Section 1: Organic Structure and Bonding

Chemistry 211
Dr. Meyer
Section 1: Organic Structure and Bonding
What is Organic Chemistry?
Compounds
containing
only
carbon
and
hydrogen,
also
__________________, are the simplest form of organic compounds.
known
Examples:
H
H H
H H H
H C H
H
H C C H
H H
H C C C H
H H H
Atoms other than C and H are referred to as __________________________.
Common heteroatoms encountered in organic chemistry: ______________________.
Example of an Organic Molecule containing a heteroatom:
Drawing Organic Molecules using Ethanol as an example:
(a) Lewis-Dot Structure
(b) Molecular Formula
(c) Condensed Structural Formula
(d) Line-Angle formula
(e) Abbreviated
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as
Chemistry 211
Dr. Meyer
Reviewing Lewis Dot Structures
In Lewis Dot Structures, the valence electrons are shown as “dots” surrounding the
atom.
Example:
In ionic bonds the electrons are transferred between atoms.
In covalent bonds the electrons are shared between atoms and are shown in the Lewis
structures as a line between atoms.
Most atoms follow the octet rule which states that an atom is in the lowest energy state
when it has eight valence electrons.
Question: Examine the Lewis structures of the following molecules:
HNO3
HNO2
H2SO4
H2CO3
CH3CO2CH3
(CH3)2CHOH
CH3CHO
CH3SCH3
Which of the Lewis structures above violate the octet rule?
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Chemistry 211
Dr. Meyer
Formal Charge
Formal charge = group number (from periodic table) – # of lines – # of dots
Question: Identify the formal charge for each of the following central atoms:
C
N
O
C
N
O
Br
C
N
O
Br
Br
Which ones violate the octet rule?
What bonding trends can we establish based on formal charge?
Isomers –
_________________________________________________
___________________________________________________________
Example: Ethanol and dimethyl ether are isomers with the molecular formula C2H6O.
Draw the structural formula of each isomer.
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Chemistry 211
Dr. Meyer
Resonance structures are different ways of drawing the Lewis structure of the same
molecule. The _________ are all in the same place, but the_____________ have
moved.
Example: Draw another resonance structure for each of the following compounds:
Common Resonance Pattern:
Common Resonance Pattern:
Common Resonance Pattern:
Determining the Best Resonance Contributors
A resonance contributor is more important when:
1.
2.
3.
4.
Identify the more important resonance contributor in each of the cases above.
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Chemistry 211
Dr. Meyer
The Resonance Hybrid
Rules for Drawing the Resonance Hybrid
1) Copy everything that is identical in all resonance contributors.
2) Add dotted bonds for any bonds that are present in only some resonance contributors
3) Add partial positive (δ+) or partial negative (δ–) charges for any charges that are
present in only some resonance contributors
4) Omit any lone pairs that are present in only some resonance contributors
Example: Draw the resonance hybrid for each compound shown in the
previous problem.
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Chemistry 211
Dr. Meyer
Simplifying Lewis Structures
Organic compounds have so many carbons and hydrogens that it can be helpful to
make them assumed in order to clean up the drawing.
Compare:
Lewis Structure
Condensed Structure
Skeletal Structure
Drawing / Interpreting Skeletal Structures
• The end of each line or a vertex represents a carbon atom that has four bonds.
These carbon atoms do not need to be shown.
•
If (and only if) the carbon atom is not shown, then the hydrogen atoms connected
to that carbon atom do not need to be shown.
•
All heteroatoms (any atom that is neither carbon nor hydrogen) must be shown
and all hydrogens connected to heteroatoms must be shown.
•
All formal charges must be shown, but lone pairs of electrons are optional.
Question: Convert the following condensed structures into skeletal structures:
(CH3)3CBr
C4H10 (straight chain)
C6H6 (benzene)
Question: Convert the following skeletal structures into Lewis structures:
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CH3CHO
Chemistry 211
Dr. Meyer
Electronegativity –__________________________________________
Trend:
Note: The electronegativity values on the “Pauling Scale” are included, above.
