Common, Possible Patterns of Resonance Resonance Implies

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 pi bonds lone pairs (alkenes, alkynes, aromatics) pattern 1 pattern 3

XC XC C C C C C C pi cation empty 2p empty 2p orbital orbitals H H H empty 2p orbital b a H H H O O O a b C O XC XC XC C O C O C C C C C C pi cation empty 2p orbital pi cation pi cation empty 2p orbital Acceptors pattern 2 pattern 4

O O C C C N C N C C CC is OK C C

pi bonds C C polar is better (all kinds) C C O N O N H H polar is better N N C C C C H C H C O N O N C C pi cation is best pi cation is best

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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).

Usually we write these examples the other way around, pi cations starting with the structure having the maximum number R R of bonds and show the expected polarization forming the minor resonance by pushing the pi electrons to the C H more electronegative atom. In these examples we are C H doing it backwards, showing a lone pair sharing with an R N R N empty 2p orbital, making a pi bond. R R R R R R C R C R C H C H R N R N R O R O

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

R C O R C O

R R C N R C N R

C C R F These structures have more R F bonds and full octets and are considered the major contributor. These structures have more bonds and full octets and are considered the major contributor.

2D resonance structures - carbonyl resonance, 3D resonance structures - carbonyl resonance this is the usual way of showing carbonyl resonance (reverse of above).

R R R R C O CO C O C O R R R R If we turn it around, it shows formation of a pi bond. R R R R CO C O C O C O R R R R

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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 O The amide nitrogen C H C H C H atom extends resonance R N R N R N to one additional atom. BCContribution: A > C > B A H H H 3D resonance structures - carbonyl resonance

O O O H H H C N C N C N H H H R 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 O O C H C H C H C H F N F N F N F N A H B H C H D H 3D resonance structures - carbonyl resonance

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

F F F 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 C C H C R C R C R O R O R R N R O R S R Cl carboxylic acids esters thioesters anhydrides acid chlorides amides H

A few examples of a carbonyl group with two adjacent lone pairs. O O O O NR O O

C C C C C C C HO OH RO OR R2N OR R2N NR2 R2N NR2 RO Cl Cl Cl

carbonic acid carbonates urethanes ureas guanidines alkyl chloroformates phosgene

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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.

2D resonance structures pushable electrons = lone pair 3D resonance structures acceptor = pi bond

Y Y X Z X Z YZ YZ

X X sp2 pi acceptor X, Y, Z are usually C, N, O in organic chemistry, The examples below can be viewed in either direction. 12R R R R

R C R C R C R R C R C F C F C O C O

R R R R

R 3 R R 4 R R C R C R R C R R C C O C N C N C O R R R R R R

R 5 R R 6 R R C R R C R R C R R C R C C C N C N C C R R R R R R equivalent

78 R R R R R C R C R C R R C R N F N F N O N O

910R R R R R C R R C R N N N N R C R C N O N O R R

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11 12 R R R R R C R R C R R C R R C R N C N C N N N N equivalentR reverse of 5 R

13 14 R R R R

C C C R C R O F O F O O O O

15R R 16 R R C R C R O N O N C C O O O O R R equivalent

17 18 R R R R C R C R C R C R O C O C O N O N R reverse of 10 R reverse of 4

19 20

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

equivalent equivalent

21 22 R R

N N N R N R O O O O O O O O equivalent

23 24

N R N R N N O N O N O F O F

There are even more possibilities! All of the above examples can be written in the reverse direction, and some are.

