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y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Common, Possible Patterns of Resonance
There are four common resonance patterns that we encounter, using two donor sites and two acceptor sites. Donor
electrons (D) can come from lone pairs and pi bonds. Acceptor sites (A) include an empty 2p orbital (almost
always carbon in our course = carbocations) and pi bonds (polar is best). Notice that pi bonds can donate and
accept electrons. In our course resonance systems will always occur through p orbitals. There is another brand of
resonance that can use sigma bonds with p orbitals, called hyperconjugation. We will not emphasize this
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).
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 better.)
Donors
lone pairs (neutral or anion)
pi bonds
(alkenes, alkynes, aromatics)
empty 2p
orbitals
Acceptors
pattern 1 pattern 3
X C X^ C
C
C
C C
C
C
X C
O
X C
H O H
pi cation
C
C
C
O
C
C
C
O
pi cation
X C X C H H
X C
O
X C
H O H
pi cation
lone pair donors
R
C
R
A R
N
A
O
A
R
N
A
O
A
F
A
R
R
a. anions (carbon, nitrogen, oxygen)
b. neutral (nitrogen, oxygen, halogens)
(S too?) (Cl,Br,I too?)
none
resonance resonance none
A = acceptor sites = empty 2p (above) or pi bond (next page)
resonance
F
C
resonance resonance^ resonance
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pi bonds
(all kinds)
pattern 2
pattern 4
C
C
N C
C
N
C
C
C
O
C
C
C
O
O
C
N O
C
N
polar is better
polar is better
C
C
C
C
C
C
N N
O
C
O O
C
O
O
C
C O
C
C
polar is better C
C
C
O
C
C
C
O
H H
Acceptors
Donors
lone pairs (neutral or anion)
pi bonds
(alkenes, alkynes, aromatics)
C
R
D N
R
R
R
C
R
D
A = acceptor groups = empty carbon 2p orbital or pi bond (polar is better)
empty 2p or pi cations
C
R
D N
R
R C
R
D N
R
R
D: = donor group = lone pair (can be neutral or negatively charged) or CC pi bond (alkene, alkyne, aromatic)
C
R
D O
R C
R
D O
R C
R
D O
R
C
R
D F
C
R
D F
C
R
D F
N
C
N
C
R R
D D
N
C
R
D
O
C
D
O
C
D
O
C
D
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Resonance Pattern 1 – Lone pair donation into pi bonds (many kinds). Add in curved arrows and formal charge
and draw 3D structures (use template on the right, just below). Decide an atom’s hybridization using a resonance
structure where it has its maximum bonds. (A key for each group is the following page.)
Y Z
X
X
Y
Z X
Y
Z
Two Dimensional Three Dimensional
Y Z
X
X, Y, Z can be C, N, O in our course sp 2 pi acceptor
pushable electrons = lone pair acceptor = pi bond
C
C
O
CH 3
C
C
F
H
H
H H
H
H
C
C
N
CH 3
CH 3
C
C
O
H
H
H
H
H
H
C
C
N
CH 3
C
C
C
CH 3
CH 3
H
H
H
H
H
H
N
C
O
CH 3
N
C
F
H 3 C
H
H 3 C
H
N
C
N
CH 3
CH 3
N
C
O
H 3 C
H
H 3 C
H
N
C
N
CH 3 N
C
C
CH 3
CH 3
H
H 3 C
H
H 3 C
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Resonance Pattern 1 – Lone pair donation into pi bonds (many kinds). Add in curved arrows and formal charge and
draw 3D structures (use template on the right, just below). Decide an atom’s hybridization using a resonance
structure where it has its maximum bonds.
