Bonding and Isomerism Nanoplasmonic Research Group Organic Chemistry Chapter 1

Organic Chemistry How would you explain “Organic Chemistry” to your ~ ? The scientific study of the structure, properties, composition, reactions, and syntheses (preparation of chemical compounds that contain carbon) Nanoplasmonic Research Group

Why & How we study Organic Chemistry ? Why –Pervasive in nature –Chemical foundation of biology –Improve standard of living (medicine, plastics,…) How –Examine structure and analyze how it governs reactivity

Background Review: Atom Nucleus + Electrons Nucleus consists of Protons and Neutrons except H Atomic Number = # of Protons Atomic Weight = # of Protons + # of Neutrons (Electrons are comparably light) Nanoplasmonic Research Group

Electron Configuration and Valence Electrons Nanoplasmonic Research Group Electron redistribution (change in configuration) is the origin of chemical change WHY? Attain lower ENERGY Process of chemical bonding: Adding or taking away outer shell electrons to gain a closed shell configuration

Ionic vs. Covalent Bonding Ionic bonding –Why NaCl can be easily dissolved in water at Room Temperature even if it has a high melting point ? Covalent bonding –Bonding Energy: Energy stored in the bond –Bond length Nanoplasmonic Research Group

Basic Chemistry of Carbon Features –Four valence electrons –Neither to gain four electrons to be C4-nor lose all to be C4+ –Tend to share all electrons to make eight valence electrons around it Polar covalent bond arises due to the different electronegativities of atoms Please refer to Table 1.4 Nanoplasmonic Research Group

Valence and Isomerism Valence –Number of bonds that an atom of the element can form Isomerism –A molecular formula can’t give an information about how atoms in the structure are arranged, but a STRUCTURAL FORMULA CAN!!! –Isomers: The same numbers and kinds of atoms but DIFFERENT arrangement Nanoplasmonic Research Group

Formal Charge How to calculate –FC = number of valence electrons of the atom in isolation – lone pair electrons on this atom in the molecule – half the total number of electrons participating in covalent bonds with this atom in the molecule Nanoplasmonic Research Group

Resonance (I) Pls take a look at the carbonate ion Resonance structure –Identical arrangement of atoms but different arrangement of electrons How to arrange ALL ELECTRONS (valence electrons in the structure + borrowed electrons from somewhere Nanoplasmonic Research Group

Resonance (II) The basic principles –Keep the relative positions of all the component atoms the same – The same number of paired and unpaired electrons Evaluating contribution –The number of covalent bonds in a structure –Formal charge separation –Electronegativity of charge bearing atomes and charge density Nanoplasmonic Research Group

Why we have to care about Resonance ?? Lots of chemical reactions occur because of the attraction of one molecule containing a region of high electron density to a second molecule containing a region of low electron density +- To predict how and when two molecule will react, need to be able to predict the regions of low and high electron density

Molecular Orbital Theory Atomic Orbitals: Unhybridized orbitals on an atom (s, p, d) Linear Combination of Atomic Orbitals (LCAO): Individual wave functions (orbitals) combine to form hybrid atomic orbitals and molecular orbitals Hybrid Atomic Orbitals: Combination of atomic orbitlas from the same atom Molecular Orbitals: Combination of atomic orbitals from different atoms

Shapes of Orbitals Spherical or Dumbbell-like

How to combine each other (sigma bond)

How to combine each other (pi bond) Any differences between two ???

If all bonding occurred between simple s- and p- orbitals, then all bond angles would be ca. 90 o Valence Shell Electron-Pair Repulsion (VSPER) theory Nanoplasmonic Research Group Point is “Electrons repel each other”

Thanks to Lewis..but Lewis theory of chemical bonding –Electron-dot structure –Limited in explaining the 3-D geometries of molecules The orbital view of bonding –Valence Shell Electron-Pair Repulsion (VSPER) theory –Hybridization Nanoplasmonic Research Group

Please take a look at Figure 1.10 How would you think about this in terms of orbitals ?????? Hybridization came out!!!

Let’s classify organic molecules (I) With respect to Molecular Framework Nanoplasmonic Research Group Does it have only chains of carbon atoms ? Acyclic compounds Heterocyclic compoundsCarbocyclic compounds Does it have heteroatoms in the ring ? YESNO YES

With respect to Functional group –Please refer to Table 1.2 & Lecture Materials Nanoplasmonic Research Group Let’s classify organic molecules (II)