Introduction to Organic Chemistry
 The Organic Molecule
 Representations of structural formulas
1
Molecular Formula vs Structural Formula
 Molecular formulas tell the number of different
atoms present in the molecule
E.g. C2H6, C3H6O, etc.
 Structural formulas show the connection between
different atoms present in the molecule
Chapter 11
2
Representations of Structural Formulas
 Dot formulas are more cumbersome to draw than dash
formulas and condensed formulas
 Lone-pair electrons are often (but not always) drawn in,
especially when they are crucial to the chemistry being
discussed
Know how to find molecular formula from structural formula
C2H6O
Chapter 11
3
Dash Formulas
 Each dash represents a pair of electrons
 This type of representation is meant to emphasize
connectivity and does not represent the 3-dimensional
nature of the molecule
The dash formulas of propyl alcohol appear to have 90o angles for
carbons which actually have tetrahedral bond angles (109.5o)
 There is relatively free rotation around single bonds so
the dash structures below are all equivalent
Chapter 11
4
Equivalent Dash Formulas
 The following dash formulas are equivalent in
representing isopropyl alcohol
Chapter 11
5
Constitutional Isomers
Are the two structural formulas equivalent?
C3H8O
C3H8O
 Constitutional isomers have the same molecular formula
but different connectivity
 Propyl alcohol (right) is a constitutional isomer of
isopropyl alcohol (left)
Chapter 11
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Condensed Structural Formulas
In these representations, some or all of the dash lines are omitted
In partially condensed structures all hydrogens attached to an
atom are simply written after it but some or all of the other bonds
are explicitly shown
In fully condensed structure all bonds are omitted and atoms
attached to carbon are written immediately after it
For emphasis, branching groups are often written using vertical
lines to connect them to the main chain
Chapter 11
7
Bond-Line Formulas
 A further simplification of drawing organic molecules is
to completely omit all carbons and hydrogens and only
show heteroatoms (e.g. O, Cl, N) explicitly
 Each intersection or end of line in a zig-zag represents a
carbon with the appropriate amount of hydrogens
Heteroatoms with attached hydrogens must be drawn in explicitly
Chapter 11
8
Bond-Line Formulas
Cyclic compounds are condensed using a drawing of the
corresponding polygon
Multiple bonds are indicated by using the appropriate number of
lines connecting the atoms
Chapter 11
9
Matching Game
Match the left with the right right.
CH3CH2C(CH3)CHCH3
C6H6
CH2CH2CH2CH2
C
H2
Chapter 11
10
Three-Dimensional Formulas
 Since virtually all organic molecules have a 3-
dimensional shape it is often important to be able to
convey their shape
 The most important factor in determining the shape of a
organic molecule is the shape of the carbon center
 The different bonding modes around each C center are:
Four single bonds
Two single bonds and one double bond
One single bond and one triple bond
Using the VSEPR theory, the corresponding shape
of each carbon center is, respectively:
tetrahedral
Trigonal planar
Chapter 11
linear
11
Three-Dimensional Formulas
 Trigonal planar arrangements of atoms can be drawn in
3-dimensions in the plane of the paper
Bond angles should be approximately 120o
These can also be drawn side-on with the central bond in the
plane of the paper, one bond forward and one bond back
 Linear arrangements of atoms are always best drawn in
the plane of the paper
Chapter 11
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Three-Dimensional Formulas
 Generally to represent a tetrahedral atom:
 Two of the bonds are drawn in the plane of the paper
about 109o apart
 The other two bonds are drawn in the opposite direction
to the in- plane bonds but right next to each other
Draw the 3-D formula for
bromomethane
Chapter 11
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