Organic Resonance Systems
Common, Possible Patterns of Resonance
Resonance implies electron delocalization, which is stabilizing because it minimizes electron-electron repulsion.
There are four common resonance patterns in organic chemistry, using two donor sites and two acceptor sites.
Donor electrons can come from lone pairs and pi bonds. Acceptor sites include an empty 2p orbital (almost always
carbon in our course = carbocations) and pi bonds (CC pi bonds are OK, polar is good, pi cations are best). Notice
that pi bonds can donate and accept electrons. In our course resonance systems will always occur through p
orbitals. There is some overlap among the various patterns depending how they are drawn. Hyperconjugation is a
brand of resonance that can use sigma bonds with p orbital, but we will not discuss such resonance.
Resonance Pattern 1 – Lone pair donation (2p orbital) into an empty 2p orbital (carbocation or pi cation).
Resonance Pattern 2 – Lone pair (2p orbital) donation into pi bonds (many kinds, CC, CN, CO, NO, NN, etc.).
Resonance Pattern 3 – Pi bond donation (from alkene, alkyne or aromatic) into an empty 2p orbital (carbocation or
pi cation).
Resonance Pattern 4 – Pi bond donation (from alkene, alkyne or aromatic) into a pi bond acceptor (alkene, alkyne,
aromatic, carbonyl, imine, nitrile, etc, polar is good, pi cations are best.)
Donors
lone pairs
pi bonds
(alkenes, alkynes, aromatics)
pattern 1
X
pattern 3
X
C
C
C
C
C
H
a b
O
X
a
C
X
H
H
O
O
b
C
X
empty 2p orbital
C
C
C
O
C
C
pattern 4
C
N
O
O
C
C
N
CC is OK
C
C
C
C
C
pi bonds
(all kinds)
C
empty 2p orbital
pattern 2
C
C
O
C
pi cation
pi cation
Acceptors
H
H
H
C
empty 2p orbital
pi cation
C
pi cation
empty 2p orbital
empty 2p
orbitals
C
C
C
polar is better
C
C
O
N
O
N
H
polar is better
H
N
N
C
C
H
O
C
C
C
C
H
N
C
C
O
N
pi cation is best
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pi cation is best
O
C
Organic Resonance Systems
Resonance Pattern 1 – Lone pair donation into an empty 2p orbital (carbocation or pi cation). This is common in
polar pi systems or pi cations (see the example at the bottom of this page).
pi cations
Usually we write these examples the other way around,
starting with the structure having the maximum number
of bonds and show the expected polarization forming
the minor resonance by pushing the pi electrons to the
more electronegative atom. In these examples we are
doing it backwards, showing a lone pair sharing with an
empty 2p orbital, making a pi bond.
R
R
C
C
H
R
H
R
N
N
R
R
R
R
C
R
C
R
R
R
N
R
C
C
R
O
C
N
R
R
R
C
N
C
H
R
O
R
C
N
H
R
C
R
C
C
R
O
R
N
R
These structures have more
bonds and full octets and are
considered the major contributor.
2D resonance structures - carbonyl resonance,
this is the usual way of showing carbonyl
resonance (reverse of above).
O
R
R
F
R
C
O
O
R
R
C
F
These structures have more
bonds and full octets and are
considered the major contributor.
R
C
C
O
C
O
R
R
If we turn it around, it shows formation of a pi bond.
R
R
R
R
C
R
C
O
R
H
O
3D resonance structures - carbonyl resonance
R
C
N
C
C
R
H
O
O
R
R
R
R
O
C
O
R
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R
Organic Resonance Systems
Electron donation can occur from 2 or even 3 sides (wherever a lone pair is adjacent to an empty 2p orbital.
2D resonance structures - carbonyl resonance with one adjacent lone pair
O
O
C
H
C
R
O
H
R
N
N
B
A
H
3D resonance structures - carbonyl resonance
O
C
R
N
C
H
H
H
N
C
C
N
H
N
H
H
R
Contribution: A > C > B
H
O
O
H
The amide nitrogen
atom extends resonance
to one additional atom.
