Organic Resonance Systems

Transcription

1 rganic esonance Systems ommon, Possible Patterns of esonance There are four common resonance patterns that we encounter, using two donor sites and two acceptor sites. onor electrons () can come from lone pairs and pi bonds. cceptor sites () include an empty 2p orbital (almost always carbon in our course = carbocations) and pi bonds (polar is best). otice 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. esonance Pattern 1 Lone pair donation (2p orbital) into an empty 2p orbital (carbocation or pi cation). esonance Pattern 2 Lone pair (2p orbital) donation into pi bonds (many kinds). esonance Pattern 3 Pi bond donation (from alkene, alkyne or aromatic) into an empty 2p orbital (carbocation or pi cation). esonance 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.) lone pairs (neutral or anion) pattern 1 onors pi bonds (alkenes, alkynes, aromatics) pattern 3 cceptors empty 2p orbitals pi cation pi cation pi cation lone pair donors = acceptor sites = empty 2p (above) or pi bond (next page) a. anions (carbon, nitrogen, oxygen) resonance resonance b. neutral (nitrogen, oxygen, halogens) resonance none (S too?) (l,br,i too?) none resonance resonance resonance y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

2 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related pi bonds (all kinds) pattern 2 pattern 4 polar is better polar is better polar is better cceptors onors lone pairs (neutral or anion) pi bonds (alkenes, alkynes, aromatics) = acceptor groups = empty carbon 2p orbital or pi bond (polar is better) empty 2p or pi cations : = donor group = lone pair (can be neutral or negatively charged) or pi bond (alkene, alkyne, aromatic)

3 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related neutral pi patterns: polar is better better, but all kinds are possible If "" atom has a lone pair If "" atom has a lone pair many variations more resonance connecting pi patterns / pi bonds, usually alkenes, alkynes, aromatics Make your own problems with various donors and acceptors. Possible connector patterns with any combination of the above donors and/or acceptors. The 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.

4 rganic esonance Systems esonance Pattern 1 Lone pair donation into pi bonds (many kinds). dd in curved arrows and formal charge and draw 3 structures (use template on the right, just below). ecide an atom s hybridization using a resonance structure where it has its maximum bonds. ( key for each group is the following page.) Two imensional pushable electrons = lone pair acceptor = pi bond Three imensional,, can be,, in our course sp 2 pi acceptor y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

5 rganic esonance Systems esonance Pattern 1 Lone pair donation into pi bonds (many kinds). dd in curved arrows and formal charge and draw 3 structures (use template on the right, just below). ecide an atom s hybridization using a resonance structure where it has its maximum bonds. Two imensional pushable electrons = lone pair acceptor = pi bond Three imensional,, can be,, in our course sp 2 pi acceptor y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

6 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

7 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

8 rganic esonance Systems Two imensional pushable electrons = lone pair acceptor = pi bond Three imensional,, can be,, in our course no resonance in this pi bond (also consistent with the example that follows) 3 y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

9 rganic esonance Systems Two imensional pushable electrons = lone pair acceptor = pi bond Three imensional,, can be,, in our course sp pi acceptor no resonance in this pi bond (also consistent with the example that follows) 3 3 y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

10 rganic esonance Systems (also consistent with the example that follows) 3 (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. ot 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. 3 skeleton and resonance structures. azide = 3 nitronium ion = 2 carbon dioxide = 2 y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

11 rganic esonance Systems (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. ot 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. 3 skeleton and resonance structures. azide = nitronium ion = 2 carbon dioxide = 2 y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

12 rganic esonance Systems esonance 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). dd in curved arrows and formal charge. 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 2 resonance structures 3 resonance structures - carbonyl resonance dditional 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. "" = nitrogen, oxygen, an alkene, alkyne or aromatic ring. 2 B The amide nitrogen atom extends resonance to one additional atom. ontribution: >>B y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

13 rganic esonance Systems esonance 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). dd in curved arrows and formal charge. 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 2 resonance structures 3 resonance structures - carbonyl resonance dditional 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. "" = nitrogen, oxygen, an alkene, alkyne or aromatic ring. 2 2 B 2 The amide nitrogen atom extends resonance to one additional atom. ontribution: >>B y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

14 rganic esonance Systems esonance Pattern 3 Pi bond donation (from alkene, alkyne or aromatic) into an empty 2p orbital (carbocation). dd in curved arrows and formal charge. We do not show this sort of resonance using or pi bonds, assuming that the more electronegative nitrogen and oxygen atoms are not as willing to share their electrons. esonance Pattern 4 Pi bond donation (from alkene, alkyne or aromatic) into a pi bond acceptor (alkene, alkyne, aromatic, carbonyl, imine, nitrile, etc.). polar pi bond acceptor is better and a pi cation is an even better acceptor. dd in curved arrows and formal charge. eutral pi systems otice the partial positive site is spread to multiple centers (two in this example). ationic pi systems otice the cationic site is spread to multiple centers (three in this example). y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

15 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related esonance Pattern 3 Pi bond donation (from alkene, alkyne or aromatic) into an empty 2p orbital (carbocation). dd in curved arrows and formal charge. We do not show this sort of resonance using or pi bonds, assuming that the more electronegative nitrogen and oxygen atoms are not as willing to share their electrons. esonance Pattern 4 Pi bond donation (from alkene, alkyne or aromatic) into a pi bond acceptor (alkene, alkyne, aromatic, carbonyl, imine, nitrile, etc.). polar pi bond acceptor is better and a pi cation is an even better acceptor. dd in curved arrows and formal charge. eutral pi systems ationic pi systems otice the partial positive site is spread to multiple centers (two in this example). otice the cationic site is spread to multiple centers (three in this example).

