Bonding and Isomerism

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Organic Chem-istry

Chapter 1

Organic Chemistry

• How would you explain “Organic Chemistry” to your ~ ?

• The scientific study of the structure, properties, composition, reac-tions, and syntheses (preparation of chemical compounds that contain carbon)

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Organic Chemistry is Rational and Sys-tematic !!!

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 Neu-

trons except H• Atomic Number = # of Protons• Atomic Weight = # of Protons + # of

Neutrons (Electrons are comparably light)

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Electron Configuration and Valence Electrons

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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 elec-

trons to gain a closed shell configuration

Ionic vs. Covalent Bonding

• Ionic bonding –Why NaCl can be easily dis-

solved in water at Room Tem-perature even if it has a high melting point ?

• Covalent bonding– Bonding Energy: Energy stored

in the bond– Bond length

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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

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Valence and Isomerism

• Valence– Number of bonds that an atom of the el-

ement can form

• Isomerism– A molecular formula can’t give an infor-

mation about how atoms in the structure are arranged, but a STRUCTURAL FOR-MULA CAN!!!

– Isomers: The same numbers and kinds of atoms but DIFFERENT arrangement

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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 co-valent bonds with this atom in the mole-cule

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Resonance (I)

• Pls take a look at the carbonate ion

• Resonance structure– Identical arrangement of atoms but different ar-

rangement of electrons

• How to arrange ALL ELECTRONS (valence elec-trons in the structure + borrowed electrons from somewhere

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Resonance (II)

• The basic principles– Keep the relative positions of all the compo-

nent 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

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Why we have to care about Reso-nance ??

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 (or-bitals) combine to form hybrid atomic or-bitals 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. 90o

• Valence Shell Electron-Pair Repulsion (VSPER) theory

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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

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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

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Does it have only chains of carbon atoms ?

Acyclic compounds

Heterocyclic compounds Carbocyclic compounds

Does it have heteroatoms in the ring ?

YES NO

NOYES

• With respect to Functional group– Please refer to Table 1.2 & Lecture Mate-

rials

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Let’s classify organic molecules (II)