H
C
R
R
2D resonance structures - carbonyl resonance with two adjacent lone pairs. We judge resonance structures with full octets and more bonds
as better, that would be A, C and D below. Because A does not separate charge we judge it to be the best. Nitrogen is a much better electron
pair donor than fluorine so C is better than D. Normally, D would be judged better than B, but because of its high electronegativity, fluorine's
resonance effect is about equally cancelled by its electron withdrawing inductive effect, so D  B (contribution :A > C > D  B).
O
O
C
H
F
O
C
N
H
F
C
N
A
H
B
3D resonance structures - carbonyl resonance
O
H
F
C
N
H
H
C
H
F
N
D
H
H
H
N
H
H
H
H
C
H
N
C
O
F
N
H
C
O
F
N
O
C
F
O
F
A few examples of a carbonyl group with one adjacent lone pair. Lone pair donation is best when all overlapping orbitals are all 2p. Larger
3p orbitals are often shown overlapping with 2p orbitals, but are not as effective at sharing their electron density.
O
O
O
O
O
O
O
C
H
R
C
O
C
R
O
carboxylic acids
R
R
R
O
R
esters
anhydrides
C
S
thioesters
A few examples of a carbonyl group with two adjacent lone pairs.
O
O
O
O
C
C
C
HO
OH RO
carbonic acid
OR
carbonates
R2N
urethanes
R2N
ureas
N
amides H
O
O
C
NR2
guanidines
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R
R
Cl
acid chlorides
C
NR2 R2N
RO
H
C
C
NR
C
OR
R
C
Cl
alkyl chloroformates
Cl
Cl
phosgene
Organic Resonance Systems
Resonance Pattern 2 – Lone pair donation into pi bonds (many kinds). Formal charge can vary depending on the
atoms and number of bonds. Decide an atom’s hybridization (shape and bond angles too) using a 2D resonance
structure where it has its maximum bonds. Use the 2D structures to decide how to draw the 3D structures. ‘R’ is
an H or carbon group below.
pushable electrons = lone pair
acceptor = pi bond
2D resonance structures
Y
3D resonance structures
Y
X
Z
X
Z
Y
Z
Y
X
sp2 pi acceptor
X, Y, Z are usually C, N, O in organic chemistry,
Z
X
The examples below can be viewed in either direction.
1
R
R
R
C
R
C
C
C
F
R
3
R
R
C
C
R
C
N
R
R
R
5
R
R
C
R
R
R
C
7
R
N
C
9
C
R
R
equivalent
F
C
R
R
R
N
C
R
O
O
R
R
C
R
N
R
N
N
R
N
R
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C
R
C
R
N
R
R
N
10
C
R
R
C
R
C
R
R
R
C
C
R
R
R
C
N
F
R
8
R
C
R
R
R
N
R
O
R
R
R
C
O
6
C
N
C
R
C
C
R
R
R
R
C
R
O
R
N
R
R
O
4
C
C
R
C
C
R
R
R
R
R
C
F
R
R
R
R
2
O
N
O
Organic Resonance Systems
11
C
R
12
R
R
N
C
R
R
N
N
C
R
R
R
R
R
N
N
R
N
C
R
equivalent
13
C
R
C
R
reverse of 5
14
R
R
C
C
O
15
C
O
F
F
O
O
O
16
R
R
O
R
C
R
C
O
N
N
C
C
O
R
O
R
18
O
R
R
O
N
O
C
R
reverse of 10
19
R
C
R
C
R
C
N
O
equivalent
R
C
O
O
R
R
17
R
C
R
O
R
R
R
R
C
R
reverse of 4
20
O
O
N
O
O
O
O
N
O
O
O
equivalent
O
equivalent
21
22
R
R
O
N
N
N
O
O
O
O
R
N
R
O
O
O
equivalent
23
24
N
R
N
O
N
O
N
R
N
O
N
F
O
F
There are even more possibilities! All of the above examples can be written in the reverse direction, and some are.
y:\files\classes\201\201 resonance related examples with 3D structures.doc
Organic Resonance Systems
pushable electrons = lone pair
acceptor = pi bond
2D resonance structures
X
Y
Z
3D resonance structures
X
Y
Z
Y
X
sp pi acceptor
X, Y, Z are usually C, N, O in our course
Y
X
Z
Z
Notice that the pi bonds are perpendicular. There is no resonance in
this second pi bond with the lone pair that is shown. If there is a
second lone pair on X then another resonance structure can be drawn
using those orbitals.
