Lecture Learning Objectives
Chem 2400
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What is the field of organic chemistry and organic compounds?
General Chemistry Review:
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What is a bond?
Electronegativity and Bond Polarity
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How do we draw/represent organic structures?
o
o
o
o
o
o
o
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Molecular Geometry
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Effects of hybridization
Formal charges
Lewis structures
Kekule
Condensed
Skeletal
Ball and Stick
Space Filling Models
Electrostatic Potential Maps
σ and π bonds
o Bond lengths
o Bond strengths
o Bond Angles
Hybrid orbitals
Chem 2400
What is Organic Chemistry?
Short Answer: the study of organic compounds
compounds/molecules that contain carbon
These molecules construct the basis of bodies
and make life possible but also:
- the food we eat
- the clothing we wear
- major energy sources we use
- plastics
- fabrics
- rubbers
- therapeutics
......and make up many other
resources we use in our daily lives
densely carbon based molecules
the second row of the periodic table
Taxol - a chemotherapuetic
isolated from the Pacific Yew tree
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Review: Bonding and Dipole Moment
Chem 2400
What is a covalent bond? Sharing of electrons between to atoms
- Sharing of electrons may not be equal
E.g.
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Electronegativity: measure of ability of an atom to pull
electrons toward itself
Nonpolar covalent bond: bonded atoms are the same or
have similar electronegativities
Polar covalent bond: bonded atoms have different
electronegativities
Ionic bond: electrons are not shared between atoms. The
resulting ions are held together by electrostatic attraction.
Electrostatic Potential Maps
Dipole moment: size of the charge x the distance between charges
e.g.
= 1.2 D
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Review: Rule of 4 and Formal Charges
Chem 2400
Each type of atom can make a finite # of bonds based on its # of available valance electrons
Recall, when neutral
Another note on bonding – Formal Charges (accounting for electrons)
Formal Charge = # of valence electrons -
(# of bonds + # lone-pair electrons)
Common formal charges on Carbon, Oxygen, Nitrogen, and Halogens
Practice: The structure for molecule called azide (N3), drawn below, is incorrect. Explain Why?
Azide
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Representations of Structural Formulas
Chem 2400
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Organic chemist represent organic
structures in many different ways.
Some for ease and speed.
Others can highlight connections between
atoms or can showcase forces or space
between atoms
Lewis Structures:
Dot structures:
Kekulé Structures:
Dash structures:
Lewis dash structure with lone pairs omitted
Condensed Structures:
Skeletal Structures:
Like Kekule but omit (most) covalent bonds
NH2
Cl
OH
O
Show the carbon-carbon bonds as lines, but do not show the carbons or the hydrogens that
are bonded to the carbons
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Chem 2400
Practice on Drawing Structures
Convert the following Lewis dash structure into a skeletal structure:
How many hydrogen atoms are represented in each of the naturally occuring molecules shown
below?
OHC
O
O
HO
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Molecular Orbitals and Hybrids
Chem 2400
Recall:
• Atomic orbital: volume of space around
an atom’s nucleus where an electron is
likely to be found
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Molecular orbital: volume of space
around a molecule where an electron is
likely to be found
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σ-bond (sigma): Overlap between to atomic
orbitals (s + s), (hybrid + hybrid), (s + hybrid)
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All purely single bonds are σ-bonds
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π-bond (pi): side to side overlap of two
parallel p orbitals
Orbital hybridization: the result from mathematically averaging of atomic orbitals
methane
Carbon bonded
to 4 atoms
carbon
Carbon bonded
to 3 atoms
4 degenerate sp3
hybrid orbitals
Carbon bonded
to 2 atoms
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Hybridization and Molecular Geometry
Chem 2400
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Hybridization of an atom determines the arrangement of the bonds around the atom and in
turn this determines the bond angles. Thus, molecular geometry is determined by hybridization
Molecular geometry: the three-dimensional arrangement of atoms that constitute a molecule
Hybridization Affects Bond
Lengths and Strengths
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The more bonds holding 2
atoms together, the stronger
and shorter it is
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The greater the electron
density in the region of
overlap, the stronger and
shorter the bond
Take home message: an ↑ in s characters = ↑ bond strength, ↑ bond angle, ↓ bond length
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Practice on Hybridization
Chem 2400
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Quick and easy way to determine hybridization
Count the # of π bonds
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The hybridization of a C, N, O, etc. atom is sp(3 - # of π bonds)
eg.
ex.
Practice:
1)Which of the atom in the following molecule is sp hybridized?
2) Circle all the sp2 hybridized atoms
in hyperforin
3a) Which atoms lie in the same plane as the
indicated olefin in morphine?
OH
Me
Me
OH
H
Me
O
Me
O
Me
OH
Me
Me
Me
O
O
Me
hyperforin
Me
Me
N
H
Me
Morphine
b) What are the hybridizations of the blue atoms
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