Pneumonic for Organic Chemistry:
Question: Use the partial charges (δ+ and δ–) to indicate the dipole moment in
HF.
VSEPR Structures
The electron geometry is determined by the number of groups of electrons (lone pairs
of electrons or bonds) around the central atom.
Linear
Trigonal Planar
Tetrahedral
The molecular geometry is determined by the position of the atoms (ignore the lone
pairs). In addition to the above, we have these possible molecular geometries:
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Chemistry 211
Dr. Meyer
Bent
Trigonal Pyramidal
Bond Angle Distortion
The bond angles shown above are only correct when all electron groups attached to a
central atom are identically sized. Otherwise, larger groups will occupy more space and
distort the bond angles.
Sizes of Electron Groups:
Groups of atoms > Lone Pairs of Electrons > Single Atoms
Recall: Single atoms get larger from top to bottom and right to left in periodic table!!!
Representing Three Dimensional Molecules on Paper
We use the following symbols to represent bonds in molecules:
Linear
Straight Line
Wedge
Dash
Bond in Plane
of Paper
Bond Extends
Out of Paper
(From L to R)
Bond Extends
Into Paper
(From L to R)
Bent
Trigonal
Planar
Tetrahedral
Trigonal
Pyramidal
Question: Draw the molecular structure for each of the following molecules, using
dashes and wedges to indicate the three dimensional nature of each molecule and label
the bond angles:
CH4
NH3
Acetic Acid (CH3CO2H)
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Chemistry 211
Dr. Meyer
Polar Molecules
A molecule is polar (has a net dipole moment) if all of the dipole moments do NOT
cancel out in three dimensions. Otherwise, the molecule is nonpolar
Question: Revisit the Lewis structure of methane and ammonia. Which one is polar?
Hybrid Orbitals
Recall: A covalent bond is formed by overlapping 2 orbitals – one from each atom.
Most of the atoms we encounter in organic chemistry are period 2 elements and
therefore have ________s and _________p orbitals.
Atomic Orbitals
Hybridization and Geometry
Shapes
sp3 = tetrahedral
______remaining p orbitals
sp2 = planar
______remaining p orbitals
s
p
sp
sp = linear
______remaining p orbitals
Hydrogen only has an ____ orbital to use to make covalent bonds.
Example: Label the atomic orbitals on each atom.
CH3
H3C C C CH
O CH3
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sp2
sp3
Chemistry 211
Dr. Meyer
Molecular Orbitals:
sigma
pi
The hybrid orbitals overlap to form ____________ bonds
The leftover p orbitals overlap to form __________ bonds
Exercise – List the atomic orbitals used in the following examples:
C
C
C
BOND NOTATION: (Atomorb – Atomorb) Ex: (Csp2 – Hs)
Exercise:
each C.
H
Describe atomic orbitals, Molecular orbitals, and geometry around
CH3
C C
H
H
Summary
If an atom has 4 electron groups, the hybridization is ____ and it can form ____ π
bonds
If an atom has 3 electron groups, the hybridization is ____ and it can form ____ π
bonds
If an atom has 2 electron groups, the hybridization is ____ and it can form ____ π
bonds
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Chemistry 211
Dr. Meyer
Question: Assign the hybridization of every atom in each of the following:
Question: Consider this molecule, H2C=C=CH2 (allene).
What is the C–C–C bond angle?
What is the H–C–H bond angle?
Why is this molecule NOT flat? Sketch a picture:
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Chemistry 211
Dr. Meyer
Bond Lengths and Bond Strengths
For each set of bonds, indicate which are longer and which are stronger:
a)
C–C
c)
Csp3–H
C=C
C≡C
Csp2–H
Csp–H
b)
C–F
C–Br
C–I
d)
C–C
C–N
C–O
Trends:
Reading: Wade Chapters 1 & 2
Homework: Chapter 1-21,22,23,25–31, 34–37, 40, 41
Chapter 2-28, 29, 30, 31
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