Common, Possible Patterns of Resonance There Are Four Common

Organic Resonance Systems

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 pi bonds lone pairs (neutral or anion) (alkenes, alkynes, aromatics)

pattern 1 pattern 3

XC XC C C C C C C

XC XC H H Acceptors C O empty 2p O H O H C O orbitals C C C C XC XC pi cation pi cation O H O H XC XC pi cation

lone pair donors A = acceptor sites = empty 2p (above) or pi bond (next page) a. anions (carbon, nitrogen, oxygen) R

C N O R A R A A F

resonance resonance resonance none b. neutral (nitrogen, oxygen, halogens)

R O F R A A C N (S too?) (Cl,Br,I too?) R A

none resonance resonance resonance y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related examples & answers, 21p.doc Organic Resonance Systems

Donors pi bonds lone pairs (neutral or anion) (alkenes, alkynes, aromatics) pattern 2 pattern 4 O O C C C C C N C N C C

C C

Acceptors polar is better C C pi bonds O N O N N N (all kinds) C C polar is better C C

C C C C O O O O H H O O

C C C C C C O C O C

polar is better C C

D: = donor group = lone pair (can be neutral or negatively charged) or CC pi bond (alkene, alkyne, aromatic) A = acceptor groups = empty carbon 2p orbital or pi bond (polar is better)

R R R R

C C R C R C R D R D N D N D N

empty 2p or pi cations R R R

R R R

C R C R C R D O D O D O

R R R

C C C D F D F D F

R R R N N N C C C D D D

O O O C C C D D D

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neutral pi patterns: polar is better better, but all kinds are possible If "D" atom has a lone pair R R R

C C C D O D O D O

R R R

C R C R C R D N D N D N If "D" atom has a lone pair

D N D N N N C R C R C C D D R R

R R R R C C C R C R C D C D C C D D R R

D D N N O more D O D resonance many variations

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.

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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.)

pushable electrons = lone pair Two Dimensional acceptor = pi bond Three Dimensional Y Y X Z X Z YZ YZ

X X X, Y, Z can be C, N, O in our course sp2 pi acceptor

H H

H C H C CH3 C F C O

H H

H C CH3 H C C N C O

H CH3 H

H H

H C CH3 H C CH3 C N C C

H H CH3

H H H C C 3 H C C CH N F 3 3 N O

H H H3C C CH3 N N H3C C N O CH3

H H

H3C C CH3 H3C C CH3 N C N N

CH3

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pushable electrons = lone pair Two Dimensional acceptor = pi bond Three Dimensional Y Y X Z X Z YZ YZ

X X X, Y, Z can be C, N, O in our course sp2 pi acceptor

H H H

H C CH H C C H C CH3 3 C F C F C O C O

H H H

H H H H H C H C CH3 H C CH3 H C C O C N C N C O H H CH3 H CH3 H

H H H H H C CH H C CH H C CH3 H C CH3 3 3 C C C N C N C C H CH H H H CH3 3

H H H

H C C C H3C C CH3 3 H C C CH N F N F 3 3 N O N O

H H H H H3C C CH3 H3C C CH3 H C C N N N N H3C C 3 N O N O CH3 CH3

H H H H

H C C CH H3C C CH3 H C C CH 3 3 H3C C CH3 3 3 N C N C N N N N

CH3 CH3

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H H

C C CH3 O F O O

H H C CH3 O N C O O CH3

H H C CH C CH 3 3 O C O N

CH3

O N O O O O

CH3

N N CH3 O O O O

N N CH3 O N O F

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H H H H

C C C CH3 C CH3 O F O F O O O O

H H H H C CH3 C CH3 O N O N C C O O O O CH3 CH3

H H H H C CH C CH3 C CH 3 3 C CH3 O C O C O N O N CH3 CH3

O O N N O O O O O O O O

CH CH3 3

N N N CH3 N CH3 O O O O O O O O

N CH N N CH3 3 N O N O N O F O F

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pushable electrons = lone pair acceptor = pi bond Two Dimensional Three Dimensional

