Organic Chemistry
Second Edition
David Klein
Chapter 2
Molecular Representations
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Klein, Organic Chemistry 2e
2.1 Represen@ng Molecules
• Given that there are three isomers of propanol
(below), which structures above are adequate to
represent only isopropanol and not its isomers?
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• The Bond-line structure is easier to read and to draw
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• It may seem like a foreign language at first, because
many of the atoms are not labeled
• This type of representa@on is THE main way that
chemists communicate, so it is a language you MUST
master to be successful in organic chemistry
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• Like Lewis structures, lines are drawn between
atoms to show covalent bonds
• Atoms are bonded at angles (zigzag) that represent
the actual geometry of the bond angles
– What is the bond angle for sp3 hybridized carbon?
• Carbon atoms are not labeled, but a carbon is
assumed to be located at every corner or endpoint
on the zigzag.
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• Prac@ce iden@fying the loca@on of and coun@ng the
number of carbon atoms in the structures below
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• Double bonds and triple bonds are represented as
you might expect
• Why is a triple bond wri[en without zigzagging?
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• You must also be able to use the bond-line structure
language to interpret the number and loca@on of H
atoms in a molecule
• H atoms are not shown, but we can assume there
are enough to complete the octet (4 bonds) for each
carbon
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• You should prac@ce bond-line structures un@l it
becomes natural for you to see all of the carbon and
hydrogen atom loca@ons.
• How many carbon and hydrogen atoms are in the
following molecule?
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• If you are given a Lewis structure or condensed
structure, you must also be able to draw the
corresponding bond-line structure
– Represent the bond angles with zigzags
– Follow VSEPR and spread out the electron pairs on a
central atom
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
- Single bonds are axes of rota@on, so be aware that they can
rotate
– Give alterna@ve bond-line structures for the molecule below
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
– Heteroatoms (atoms other than C and H) should be labeled
with all hydrogen atoms and lone pairs a[ached
– NEVER draw a carbon with more than 4 bonds!!
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• Draw bond-line representa@ons for the following Lewis
structures
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Klein, Organic Chemistry 2e
2.2 Bond-line Structures
• Draw bond-line representa@ons for 3 possible isomers
given the formula: C5H9ClO
• Draw bond-line structures for 3 different rota@onal
conforma@ons for the molecule: CH2CH(CH2)4CH3
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Klein, Organic Chemistry 2e
2.3 Inden@fying Func@onal Groups
• When certain atoms are bonded together in specific
arrangements, they undergo specific chemical reac@ons
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Klein, Organic Chemistry 2e
2.3 Inden@fying Func@onal Groups
• More func@onal groups are listed in table 2.1
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Klein, Organic Chemistry 2e
2.4 Bond-line Structures with
Formal Charge
• Formal charge (sec@on 1.4) affects the stability and
reac@vity of molecules, so you must be able to iden@fy
formal charges in bond-line representa@ons
• Label all of the formal charges in the following molecule
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Klein, Organic Chemistry 2e
2.4 Bond-line Structures with
Formal Charge
• Most carbon atoms will have 4 covalent bonds and no
lone pairs to avoid carrying a formal charge
• Some@mes carbon will have a +1 charge. In such cases,
the carbon will only have 3 bonds.
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.5 Bond-line Structures and Lone
Pair Electrons
• If the formal charge is indicated on an atom, you can
determine how many lone pairs are present
• To calculate the number of lone pair electrons for an
atom, compare the number of valence electrons that
should be associated with the atom to the number of
valence electrons that are actually associated with an
atom (sec@on 1.4)
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Klein, Organic Chemistry 2e
2.5 Bond-line Structures and Lone
Pair Electrons
• You can also determine the formal charge on an O atom
by matching its bonding pa[ern with its formal charge
according to table 2.2
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Klein, Organic Chemistry 2e
2.5 Bond-line Structures and Lone
Pair Electrons
• The formal charge on a N atoms can be calculated the
same way or by matching its bonding pa[ern with its
formal charge according to table 2.3
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.6 3D Bond-line Structures
• The vast majority of molecules are 3-dimensional, but it
is difficult to represent a 3D molecule on a 2D piece of
paper or blackboard
• We will use dashed and solid wedges to show groups
that point back into the paper or out of the paper
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Klein, Organic Chemistry 2e
2.7 Resonance
• Drawing lines between atoms inadequately represents
covalent bonds in molecules with resonance
• Remember from General Chemistry, what is resonance?
• Consider the allyl carboca@on:
• How is the bond-line structure inadequate in
represen@ng the allyl carboca@on’s TRUE structure?
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Klein, Organic Chemistry 2e
2.7 Resonance
• Let’s look at the hybridiza@on of the carbons in the allyl
carboca@on
– Calculate the steric number (# of σ bonds + lone pairs)
– When the steric number is 3, it is sp2 hybridized
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Klein, Organic Chemistry 2e
2.7 Resonance
• If all of the carbons have unhybridized p orbitals, they
can overlap
• All three overlapping p orbitals allow the electrons to
move throughout the overlapping area simultaneously
• That’s RESONANCE
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Klein, Organic Chemistry 2e
2.7 Resonance
• How do we represent the complete picture of the allyl
carboca@on provided by valence orbital and MO
theories using a bond-line structure?
• The pi electrons can exist on both sides of the molecule,
so we can use two resonance contributors to represent
the structure
• The brackets indicate that both resonance contributors
exist simultaneously
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Klein, Organic Chemistry 2e
2.7 Resonance
• Because neither of the contributors exists (look at MOs),
the average or hybrid is much more appropriate
vs.
δ+
δ+
two contributors
resonance hybrid
• How is a resonance arrow different from equilibrium?