Y Z
X
X
Y
Z X
Y
Z
Two Dimensional Three Dimensional
Y Z
X
X, Y, Z can be C, N, O in our course sp 2 pi acceptor
pushable electrons = lone pair acceptor = pi bond
C
C
O
CH 3
C
C
F
H
H
H H
H
H
C
C
F C
C
O
CH 3
H
H
H
C
C
N
CH 3
CH 3
C
C
O
H
H
H
H
H
H
C
C
N
CH 3
CH 3
H
H
H
C
C
O
H
H
H
C
C
N
CH 3
C
C
C
CH 3
CH 3
H
H
H
H
H
H
C
C
N
CH 3
H
H
H
C
C
C
CH 3
CH 3
H
H
H
N
C
O
CH 3
N
C
F
H 3 C
H
H 3 C
H
N
C
F N
C
O
H 3 C CH 3
H
N
C
N
CH 3
CH 3
N
C
O
H 3 C
H
H 3 C
H
N
C
N
CH 3
CH 3
H 3 C
H
N
C
O
H 3 C
H
N
C
N
CH 3 N
C
C
CH 3
CH 3
H
H 3 C
H
H 3 C
N
C
N
CH 3
H
H 3 C N
C
C
CH 3
CH 3
H
H 3 C
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
O
C
O
CH 3
O
C
F
H H
O
C
F
H
O
C
O
CH 3
H
O
C
N
CH 3
CH 3
O
C
O
H
H
O
C
O
H
O
C
N
CH 3
CH 3
H
O
C
N
CH 3
O
C
C
CH 3
CH 3
H
H
O
C
N
CH 3
H
O
C
C
CH 3
CH 3
H
O
O
O O
N
O O
O
O O
N
O
O
N
O
CH 3
O
N
O
CH 3
O
N
O
CH 3
O
N
O
CH 3
O
N
N
CH 3
O
N
F O
N
O F
N
N
CH 3
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Two Dimensional
X Y Z
X, Y, Z can be C, N, O in our course
pushable electrons = lone pair acceptor = pi bond
X Y Z X Y Z
X Y Z
no resonance in this pi bond
Three Dimensional
F C C H
O C C H
H 3 C
N C C H
H 3 C
H 3 C
O C C H
N C C H
H 3 C
C C C H
H 3 C
H 3 C
F C N O C^ N
H 3 C
N C N
H 3 C
H 3 C
O C N
N C N
H 3 C
C C N
H 3 C
H 3 C
(also consistent with the example that follows)
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
F C O O C^ O
H 3 C
N C O
H 3 C
H 3 C
C N N
H
H
N C O
H 3 C
N N N
H 3 C
(also consistent with the example that follows) (^) (also consistent with the example that follows)
Perpendicular lone pairs can be parallel to perpendicular pi bonds, in effect having two independent resonance
patterns in the same structure. Not everyone would write it this way. 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, in this case sp.
azide = N 3
nitronium ion = NO 2
X Y Z X Y Z X Y Z
3D skeleton and resonance structures.
carbon dioxide = CO 2
N N N N^ N N N N^ N
O N O O^ N O O N^ O
O C O O^ C O O C^ O
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
F C O O C^ O
H 3 C
F C O
O C O
H 3 C
N C O
H 3 C
H 3 C
C N N
H
H
N C O
H 3 C
H 3 C
C N N
H
H
N C O
H 3 C
N N N
H 3 C
(also consistent with the example that follows) (^) (also consistent with the example that follows)
N N N
H 3 C
N C O
H 3 C
Perpendicular lone pairs can be parallel to perpendicular pi bonds, in effect having two independent resonance
patterns in the same structure. Not everyone would write it this way. 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.
azide = N 3
nitronium ion = NO 2
X Y Z X Y Z X Y Z
3D skeleton and resonance structures.
carbon dioxide = CO 2
N N N N^ N N N N^ N
O N O O^ N O O N^ O
O C O O^ C O O C^ O
-2 -
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Resonance Pattern 2 – 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). Add in curved arrows and formal charge.
C
R
R N
H
R
C
R
R O
C
R
R F
H
C N
R C O
C
R
R N
R
C
R
R O
C
R
R F
H
C N
R C O
H
R H R H
C
R
R N
R
C
R
R O
C N
C
R
R N
C
R
R O
C N
R
R R
Usually we write the first three examples the other
way around, starting with the neutral structure and
showing the expected polarization using the minor
resonance structure. In these examples we are
showing a lone pair sharing with an empty 2p orbital.
pi cations
C
R
R
O C O
R
R
C O
R
R
C O
R
R
2D resonance structures 3D resonance structures - carbonyl resonance
Additional resonance is possible with the positively charged carbon, if it is connected to an atom with a
lone pair of electrons or another pi bond, e.g. "R" = nitrogen, oxygen, an alkene, alkyne or aromatic ring.
R
C
O
NH 2 R
C
O
NH 2 R
C
O
NH 2
The amide nitrogen
atom extends resonance
to one additional atom.
Contribution: A>C>B
A B C
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Resonance Pattern 3 – Pi bond donation (from alkene, alkyne or aromatic) into an empty 2p orbital (carbocation).
Add in curved arrows and formal charge. 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 share their electrons.
C
C
C
R
R
R
R
R
R C C C
R
R
C
C
C
C
C
C
C
R
R R
R
R
R
R
Resonance Pattern 4 – Pi bond donation (from alkene, alkyne or aromatic) into a pi bond acceptor (alkene, alkyne,
aromatic, carbonyl, imine, nitrile, etc.). A polar pi bond acceptor is better and a pi cation is an even better acceptor.
Add in curved arrows and formal charge.
C
C
C
O
R
R
R
R
N C C
C
R
R
R
Neutral pi systems
Cationic pi systems
C
C
C
O
R
R
R
R
N C C
C
R
R
R
H
H
Notice the partial positive site is spread to multiple centers (two in this example).
Notice the cationic site is spread to multiple centers (three in this example).