X Y Z X Y Z X YZ X YZ

no resonance X, Y, Z can be C, N, O in our course in this pi bond

O CCH F CCH

H3C

H3C N CCH O CCH H3C

H3C N CCH C CCH H C 3 H3C

F CN O CN

H3C

H3C N CN O CN

H3C

H3C N CN C CN

H3C H3C (also consistent with the example that follows)

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pushable electrons = lone pair acceptor = pi bond Two Dimensional Three Dimensional

X Y Z X Y Z X YZ X YZ

X, Y, Z can be C, N, O in our course no resonance sp pi acceptor in this pi bond

O CCH F CCH F CCH O CCH

H3C H3C

H3C H3C N CCH N CCH O CCH O CCH

H3C H3C

H3C H3C N CCH N CCH C CCH C CCH H C H3C 3 H3C H3C

F CN F CN O CN O CN

H3C H3C

H3C H3C N CN N CN O CN O CN H3C H3C

H C H3C 3 N CN N CN C CN C CN

H3C H3C H C H3C 3 (also consistent with the example that follows)

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F CO O CO

H3C

H3C H

N CO C NN

H3C H

N NN N CO H C 3 H3C (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.

3D skeleton and resonance structures.

X Y Z X Y Z X Y Z

azide = N3 N N N N N N N NN nitronium ion = NO2

O N O O N O O NO

carbon dioxide = CO2

O C O O C O O CO

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O CO F CO F CO O CO H C H3C 3

H C H3C 3 H H

N CO N CO C NN C NN H C H3C 3 H H

N NN N NN N CO N CO H C H C 3 3 H3C H3C (also consistent with the example that follows) (also consistent with the example that follows)

3D skeleton and resonance structures.

X Y Z X Y Z X Y Z

azide = N3

-2 -2 N N N N N N N NN nitronium ion = NO2

O N O O N O O NO

carbon dioxide = CO2

O C O O C O O CO

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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.

Usually we write the first three examples the other pi cations way around, starting with the neutral structure and showing the expected polarization using the minor R resonance structure. In these examples we are showing a lone pair sharing with an empty 2p orbital. C H R N R R C R R N R

C H R O R

C R O R C N H

R C N R C O

C R F

2D resonance structures 3D resonance structures - carbonyl resonance

R R

C O CO R R C O C O R R R R

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.

The amide nitrogen O atom extends resonance to one additional atom. Contribution: A>C>B C R NH2 A C B

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Usually we write the first three examples the other pi cations way around, starting with the neutral structure and R showing the expected polarization using the minor R resonance structure. In these examples we are showing a lone pair sharing with an empty 2p orbital. C H C H R N R N R R R R C R C R R N R N R R

C H C H R O R O R R

C C R O R O R C N H R C N H

R C N R C N R C O R C O

R R

C C R F R F

2D resonance structures 3D resonance structures - carbonyl resonance

R R

C O CO R R C O C O R R R R

The amide nitrogen O O O atom extends resonance to one additional atom. Contribution: A>C>B C C C R NH2 R NH2 R NH2 A C B

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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. R

R C R R C C R C C C

R R R

R C R C C

C C R R C C

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.

Neutral pi systems R

R C O C C

R R

Notice the partial positive site is spread to multiple centers (two in this example).

NCC

C R

R Cationic pi systems R

R C O C C H

R R

Notice the cationic site is spread to multiple centers (three in this example).

H NCC

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R R R R

R C R R C R R C R R C R C C C C C C C C

C C R C C R C C R C C R R C C R C C R C C R C C

R R R R R R R R

Neutral pi systems

R R R

R C O R C O R C O C C C C C C

R R R R R R

Notice the partial positive site is spread to multiple centers (two in this example). R R R NCC NCC NCC C R C R C R R R R Cationic pi systems

R R R

R C O R C O R C O C C H C C H C C H

R R R R R R Notice the cationic site is spread to multiple centers (three in this example).