16 rganic esonance Systems y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related Three variations with resonance in imidazole: neutral, anionic and cationic (add in necessary arrows) eutral imidazole: elocalization of p orbital electrons in a neutral ring structure. dditional structures create charge, but have the same number of bonds and full octets. nionic imidazole: elocalization 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 K K K better

17 rganic esonance Systems ationic imidazole: elocalization 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. ne extra structure is shown with positive charge on carbon, but it has an incomplete octet. 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. better better K 3 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. K 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 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

18 rganic esonance Systems Three variations with resonance in imidazole: neutral, anionic and cationic eutral imidazole: elocalization of p orbital electrons in a neutral ring structure. dditional structures create charge, but have the same number of bonds and full octets. nionic imidazole: elocalization 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 K K K better ationic imidazole: elocalization 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. ne extra structure is shown with positive charge on carbon, but it has an incomplete octet. 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. better better K 3 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. K 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 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

19 rganic esonance Systems Specific Examples lone pairs donators (excess of electrons) pi bonds can donate or accept (balance of electrons) / pi bonds can accept (push to polar atom) carbocation electron pair acceptors (lack of electrons) =anion with lone pair (,,) = neutral atom with lone pair (,,) anion lone pair into empty 2p lone pair of electrons can only donate to a. an empty adjacent 2p orbital (+) b. an adjacent pi bond neutral lone pair into empty 2p neutral pi bonds can a. donate to an empty adjacent 2p orbital (+) 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 pi bonds alkenes alkynes aromatics anion lone pair (,,) into a pi bond neutral lone pair (,,) into pi bond pi bond into pi bond sp 2 + is more common sp + is less common + needs electrons, has to overlap with a. an adjacent 2p lone pair with electrons (see first column) b. an adjacent pi bond (see below) This is just a sampling. There are too many variations to show all possibilities. 2 3 pi bond into empty 2p We usually don't separate the electrons (form charge) unless one of the atoms of the pi bond is a heteroatom (,). 3 In this slide, = or» S» l, Br, I y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

20 rganic esonance Systems donors (:) lone pair / pi patterns lone pairs a. neutral b. anion / pi bonds alkenes, alkynes, aromatics Generic Examples connecting pi patterns (possible) acceptors () empty 2p / pi patterns Example of 3 representation (below) donor site(s)? acceptor site(s)? connector site(s)? donor = acceptor = In this slide, = or S l, Br, I Just a sampling. There are too many variations to show all possibilities. 3 Where are + and - sites? (substitutions? insertions?) This 3 framework fits all resonance structures above. elocalization occurs through parallel 2p orbitals y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

21 rganic esonance Systems 1. rganic Semiconductor Materials e electron poor S S electron rich n-type material (excess electrons) S S electron poor S electron rich p-type material (electron deficiency) S electron poor S S electron rich 2. Sun Screen Materials M LUM gap in the ultra violet portion of the spectrum cinnamate esters and related compounds 3 3 resonance can show some of the interaction among the orbitals, communication of one end of the molecule with the other end LUM M h ultra violet excitation LUM M absorption of high energy UV photon protects your body from that energy y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

22 rganic esonance Systems 3. electron transport in living systems, electron conduction through membranes, photosynthesis, etc lose electron 2. lose proton 3. lose electron 4. lose proton e capture electron ( = more carbons) electrons transfer out and protons are lost in a reverse of what is shown here to move electrons along in an organic environment B pick up proton pick up proton B 3 3 repeat processes a 2nd time, capture electron and pick up proton 3 3 e capture electron electron is external to membrane (assume mitochondrial membrane) ow does it get across to do chemistry? rganic Structure e high energy electron specie on outside of membrane moves via M/LUM transitions across the membrane to be deposited as a high energy electron specie on the inside of the membrane membrane in organism electron is internal to membrane rganic Structure e y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related

23 rganic esonance Systems enol structure - has resonance contributor showing an electron rich double bond. vitamin ascorbic acid one electron oxidation one electron oxidation helps to reduce free radical one electron oxidation radical cation has resonance reduced one electron reduction of reactive free radical by vit acid/base reaction B B dangerous free radical is stabilized radical cation has resonance again vit dehydroascorbic acid washes out of body acid vitamin E - for comparison enol-like structure rom a recent hemical and Engineering ews (&E ews) article: March 11, 2013, p 4 esveratrol 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 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.. If vit is the last to donate its electron(s), its water solubility allows it to be eliminated from the body in the urine. esveratrol has 3 parts that are similar to the enol part of vit. 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, 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). ould we design an even more electron rich version of resveratrol to test for antioxidant properties? int: can you add one or two additional "" 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? ou may have to draw some resonance structures to decide which would work better. linical trials on resveratrol were halted in 2010 because some of the patients developed kidney failure. owever, many variations are being evaluated in human volunteers for diabetes, psoriasis, inflamation and more. Modify one of the rings with one or two "" groups to make resveratrol a better antioxidant. Where would you put it or them? resveratrol? y:\files\classes\0 rganic Topics - latest\315 topics\ lecture notes, \314 eview Problems\03 resonance related