The examples below can be viewed in either direction. 'R' is an H or carbon group below.
1
2
F
C
C
R
F
C
C
O
R
C
C
R
R
O
C
C
R
R
4
3
R
R
N
C
C
R
R
N
C
C
R
O
C
C
R
O
C
C
R
R
6
5
R
N
C
C
N
R
C
C
C
R
R
R
R
C
C
R
R
7
C
C
C
R
R
8
F
C
N
F
C
O
N
C
N
C
N
N
C
N
O
C
O
N
C
N
R
R
12
11
H3C
H3 C
N
H3C
N
10
R
R
C
R
R
9
O
N
C
N
N
H3C
C
N
C
H3 C
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C
C
N
H3C
C
N
Organic Resonance Systems
13
14
F
C
O
F
C
O
O
C
O
O
O
R
R
15
C
16
R
R
N
C
R
N
O
C
O
C
R
R
R
N
N
R
17
C
N
N
N
C
O
R
18
N
N
N
N
N
N
N
R
R
C
O
R
R
(also consistent with the examples below)
(also consistent with the examples below)
Triple bonds have perpendicular pi bonds. Lone pairs can be parallel to either pi bond, in effect having two
independent resonance patterns in the same structure. The middle structures look like the end atoms would be
hybridized as sp2, but in the first and last structures they look like sp. They can’t be both. We assume an atom’s
hybridization to be consistent with whichever structure shows the most bonds for that atom, in this case sp. Not
everyone would write these this way, but sometimes it makes sense.
3D skeleton and resonance structures.
X
Y
Z
N
N
X
Y
Z
X
Y
Z
azide = N3
-2
-2
N
N
N
N
N
N
N
O
N
O
O
N
O
O
C
O
O
C
O
nitronium ion = NO2
O
N
O
carbon dioxide = CO2
O
C
O
y:\files\classes\201\201 resonance related examples with 3D structures.doc
Organic Resonance Systems
Resonance Pattern 3 – We only show pi bond donation from an alkene, alkyne or aromatic into an empty 2p orbital
(carbocation). By spreading out the electrons the positive charge is stabilized. We do not show this sort of resonance
using CO or CN pi bonds, assuming that the more electronegative nitrogen and oxygen atoms are not as willing to
give away their electrons (less than an octet). The second column (right side) compares pattern 2 with this pattern.
2D Lewis structure alkenes as donors to empty 2p carbocations. 2D Lewis structure alkenes as acceptors (donor atoms can
be anions of carbon, nitrogen and oxygen or neutral atoms
R
R
R
R
of nitrogen, oxygen and halogens.
C
C
R
C
R
R
C
C
pattern 3
R
R
C
C
R
R
R
C
R
R
R
C
R
R
C
C
C
3D Lewis structure
R
R
R
R
R
R
R
R
C
pattern 2
C
C
3D Lewis structure
C
R
R
C
R
R
C
C
R
R
R
R
R
R
C
C
R
C
R
C
R
C
R
R
2D Lewis structure alkynes as donors to empty 2p carbocations. 2D Lewis structure alkynes as acceptors (donor atoms can
be anions of carbon, nitrogen and oxygen or neutral atoms
R
R
of nitrogen, oxygen and halogens.