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pushable electrons = lone pair acceptor = pi bond 2D resonance structures 3D resonance structures

X Y Z X Y Z X YZ X YZ

sp pi acceptor X, Y, Z are usually C, N, O in our course 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

O CCR F CCR F CCR O CCR R R

3 4 R R

N CCR N CCR O CCR O CCR R R

5 6 R R N CCR N CCR C CCR C CCR R R R R

F CN F CN O CN O CN R R

9 10 R R

N CN N CN O CN O CN R R

11 12 H C H3C 3 N CN N CN C CN C CN

H3C H3C H C H3C 3

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13 14

O CO F CO F CO O CO R R

15 16 R R R R

N CO N CO C NN C NN

R R R R

17 18

N NN N NN N CO N CO

R R 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 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 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 R R R R be anions of carbon, nitrogen and oxygen or neutral atoms of nitrogen, oxygen and halogens. C C pattern 3 C C R R R R

RC R R C R C C pattern 2 C C

R R RC R R C R 3D Lewis structure R R R R 3D Lewis structure R R C C R R C C R R R C C R RC C C RC R R R RC R RC 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. pattern 3 R R R C CC R C CC pattern 2 R C CC R C CC R R 3D Lewis structure - notice the pi bonds are perpendicular 3D Lewis structure R R

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

2D Lewis structure aromatic rings as donors (Pattern 3) H H H H H H H H C C R CC R C C R CC R H C C C H C C C H C C C H C C C CC R C C R CC R C C R H H H H H H H H 3D Lewis structure H H H H H H H H C C C C C C C C R R R R H C C C H C C C H C C C H C C C R R CC R R CC CC CC H H H H H H H H

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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 H H H H H H C C R CC R C C R CC R H C C C H C C C H C C C H C C C CC R C C R CC R C C R H H H H H H H H 3D Lewis structure H H H H H H H H C C C C C C C C R R R R H C C C H C C C H C C C H C C C R R CC R R CC CC CC H H H H H H H 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 BCR R

R C O R C O C C R C O C C C C R R R R R R 3D Lewis structure

R R R R R R C C C C C C R C O R C O R C O R R R R E R F R G NCC NCC NCC C R C R C R R R R 3D Lewis structure

R R R N C C N C C N C C

C R C R C R

R R R

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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 B R C R

R C O R C O C C H R C O C C H C C H R R R R R R 3D Lewis structure

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

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 BC R R R

H NCC HN C C HN C C

C R C R C R

R R R 3D Lewis structure

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

C R C R C R

R R R

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Combination of All 4 Patterns

donor site(s)? acceptor site(s)? There are other ways to show resonance in these examples beyond what is shown below. connector site(s)?

H C CH H C CH CH A 3 3 BC3 3 3 N N O

H C H C pattern 2 H C C NH2 C NH2 C NH2 C C C C C C O C O C Move lone pair O C Move lone pair into empty p orbital. into pi cation. CH CH CH 3 pattern 1 3 3 Move lone pair Move lone pair into pi bond. into pi bond. pattern 2 pattern 2 pattern 2 CH CH3 CH3 3 F O E O D O pattern 3 H C H C pattern 4 H C C NH C NH2 C NH2 2 C C C C C C O C O C O C Move CC pi bond Move CC pi bond CH CH3 into empty p orbital. CH3 into polar pi bond. 3 Example of 3D representation (below)

H H

O C O CH3 O C O CH3 C CCC C CCC H3C N H H C 3 N H H A D H

This 3D framework fits all resonance structures above (and more). Delocalization occurs through parallel 2p orbitals.

O C O CH3 C CCC H C 3 N H This pi bond does not participate H in resonance in these examples.

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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 H H H H

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

Best H OKH OKH OK H OK 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 H H H H

N N N N N N N N N N

H 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 H The 3rd structure is not as good as the first two structures. It has fewer bonds and an N H N H N H N incomplete octet, but is still a resonance 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 Example 1 H Example 2 Example 3 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 electron charges in a neutral structure high energy negative charge density to electron deficient sites.

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Generic Examples donors = D acceptors = A lone pair / pi patterns donors = D empty 2p / pi patterns acceptors = A R lone pairs a. neutral (N, O, F) In this slide, C b. anion (C, N, O) D R R C/C pi bonds R = C or H alkenes, alkynes, aromatics O = S F = Cl, Br, I C R D N R connecting R C pi patterns R A R

R C R N D O R A D C C A R O A R C R D F D C CA N D C NR R A D O D C O R A

A F D C N A R R R D N C A C C R R C D O D C C A R (possible) O R R N R C CA D O D C C R Just a sampling. There N are too many variations D to show all possibilities. D O

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.

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