Section 1: Organic Structure and Bonding
What is Organic Chemistry?
Compounds containing only carbon and hydrogen, also known
__________________, are the simplest form of organic compounds.
as
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
Formula
(c)
Condensed
(d) Line-Angle formula
(e) Abbreviated
Structural
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:
CH3CO2CH3
(CH3)2CHOH
CH3CHO
CH3SCH3
Formal Charge
Formal charge = group number (from periodic table) – # of lines – # of dots
Question:
atoms:
C
Identify the formal charge for each of the following central
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.
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:
compounds:
Draw another resonance structure for each of the following
Common Resonance Pattern:
Common Resonance Pattern:
H
H3C
H
C
O
H
C
CH3
Common Resonance Pattern:
Common Resonance Pattern:
Determining the Best Resonance Contributors
A resonance contributor is more important when:
Identify the more important resonance contributor in each of the cases above.
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.
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
CH3CHO
C4H10 (straight chain)
C6H6
(benzene)
Question: Convert the following skeletal structures into Lewis structures:
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:
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
Trigonal
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
Tetrahedral
Planar
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
(CH3CO2H)
NH3
Acetic
Acid
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
sp2
sp3
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: Describe atomic orbitals, Molecular orbitals, and geometry
around each C.
H
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
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:
Bond Lengths and Bond Strengths
For each set of bonds, indicate which are longer and which are stronger:
a)
C–C
c)
Csp3–H
Trends:
C=C
C≡C
Csp2–H
b)
Csp–H
C–F
C–Br
C–I