R
C
C
pattern 3
C
R
R
C
C
R
R
C
C
C
R
C
C
R
C
C
C
R
R
3D Lewis structure - notice the pi bonds are perpendicular 3D Lewis structure
R
R
pattern 2
C
R
C
R
C
C
R
R
C
R
R
R
C
C
R
C
R
C
C
C
R
R
2D Lewis structure aromatic rings as donors (Pattern 3)
H
H
C
H
C
C
C
H
R
C
C
H
H
C
C
C
H
R
H
R
C
C
H
C
C
C
H
H
H
H
C
C
C
C
C
H
C
C
H
C
R
C
C
H
C
H
C
C
C
R
C
C
R
H
H
H
C
C
R
H
C
C
C
H
C
H
C
R
C
H
C
H
R
H
C
C
C
H
C
H
R
R
C
C
H
R
H
C
H
H
C
C
C
H
R
3D Lewis structure
H
H
H
y:\files\classes\201\201 resonance related examples with 3D structures.doc
C
R
H
C
C
R
C
C
H
H
C
R
C
H
Organic Resonance Systems
2D Lewis structure aromatic rings as acceptors (donor atoms can be anions of carbon, nitrogen and oxygen or neutral atoms of nitrogen, oxygen
and halogens. (Pattern 2)
H
H
C
H
H
C
C
C
C
C
H
R
C
C
H
C
C
H
R
H
H
H
3D Lewis structure
H
H
H
C
C
C
H
C
C
C
H
H
C
C
C
H
H
C
C
R
H
H
H
C
C
C
H
H
C
H
C
C
C
C
H
H
R
C
C
H
R
C
R
R
C
R
C
R
R
C
C
C
H
H
C
R
H
C
C
C
H
H
C
R
C
C
H
H
C
C
R
C
H
C
C
C
R
H
H
C
C
R
C
C
H
H
C
R
C
H
Resonance Pattern 4 – Pi bond donation (from alkene, alkyne or aromatic) into a pi bond acceptor (alkene, alkyne,
aromatic, carbonyl, imine, nitrile, etc.). A CC bond is OK, a polar pi bond acceptor is better and a pi cation is the
best acceptor.
Neutral pi systems - In resonance pi systems move electrons towards electronegative atoms (oxygen or nitrogen).
Notice the partial positive site is spread to multiple centers (two atoms in these examples). Structure A can be
shown going directly to C and structure E can be shown going directly to G using 2 arrows.
R
A
R
R
B
O
C
C
C
R
R
R
C
O
C
C
C
R
R
R
R
C
R
C
R
C
R
C
R
O
C
R
R
R
O
C
R
C
R
E
N
C
C
C
R
C
R
C
O
R
F
R
C
N
C
O
C
3D Lewis structure
R
R
C
N
C
R
C
R
R
G
R
C
C
C
R
R
R
R
3D Lewis structure
R
R
N
C
N
C
C
R
C
R
C
N
C
R
C
C
R
R
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C
R
R
Organic Resonance Systems
Cationic pi systems: notice the cationic site is spread to multiple centers (three in these examples).
Structure A can be shown going directly to C using 2 arrows.
R
A
C
R
R
B
O
C
C
R
R
R
O
C
H
C
C
R
R
C
R
H
3D Lewis structure
R
R
C
R
R
C
C
R
C
R
O
R
H
C
R
R
H
R
C
C
O
C
R
C
R
O
R
C
C
R
C
H
O
H
R
Cationic pi systems: notice the cationic site is spread to multiple centers (three in these examples). Move
electrons towards the positive charge. Structure A can be shown going directly to C using 2 arrows.
A
H
N
C
B
R
C
HN
C
C
C
R
C
R
HN
C
R
C
R
C
R
C
R
R
R
3D Lewis structure
R
R
H
N
C
H
C
C
R
N
C
R
C
H
C
R
R
y:\files\classes\201\201 resonance related examples with 3D structures.doc
N
C
C
R
C
R
R
Organic Resonance Systems
Combination of All 4 Patterns
donor site(s)?
acceptor site(s)?
connector site(s)?
There are other ways to show resonance in these examples beyond what is shown below.
H3C
A
CH3
H
C
C
C
O
Move lone pair
into empty p orbital.
CH3
Move lone pair
into pi bond.
pattern 2
H
pattern 3
C
C
C
O
O
C
O
H
pattern 4
C
C
NH2
O
C
Move CC pi bond
into polar pi bond.
CH3
NH2
C
C
C
C
Move CC pi bond
into empty p orbital.