• Analogy: a nectarine is a hybrid formed by mixing a
peach and a plum. A nectarine is NOT some@mes a
peach and some@mes a plum.
It is always a nectarine.
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Klein, Organic Chemistry 2e
2.7 Resonance
• Resonance makes a molecule MORE stable
• Delocaliza@on of electrons
– Electrons exist in orbitals that span a greater distance giving
the electrons more freedom minimizing repulsions
– Electrons spend @me close to mul@ple nuclei all at once
maximizing a[rac@ons
• Delocaliza@on of charge
– The charge is spread out over more than one atom. The
resul@ng par@al charges are more stable than a full +1 charge.
δ+
δ+
resonance hybrid
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Klein, Organic Chemistry 2e
2.8 Curved Arrows in Resonance
• Throughout Organic Chemistry, we will be using curved
arrows to show electron movement
• The sooner you master this skill, the easier the course
will be
• Curved arrows generally show electron movement for
pairs of electrons
– The arrow starts where the electrons are currently located
– The arrow ends where the electrons will end up aper the
electron movement
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Klein, Organic Chemistry 2e
2.8 Curved Arrows in Resonance
• Rules for using curved arrows to show RESONANCE
1. Avoid breaking a single bond
• Single bonds can break, but NOT in RESONANCE
• Resonance occurs for electrons exis@ng in overlapping
p orbitals, while electrons in single bonds are
overlapping sp, sp2, or sp3 (sigma) orbitals.
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Klein, Organic Chemistry 2e
2.8 Curved Arrows in Resonance
• Rules for using curved arrows to show RESONANCE
2. Never exceed an octet for 2nd row elements (B, C, N, O, F)
• Atoms in the 2nd row can only have four 2nd energy
level orbitals holding a max. of 8 electrons
• Examples of arrows that violate rule 2.
• How does this arrow look?
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Klein, Organic Chemistry 2e
2.8 Curved Arrows in Resonance
• Rules for using curved arrows to show RESONANCE
3. 2nd row elements (B, C, N, O, F) will rarely but some@mes
have LESS than an octet
• What will the resonance hybrid look like for this
structure?
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.9 Formal Charge in Resonance
• When using curved arrows to show RESONANCE, open
structures will carry a formal charge that must be shown
• Draw the resonance contributor indicated by the arrows
below
• Show any formal charges on the contributors
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Klein, Organic Chemistry 2e
2.9 Formal Charge in Resonance
• In the resonance, the arrows tell us how to move the
electrons to create the other contributor
• Draw arrows showing the resonance in the reverse
direc@on
• You can also think of the arrows as showing the
direc@on that charge will flow
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
• There are 5 main bonding pa[erns in which resonance
occurs. Recognize these pa[erns to predict when
resonance will occur
1.
2.
3.
4.
Allylic lone pairs
Allylic posi@ve charge
Lone pair of electrons adjacent to a posi@ve charge
A pi bond between two atoms with different
electronega@vi@es
5. Conjugated pi bonds in a ring
• We will see many examples in the next few slides
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
• Vinyl and allyl refer to posi@ons directly bonded to or
one atom away from a C=C double bond
• Label the vinylic chlorides and the allylic chlorides
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
1. Iden@fying allylic lone pairs
•
Circle all of the allylic lone pairs
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
1. Iden@fying allylic lone pairs
• For each, show the resul@ng resonance contributor
and all formal charges
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
2. Dealing with allylic posi@ve charge
• Only one curved arrow is needed
•
If there are mul@ple double bonds (conjugated), then
mul@ple contributors are possible. Show the
resonance contributors and curved arrows below
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
3. A lone pair adjacent to a posi@ve charge
• Only one arrow is needed
• Explain how the formal charges are affected by the
electron movement in the following examples
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
4. A pi bond between atoms of different electronega@vity
• The pi electrons will be more a[racted to the more
electronega@ve atom
• Explain how the formal charges are created by the
electron movement in the following examples
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
5. Conjugated pi bonds in a ring
• Each atom in the ring MUST have an unhybridized p
orbital that can overlap with its neighbors
•
Electrons can be shown to move clockwise or
counterclockwise
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Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
•
Summary figure 2.5
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Klein, Organic Chemistry 2e
2.10 Pa[erns in Resonance
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Klein, Organic Chemistry 2e
2.11 Stability of Contributors
•
How do we assess the stability of resonance
contributors?
1. Formal charge generally DECREASES stability, especially a +1
charge on an electronega@ve atom or -1 on a low
electronega@vity atom
2. COMPLETE OCTETS INCREASE stability
•
Draw the three resonance contributors for ace@c acid
•
Assess the stability of each contributor, and draw a
resonance hybrid
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Klein, Organic Chemistry 2e
2.11 Stability of Contributors
•
•
The octet rule is usually a bigger factor than formal
charge when assessing stability
For each structure, assess the stability of each
contributor, and draw a resonance hybrid
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Klein, Organic Chemistry 2e
2.12 Delocalized vs. Localized
•
•
Localized – electrons are NOT in resonance
Delocalized – electrons ARE in resonance
–
•
Delocaliza@on increases stability
There are a couple ways to recognize electrons that
are delocalized through resonance?
1. To be delocalized, electrons must exist in an unhybridized p
orbital that can overlap with p orbitals on neighboring atoms
2. To be delocalized, electrons must be on an sp or sp2
hybridized atom
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Klein, Organic Chemistry 2e
2.12 Delocalized vs. Localized
•
Does the delocaliza@on of the electrons in the amide
create a more or less stable contributor?
•
Delocaliza@on must be a very stabilizing force if it can
favor sp2 hybridiza@on over sp3 even when the
delocaliza@on doesn’t involve two equally stable
contributors
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Klein, Organic Chemistry 2e