the Covalent Bond: Molecular Orbitals 32

33 Orbital InteracDon Diagram 1.8 Another Look at the Covalent Bond: Molecular Orbitals 33

34 Bonding in Methane The MO picture of methane distributes electrons throughout the en,re molecule Total electron density in methane: 1.9 Hybrid Orbitals 34

35 ConstrucDng Methane A more useful (but less accurate) picture of bonding in methane requires hybrid orbitals The mixing process applied to AOs to make MOs is called hybridizadon 1.9 Hybrid Orbitals 35

36 Bonding in Methane Mixing s + p x + p y + p z Four sp 3 MOs 1.9 Hybrid Orbitals 36

37 Bonding in Methane Each sp 3 MO can then combine with a H atom 1.9 Hybrid Orbitals 37

38 Bonding in Ammonia The hybrid orbital picture accommodates molecules with lone pairs too (e.g., :NH 3 ) 1.9 Hybrid Orbitals 38

39 Bonding in Ammonia The hybrid orbital descrip,on of ammonia Note, the small rear lobes of each sp 3 orbital are commonly omioed for clarity 1.9 Hybrid Orbitals 39

Experiment 21 Lewis structures and VSEPR Theory

Experiment 21 Lewis structures and VSEPR Theory Introduction 1. Lewis Structures and Formal Charge LG.N. Lewis, at the University of California at Berkeley devised a simple way to understand the nature