CH3
D
C
C
C
CH3
CH3
H
C
Move lone pair
into pi cation.
pattern 2
O
NH2
C
CH3
E
O
F
NH2
C
O
CH3
Move lone pair
into pi bond.
pattern 2
CH3
C
C
C
C
pattern 1
O
H
pattern 2
NH2
C
C
CH3
C
C
NH2
C
O
CH3
N
H
C
C
H3C
B
N
CH3
Example of 3D representation (below)
H
H
O
C
O
C
C
C
O
C
C
H3C
O
C
N
A
CH3
C
C
C
H3C
H
H
CH3
N
D
H
H
This 3D framework fits all resonance structures above (and more). Delocalization occurs through parallel 2p orbitals.
H
O
C
O
C
C
C
C
H3C
This pi bond does not participate
in resonance in these examples.
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CH3
N
H
H
Organic Resonance Systems
Three variations with resonance in imidazole: neutral, anionic and cationic
Example 1 - Neutral imidazole: Delocalization of p orbital electrons in a neutral ring structure. Additional structures create
charge, but have the same number of bonds and full octets and all are reasonable contributors.
H
H
H
H
H
H
N
H
N
OK
H
H
N
N
N
Best
H
H
H
N
N
H
H
OK
H
N
H
OK
H
H
N
N
OK
H
H
Example 2 - Anionic imidazole: Delocalization of p orbital electrons in a anionic ring structure. The first and last structures are
equivalent and the most important contributors because of the greater electronegativity of nitrogen.
H
H
H
H
H
H
N
N
N
N
N
N
H
H
H
N
N
H
H
N
N
H
H
H
H
better
OK
OK
OK
better
Example 3 - Cationic imidazole: Delocalization of p orbital electrons in a cation ring structure. The first two structures shown
are best because they have an extra bond and full octets. One extra structure is shown with positive charge on carbon, but it has
an incomplete octet.
H
H
H
H
H
N
H
H
N
N
N
H
N
H
N
H
H
H
H
H
The 3rd structure is not as good as the first
two structures. It has fewer bonds and an
incomplete octet, but is still a resonance
contributor and provides information about
the chemical reactivity of this cation.
H
better
OK
better
3D template for all resonance structures above. Lone pairs or groups perpendicular to the p orbitals are not part of the resonant
system. In some structures the group to the side of the sp2 nitrogens is a lone pair and in some structures it is a hydrogen atom.
In all cases the side group is using an sp2 orbital. The p orbitals are all part of the resonant system in this problem.
H
H
H
H
H
H
C
C
N
N
C
C
H
C
C
N
N
N
N
H
H
C
C
H
Example 1
C
H
Example 2
H
H
H
H
H
C
C
C
C
N
N
H
Example 3
H
H
N
N
C
C
N
N
H
H
C
C
H
OK resonance, but creates
charges in a neutral structure
C
H
Excellent resonance, spreads out
high energy negative charge
y:\files\classes\201\201 resonance related examples with 3D structures.doc
H
Excellent resonance, spreads out electron
density to electron deficient sites.
Organic Resonance Systems
Generic Examples
donors = D
lone pair / pi patterns
acceptors = A
empty 2p / pi patterns
donors = D
acceptors = A
lone pairs
a. neutral (N, O, F)
b. anion (C, N, O)
R
C
In this slide,
D
R = C or H
O = S
F = Cl, Br, I
C/C pi bonds
alkenes, alkynes, aromatics
R
R
C
R
R
N
connecting
pi patterns
C
A
R
R
R
C
N
R
A
C
D
O
R
A
O
C
R
A
R
D
C
D
R
D
N
R
R
D
C
C
F
A
D
A
C
N
R
D
O
R
D
A
D
F
A
O
R
A
C
D
C
C
A
C
O
R
D
R
(possible)
C
O
R
R
C
N
N
D
C
R
C
R
R
R
C
A
N
C
A
D
Just a sampling. There
are too many variations
to show all possibilities.
O
D
O
D
C
C
R
N
D
A
connecting pi patterns C/C pi bonds,
usually alkenes, alkynes, aromatics
C
C
C
C
Possible connector patterns with any combination of the above donors and/or acceptors. The CC pi systems (alkenes, alkynes
and aromatics) are similar to a wire that allows electrons to flow throughout. The electron density distributes itself in a manner
to optimize the electron-electron repulsion. We call this delocalization or resonance.
Make your own problems with
various donors and acceptors.
y:\files\classes\201\201 resonance related examples with 3D structures.doc