Dr M. Mehrdad University of Guilan, Department of Chemistry,

Rasht, Iran

References: 1. E. V. Anslyn, D. A. Dougherty, Modern Physical Organic

Chemistry, 2006. 2. F. A. Carey and R. J. Sundberg, Advanced Organic Chemistry,

5th Ed.Part A: Structure and Mechanisms, F.A., 2007. 3. J. M. Harris and C. C. Wamser, Fundamentals of Organic

Reaction Mechanisms, John Wiely & son LTD. New York, 1976.

4. N. S. Isaacs, Physical Organic Chemistry, 2nd Ed, Prentice Hall, 1996.

5. T. H. Lowry, K. S. Richardson, Mechanism and Theory in Organic Chemistry, 3rd Ed, Benjamin-Cummings, New York, 1987.

6. M. B. Smith and J. March, March’s Advanced Organic Chemistry Reactions, Mechnism and Structures, 5th ed. John Wiely & son LTD. New York, 2000.

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Chemical bonding

1.1 A review of basic bonding concepts

Solutions of Schrödinger equation:

Wavefunctions atomic or molecular orbitals

Principal quantum number: n = 1, 2, 3,..

Magnetic quantum number:

p orbital; -1, 0, 1 (px, py, pz),

d orbital; -2, -1, 0, 1, 2 (dxy, dxz, dyz, dz2, dx2-y2)

Spin quantum number: +1/2, -1/2

structure, stability and reactivity

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Principle Quantum Number

n= 1, 2, 3, 4…

indexes energy and size of orbital

lower n: closer to nucleus (on average), lower in energy

The principal quantum number

(n) must be a positive integer

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Azimuthal Quantum Number

s sub-shell: l = 0

p sub-shell: l = 1

d sub-shell: l = 2

f sub-shell: l = 3

l = 0, 1, 2, …n-1

Indexes shape of orbital

The azimuthal quantum number (l) can be zero or

any positive integer smaller than n

Courtesy Andy Washnik 5

Fig 6-16

Magnetic Quantum Number

ml = 0, 1, 2,... l

Indexes orientation of orbital

The number of allowed values of ml for

that particular value of l determines the

number of degenerate orbitals.

Degenerate: having equivalent energy

The magnetic quantum number (ml) can

have any positiven or negative integral

value between 0 and l.

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ms= +1/2 or - 1/2

Indexes spin direction of an e- within an orbital

Spin Orientation Quantum Number

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Electron configuration: C, 1s2 2s2 2p2 excitation; 1s2 2s1 2p3

Valence number: the number of bonds that an atom can make (e.g. C; 4, N; 3, O; 2, H; 1)

Octet rule: which states that atoms are most stable when their valence shell is full

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a notation that allows us to use the valence electrons of atoms in a molecule to predict the bonding in that molecule.

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such charges derive from the Lewis structure, and these full charges on atoms, denoting they are more of a formality than a reality.

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Example: HCN

Step 1. Count the total number of valence electrons

C has 4

N has 5

H has 1

4+5+1= Total of 10

Step 2. Place one e- pair between each BONDED atom

H C N

We have 10 - 4 = 6 e- left

_ _

All atoms must have an octet or duet

Building Lewis (ELECTRON DOT) Structures of Molecules

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Add 6 electrons in pairs to give the N an octet.

Step 3. Add remaining electrons to terminal atoms first

H C N

Step 4. Add any electrons left over to central atom

Do all atoms have an octet?

We have none left!

Step 5. Check for an acceptable Lewis Structure

IN THIS CASE

_ _

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Step 5. Check for an acceptable Lewis Structure

bring electron pairs from outer N atom to form shared pairs to give C its octet!!!

Still no octet on C Do it again!!!!

three electron pairs between the C and N………...

H C N _ _

H C N _ = H C N

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Another possible structure is….

H C N

How can we choose?

FORMAL CHARGE

H C N

H N C

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All possible Lewis structures with stable

electronic configurations for HCN and HNC.

Calculate formal charge for this one

FC on C = 4 - 0 - 1/2 (8) = 0

_ 1/2#shared electrons

{ } Formal charge =

#valence electrons

_ #unshared electrons { }

FC on N = 5 - 2 - 1/2 (6) = 0

Hydrogen is zero

H C N H N C

H C N

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All possible Lewis structures with stable

electronic configurations for HCN and HNC.

0 0 0 0 +1 -1

WE CHOOSE THE STRUCTURE WITH THE

FORMAL CHARGES CLOSEST TO ZERO

AND ANY NEGATIVE FORMAL CHARGES ON THE MOST ELECTRONEGATIVE ELEMENTS

H C N H N C

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(valence-shell electron-pair repulsion) rule: all groups emanating from an atom will be spatial positions that are as far apart from another as possible.

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the method of adding and substrating atomic orbitals on the same atom.

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C: 2s 2px 2py 2pz 4 sp3 4 equivalent bonds The highly directional sp3 orbitals provide for more effective overlap and stronger bonds

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The hybridization index:

The observed bond angle, q, is used in the equation below to solve for i, where the hybridization is spi

1 + i Cos q = 0

Thus, for sp3 hybridization, Cos q = -1/3 and q = 109.5o

Solving for q for H2O: i = -1/Cos (104.5) and i = 4.0

Thus sp4 is the hybrization for H2O, and the OH bonds have 80% p-character.

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the magnitude of 13C-H coupling constants are directly related to the fraction of s-character in the orbital bound to H.

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Two models for considering chemical bonding in organic molecules

- Valence bond theory (VBT): most atoms of the binding energy between the atoms at the most stable internuclear separation results from exchange (resonance) of electrons between the two nuclei. electrons are localized between specific atoms in a molecule.

- Molecular orbital theory (MOT): electrons are distributed among a set of molecular orbitals of discrete energies which can extend over the entire molecules.

The key ideas that are used to adapt the concepts of VBT to complex molecules are hybridization and resonance.

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