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Elemental of Organic Chemistry
CHM 201
Ms. Dang
CHAPTER 1 – COVALENT BODNING AND SHAPES OF MOLECULES
Organic compound
Examples
• is a compound made from carbon
atoms
• foods, flavors and fragment
• Medicines, toiletries and
cosmetics
• has one or more C atoms.
• has many H atoms.
• Plastics, films, fibers and
resins
• may also contain O, S, N, and
halogens
• All other living organisms
Where do we obtain organic molecules?
 Two principal ways:
◦ isolation from nature
◦ synthesis in the laboratory
Typical organic compounds
• have covalent bonds.
• are flammable.
• have low melting points.
• are soluble in nonpolar solvents.
• have low boiling points.
• are not soluble in water.
General Chemistry Review ( B&P 1.1 – 1.2)
Atomic number (Z) = number of protons = number of electrons (in a neutral atom)
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Orbitals: Space in which electron resides mass
An orbital can have 0, 1 and/or 2 e- (opposite spin – Paulii Exclusion Principle)
Shape is geometrically described s, p, d, f orbtials
Electrons (e-) are held in atomic orbtials
around the nucleus (s, p, d, f) and s orbital
are more stable (lower Energy) than p
orbital). Relative energies of atomic
orbital
Fluorine is the most electronegative
element (pulls electrons density toward
itself). Which is more electronegative, C
or O?
Na+ = 1s2 s2 2p6
O2- = 1s2 s2 2p6
All elements want to “look like” the Noble
gases (have same electronic
configuration). Atom is stable if it has a
filled valence shell (Octet’s Rule)
ns2np6
Electron Configuration for those main groups
H
B
C
N
O
F
Al
Si
P
S
Cl
Br
I
s1
s2p1
s 2 p2
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s 2 p3
s2 p 4
s 2 p5
Bonding:
Ionic bond: e- transferred (metal to nonmetal)
Covalent bonds: sharing e- ( nonmetal to nonmetal)
Polar Covalent bond arise if there is a difference in electronegativies between atoms
δ+
δ-
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Polarity of molecule depends on geometry (more in chapter 2)
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Method of Drawing Structures
Lewis Structure
Condensed Structure
Bond-line Drawings (B &P 3.1) (or stick figures) are short hand way to draw carbon
structures
 end points and intersections represent C atoms
 omit H’s attached to C’s
More examples
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Lewis Structures and Formal Charge
 represents how an atom’s valence
electrons are distributed in a
molecule
 Show the bonding involves (the
maximum bonds can be made)
 Try to achieve the noble gas
configuration
 The valence-shell electron-pair
repulsion (VSEPR) model tell us the
common angles about the carbon
atoms
Carbon normally forms four covalent bonds and has no unshared pairs of electrons
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E.g
Write a Dot Lewis structure for
CH3NH2
E.g Draw a dot Lewis structure and assign a formal charge to each atom (except
Hydrogen)
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Rules of Resonance (B&P 1.5)
1. Any compound for which more than one Lewis structure may be written is
accurately described by no single structure. The actual structure is a resonance
hydrid of them all (NOT “flipping back and forth” between resonance forms). The
various structures are called contributing structure or resonance forms
2. The stability of a resonance hybrid is greater than that which would be expected of
any of the contributing structures. The hybrid is therefore said to be stabilized by
resonance or resonance stabilized (by an amount of energy called the resonance
energy)
3. The greater the stability (the lower the energy) of a contributing structure, the
greater will be its contribution to the total structure of the hybrid
4. Resonance is the interaction of electrons in p orbitals. Only pi and nonbonded
electron density is reorganized in resonance (no σ bonds break). No atoms move
(no change in bond length, angles). Since electrons are not being added or removed,
there is no change in overall charge.
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Rules of Estimating Stability of Resonance Structures
1. The greater the number of filled octets, the greater the stability. You might notice
that a structure with more covalent bonds is more stable (since more atoms will
have complete octets).
2. The structure with the least (or no) separation of charge is more stable
3. Other things being equal, a structure with a negative charge on the more
electronegative element will be more stable. (Similarly, positive charge on the least
electronegative element are more stable)
4. Resonance forms that are equivalent have no difference in stability and contribute
equally
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Types of Resonance
Allyl carbanion-like resonance (lone pair next to a π bond (allylic lone pair)
Allyl carbocation-like resonance (vacancy, missing octet, next to a π bond)
Carbonyl-like resonance
Benzene-ring like resonance (Chapter 8)
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E.g Fill in the missing electrons then draw and rank resonance forms
Orbitals and Covalent Bonding (B&P 1.6)
How are the bonds in methane, CH4, formed?
Carbon’s atomic orbital (AOs) contain
_________valence electrons
Are those valence electrons identical? ___________
But, CH4 has four identical bonds. How can that
be?
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Hybridization describes the mixing of atomic orbitals to form special orbital for bonding.
In organic chemistry, our orbital mixtures will be simple combinations of valence
electrons in the 2s and 2p orbital on a single carbon atom.
We will mix these orbitals three ways to generate the three common shape of organic
chemistry: linear (2s + 2p), trigonal planar (2s+2p+2p) and tetrahedral
(2s+2p+2p+2p)
Two special names for covalent bonds of Organic molecules
Sigma (σ) bonds
Created when “head on” overlap occurs of
orbital
Pi (π) bonds
Created when “side on” overlaps occurs of
orbital (p orbitals)
Pi bonds are usually weaker than sigma bon. From the perspective of quantum mechanics,
this bond's weakness is explained by significantly less overlap between the component porbitals due to their parallel orientation. This is contrasted by sigma bond which form
bonding orbitals directly between the nucleus of the bonding atoms, resulting in greater
overlap and a strong sigma bond.
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Determining Hybridization
Examples
Regions
of edensity
Hybridi- s p p p
zation
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Geometry
(VSEPR)
3-D Sketch of Molecules (B&P 1.3, 1.6)
The Hybridization Model for Atoms in Molecules
The following molecules provide examples of all three basic shapes found in organic
chemistry. In these drawings, a simple line indicates a bond in the plan of the paper, a
wedged line indicates a bond coming out in front of the page and a dashed line indicated a
bond projecting behind the page
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E.g.
Fill in the missing electrons and provide 3D sketch of given molecule
Draw the Lewis formula of nitric acid, HNO3, then
- Assigned hybridization and formal charge
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