R R R

H NCC H NCC H NCC

C R C R C R R R R y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related examples & answers, 21p.doc Organic Resonance Systems

Three variations with resonance in imidazole: neutral, anionic and cationic (add in necessary arrows) 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. H H H H H H H H H H

N H N H N H N H N H N N N N N

H H 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. H H H H H H H H H H

N N N N N N N N N N

H H H H H better OK OK OK better

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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 H The 3rd structure is not as good as the first N H two structures. It has fewer bonds and an N H N H incomplete octet, but is still a resonance N N N contributor and provides information about H H H the chemical reactivity of this cation. H H H better better OK 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 C C C C

N H N N H N N N H C C C

H H H

H H H H H H C C C C C C

N H N N H N N N H C C C

H H H OK resonance, but creates Excellent resonance, spreads out Excellent resonance, spreads out charges in a neutral structure high energy negative charge high energy positive charge

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Three variations with resonance in imidazole: neutral, anionic and cationic 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. H H H H H H H H H H

N H N H N H N H N H N N N N N

N N N N N N N N N N

H H H H H better OK OK OK better 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 H The 3rd structure is not as good as the first N H two structures. It has fewer bonds and an N H N H incomplete octet, but is still a resonance N N N contributor and provides information about H H H the chemical reactivity of this cation. H H H better better OK 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 C C C C

N H N N H N N N H C C C

H H H

H H H H H H C C C C C C

N H N N H N N N H C C C

H H H OK resonance, but creates Excellent resonance, spreads out Excellent resonance, spreads out charges in a neutral structure high energy negative charge 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 examples & answers, 21p.doc Organic Resonance Systems

carbocation electron pair acceptors Specific Examples CC pi bonds can CO / CN pi bonds donate or accept can accept (lack of electrons) lone pairs donators (balance of electrons) (push to polar atom) (excess of electrons)

CC CO C C X neutral CC pi bonds can sp2 R+ is more common a. donate to an empty adjacent 2p orbital (R+) lone pair of electrons can only donate to b. donate to an adjacent pi bond a. an empty adjacent 2p orbital (R+) c. accept from an adjacent lone pair b. an adjacent pi bond d. accept from an adjacent pi bond e. polar pi bonds generally only accept (don't donate) CC from lone pairs or CC pi bonds R X X R + X R R R sp R is less common C R CC R+ needs electrons, has to overlap with R C CC a. an adjacent 2p lone pair with electrons 3D R R 2D 3D (see first column) R R R X =anion with lone pair (C,N,O) b. an adjacent pi bond (see below) 2D 3D 3D X = neutral atom with lone pair (N,O,F) R R anion lone pair R R R R into empty 2p C R C R C R R C C C C R R C R C R R R C R C R R alkenes R alkynes aromatics 2D R 3D R R 3D R R anion lone pair (C,N,O) into a pi bond pi bond into empty 2p C C R R R R N R N R C C C C R R O R C R R R O R C R

R R C C R R O R O R C C C R C R R R C C R C C R R C C R C C R R R R NCR NCR neutral lone pair (N,O,F) into pi bond neutral lone pair R R into empty 2p R R C O C R R C O C R C R R C R R C R C R R R R N R N R R R C C R R C R C R R R N N C R C C C R R R R O N N R O R pi bond into R pi bond R R R C O R C O C C C C C C R F R F R R R R R R C C We usually don't separate the electrons C N C N In this slide, R N C R R N C R (form charge) unless one of the atoms C R C R of the pi bond is a heteroatom (N,O). R = C or H O » S R R F » Cl, Br, I OCR OCR This is just a sampling. There are too many variations to show all possibilities. y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related examples & answers, 21p.doc Organic Resonance Systems

connecting acceptors (A) Example of 3D representation (below) Generic Examples pi patterns empty 2p / pi patterns H donors (D:) R R lone pair / pi patterns O C O CH3 D C C lone pairs D R C CCC a. neutral C A R b. anion H3C N H R C/C pi bonds C R H alkenes, alkynes, aromatics D N D C CA donor site(s)? CH R R acceptor site(s)? 3 O R connector site(s)? C D R A H C C R C NH2 D O C C A R O C N R A C CH3 CH3 D N D F O C A H C O D C NR C NH A R 2 C C R (possible) O C