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
Three variations with resonance in imidazole: neutral, anionic and cationic (add in necessary arrows)
N
N H
H H
H
N
N H
H H
H
N
N H
H
H
H
N
N H
H H
H
N
N H
H H
H
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.
N
N
H H
H
N
N
H H
H
N
N
H H
H
N
N
H
H
H
N
N
H H
H
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.
better OK^ OK^ OK^ better
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
N
N
H H
H
better OK
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
better
N
N
H
H
H
H
H
N
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.
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 sp^2 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.
C
N
C
N
C
H
H
H
H
C
N
C
N
C
H
H
H
C
N
C
N
C
H
H
H
H
H
C
N
C
N
C
H
H
H
H
C
N
C
N
C
H
H
H
C
N
C
N
C
H
H
H
H
H
OK resonance, but creates charges in a neutral structure
Excellent resonance, spreads out high energy negative charge
Excellent resonance, spreads out high energy positive charge
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
C C
sp 2 R+^ is more common
C C
sp R+^ is less common R+^ needs electrons, has to overlap with a. an adjacent 2p lone pair with electrons (see first column) X =anion with lone pair (C,N,O) b. an adjacent pi bond (see below)
C
R
R R
C
R
R C
C
X
R
R
C C
R R
R R
C
R
R
C C
R
R
X = neutral atom with lone pair (N,O,F)
C
R
R N
C
R
R O
C
R
R F
R
R
R
C
R
R N
C
R
R O
C
R
R F
R
R
R
carbocation electron pair acceptors (lack of electrons)
R R
X R
R
R
R
lone pairs donators (excess of electrons)
X
X
lone pair of electrons can only donate to a. an empty adjacent 2p orbital (R+) b. an adjacent pi bond
CC pi bonds can donate or accept (balance of electrons)
C C
neutral CC pi bonds can a. donate to an empty adjacent 2p orbital (R+) b. donate to an adjacent pi bond c. accept from an adjacent lone pair d. accept from an adjacent pi bond e. polar pi bonds generally only accept (don't donate) from lone pairs or CC pi bonds
2D 3D
2D
2D
3D
3D 3D
3D 3D
C
R
R C
R
R
R C C R alkenes alkynes^ aromatics
R C
C
R
R C C
N
C
R
O
C
R N C
N C
R R R R R R
R C
R
C R
R
N
R
C R
R
O
C R
R
R
R
R
R C C R
R N C R
O C R
N C R
O C R
C
R
R C
C
R
R
R
R C C C
R
R
C
R
R
C
R
R C
C
R
R
R
R C C C
R
R
C
R
R
O (^) O
C (^) C
R
R R
R
C
R C
R
R
C
O
R
C
R C
R
R
C
O
R C C C
N
R
R C C C
N
R
C N
C N
anion lone pair into empty 2p
pi bond into pi bond
pi bond into empty 2p
neutral lone pair into empty 2p C
R
R C
O R R C C N
R
R
C
R
R C
O
R
R
R C C N
R
R
R
neutral lone pair (N,O,F) into pi bond
anion lone pair (C,N,O) into a pi bond
R
This is just a sampling. There are too many variations to show all possibilities.
In this slide, R = C or H O » S F » Cl, Br, I
CO / CN pi bonds can accept (push to polar atom)
C O
R R
We usually don't separate the electrons (form charge) unless one of the atoms of the pi bond is a heteroatom (N,O).
Specific Examples
y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related
C
R
D N
R
R
R
C
R
D
Just a sampling. There
are too many variations
to show all possibilities.
In this slide,
R = C or H
O S
F Cl, Br, I
acceptors (A) empty 2p / pi patterns
connecting pi patterns
donors (D:) lone pair / pi patterns
lone pairs a. neutral b. anion
C/C pi bonds alkenes, alkynes, aromatics
C
R
D O
C
R
D F
R
D C N R
D C O
O
C
D
R
D C N
donor = D
acceptor = A
O
N
D
O
C
C
D
R
R
R
D
O
N
D
C
C
A
R
D
R
D C C A
D
A
C
N
A
D
R
R
C
R
A
R
N
A
O
A
R
N A O A F A
R
R
C
C
A
R
R
R
R C C A
A
O
C
C
C
C
C
O
NH 2
CH 3
CH 3
H
donor site(s)? acceptor site(s)? connector site(s)?
Example of 3D representation (below)
C C C C
H 3 C
O C O
N
CH 3
H
H
This 3D framework fits
all resonance structures above.
Delocalization occurs
through parallel 2p orbitals.
Where are + and - sites?
(substitutions? insertions?)
C C C C
H 3 C
O C
H
O
N
CH 3
H
H
D C CR
Generic Examples
H
O
C
C
C
C
C
O
NH 2
CH 3
CH 3
H
O
C
C
C
C
C
O
NH 2
CH 3
CH 3
H
O
C
C
C
C
C
O
NH 2
CH 3
CH 3
H
(possible)