N D C O CH CH3 3 R A O D C N donor = D H C O C NH R A acceptor = A R 2 C C O C In this slide, C F D O A CH3 R = C or H CH3 R O  S O O F  Cl, Br, I R C N H C D O C A C NH2 C C R N O C D O CH3 R C CA R Where are + and - sites? C R (substitutions? insertions?) D C This 3D framework fits all resonance structures above. R Delocalization occurs Just a sampling. There through parallel 2p orbitals. A are too many variations D C C R to show all possibilities. H O C O CH D 3 C CCC H C 3 N H H

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1. Organic Semiconductor Materials

n-type material p-type material (excess electrons) (electron deficiency) F F F F F F F F S S S F S S S F F S F F F electron rich S electron rich F F electron poor F electron poor F electron poor electron rich

2. Sun Screen Materials – HOMO LUMO gap in the ultra violet portion of the spectrum cinnamate esters and related compounds

H3C O O

OR LUMO LUMO resonance can show some of the interaction among the orbitals, h absorption of high energy UV communication of one end of photon protects your body from the molecule with the other end ultra violet that energy excitation H3C O O HOMO HOMO

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3. electron transport in living systems, electron conduction through membranes, photosynthesis, etc. O H O O H C O H B H3C O H3C O 3 e capture R R R H C O pick up H3C O H3C O electron 3 proton O O O (R = more carbons) repeat processes a capture 2nd time, capture e electron electron and pick 1. lose electron up proton H 2. lose proton O H O 3. lose electron H3C O pick up H3C O 4. lose proton proton R electrons transfer out and protons are lost R H C O H3C O in a reverse of what is shown here to move 3 BH O electrons along in an organic environment O H

high energy electron specie on outside of membrane electron is external to membrane moves via HOMO/LUMO transitions across the Organic e (assume mitochondrial membrane) Structure membrane to be deposited as a high energy electron How does it get across to do chemistry? specie on the inside of the membrane

membrane in organism

electron is internal to membrane Organic e Structure

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H H one electron H oxidation helps R O C O C to reduce O C R free radical H C H H C H H C H reduced H H H enol structure - has resonance contributor showing an electron rich double bond. radical cation has resonance vitamin C ascorbic acid H HO H HO H H H HO H one O O O HO O electron HO O one HO O oxidation electron oxidation

H H H H O O O O O O radical cation one electron has resonance dehydroascorbic acid reduction of HO H washes out of body reactive free H R O radical by vit C HO O again R vit C acid acid/base R R RH reaction BH H H dangerous O O RH free radical is stabilized B vitamin E - for comparison HO enol-like structure

From a recent Chemical and Engineering News (C&E News) article: March 11, 2013, p 4 Resveratrol is a natural product is found in red grapes and red wine. It became famous when it was proposed to activate antiaging effects in the body. Some feel that activates sirtuin enzymes, which clip acetyl groups from proteins, which regulate a host of aging associated enzymes. It is also considered to be in a class of antioxidant molecules which help to quench damaging free radicals that form in the body. Such molecules are usually very electron rich (vit E and vit C are similar examples). They can donate a single electron to stabilize reactive free radicals and become stable free radicals that do not agressively react further, as proposed above for vit. C. If vit C is the last to donate its electron(s), its water solubility allows it to be eliminated from the body in the urine. Resveratrol has 3 parts that are similar to the enol part of vit C. Which ring do you think would be more likely to donate an electron, forming a semi-stable free radical that could either be eliminated from the body or accept an electron back from vit C, which could then be eliminated from the body. In 2008 GlaxoSmithKline (GSK) paid $720 million for the company that had the rights to resveratrol (Sirtris Pharmaceuticals). Could we design an even more electron rich version of resveratrol to test for antioxidant properties? Hint: can you add one or two additional "HO" groups to one of the rings that could work together to stabilize the initial cation formed when the electron is given away? Would you want to use the left ring or the right ring? You may have to draw some resonance structures to decide which would work better. Clinical trials on resveratrol were halted in 2010 because some of the patients developed kidney failure. However, many variations are being evaluated in human volunteers for diabetes, psoriasis, inflamation and more. O H Modify one of the rings O with one or two "HO" H groups to make resveratrol a better antioxidant. Where would you put it or them? resveratrol ?

O H y:\files\classes\0 Organic Topics - latest\315 topics\20 315 lecture notes, 7-9-15\314 Review Problems\03 resonance related examples & answers, 21p.doc