Organic Chemistry 1 · Structure and electron accounting
Resonance and electron distribution: practice problems
113 problems, each with its worked solution. Open one to read every step, or filter by tag.
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- Redistributing a carboxylate pair
- A π pair next to an empty orbital
- Spread a benzylic positive charge
- Acid switches on a carbonyl
- An amide nitrogen shares its lone pair
- Why the far end of an enone is electron-poor
- Three equal drawings of carbonate
- Push a carbanion pair through an alkene
- Unequal allylic contributors
Draw the mechanism 85
- Redistributing a carboxylate pairDraw simultaneous resonance arrows without overfilling carbon.
- A π pair next to an empty orbitalMove an allylic π pair while relocating a carbocation formal charge.
- Spread a benzylic positive chargeGenerate every contributor of a benzylic carbocation by moving ring π pairs toward the empty orbital.
- Acid switches on a carbonylProtonate a ketone oxygen and draw the carbocation contributor that explains why acid makes the carbonyl carbon more electrophilic.
- An amide nitrogen shares its lone pairDelocalize an amide nitrogen lone pair into the carbonyl and use the result to explain why an amide nitrogen is planar and only weakly basic.
- Why the far end of an enone is electron-poorRelay carbonyl polarization through a conjugated C=C bond to show why the β carbon of an α,β-unsaturated ketone is electrophilic.
- Three equal drawings of carbonateGenerate all three equivalent carbonate contributors and convert them into a fractional C–O bond order and oxygen charge.
- Push a carbanion pair through an alkeneMove a carbanion lone pair through an adjacent π bond with the two coordinated arrows an allyl anion requires, and locate its charge-bearing carbons.
- Unequal allylic contributorsShift the π bond of the 1-methylallyl cation to move the charge to the primary carbon and decide which contributor dominates.
- 4-Aminoacetophenone: amino donor to acetyl acceptorDraw the push-pull contributor of 4-aminoacetophenone with one set of four arrows running from the amino lone pair to the acetyl oxygen.
- 4-Methoxybenzaldehyde: methoxy donor to formyl acceptorPush a methoxy lone pair of 4-methoxybenzaldehyde through the ring into the formyl C=O to draw the quinoid oxonium–alkoxide contributor.
- 4-Methoxybenzyl cation: oxygen stabilizes the cationMove the 4-methoxybenzyl cation's positive charge through the ring to the methoxy-bearing carbon, then let the oxygen lone pair fill it to draw the oxocarbenium contributor.
- 4-Nitroaniline: amino donor feeding the nitro groupPush the amino lone pair of 4-nitroaniline through the ring into the para nitro group to draw the charge-separated quinoid contributor.
- 4-Nitrobenzyl anion: charge delocalized onto nitroPush the benzylic lone pair of the 4-nitrobenzyl anion through the ring into the nitro group to draw the quinoid contributor with negative nitro oxygens.
- 4-Nitrophenoxide: push-pull quinoid contributorPush the phenoxide lone pair across the ring into the para nitro group to draw the quinoid contributor of 4-nitrophenoxide.
- 4-Pyridone: the aromatic pyridinium-olate contributorDraw the zwitterionic contributor of pyridin-4(1H)-one with three arrows that relay the N–H nitrogen lone pair through the ring onto the carbonyl oxygen.
- Enolate: O⁻ and C⁻ contributorsConvert the oxygen-anion form of the acetaldehyde enolate into its carbanion contributor with two curved arrows.
- Amide nitrogen lone pair into C=ODraw the charge-separated contributor of acetamide by pushing the nitrogen lone pair into the C–N bond and the C=O π pair onto oxygen.
- Acetanilide: amide nitrogen donating into the ringPush the acetanilide nitrogen lone pair into the ring and on to the para carbon, and explain why this donation is weaker than in aniline.
- Acetic anhydride: one oxygen shared by two C=ODraw the charge-separated contributor of acetic anhydride in which the bridging oxygen donates a lone pair into one of the two carbonyl groups.
- Acetonyl radical: carbon and oxygen radical contributorsUse three fishhook arrows to move the unpaired electron of the acetonyl radical from the CH2 carbon onto the carbonyl oxygen.
- Acetophenone: acetyl withdraws from the ortho carbonPull the ipso–ortho π pair of acetophenone into the acetyl group to draw the enolate-like contributor with a positive ortho carbon.
- Acetyl chloride: weak chlorine donation into C=OPush a chlorine lone pair of acetyl chloride into the carbonyl to draw its chloronium–alkoxide contributor and judge its weight.
- Acetylacetonate anion: carbanion to enolate oxygenUse two curved arrows to move the negative charge of the acetylacetonate anion from C3 onto a ketone oxygen.
- α,β-Unsaturated carbonyl: electrophilic β-carbonDraw two successive contributors of acrolein: first move the C=O π pair onto oxygen, then shift the C=C π bond to put the positive charge on the β-carbon.
- Acylium ionFill the empty orbital of the acylium ion with an oxygen lone pair to reach the C≡O⁺ contributor.
- Allyl anionDraw the second contributor of the allyl anion with two curved arrows: carbanion lone pair into a new π bond, existing π pair onto the far carbon.
- Allyl radical (fishhook arrows)Draw the second contributor of the allyl radical with three single-electron (fishhook) arrows: the unpaired electron and one electron of the π bond pair up, and the other π electron moves to the far carbon.
- Aniline: amino lone pair puts charge on the ortho carbonPush the aniline nitrogen lone pair into the ring to draw the iminium contributor with a negative charge on an ortho carbon.
- Methoxy group donates into the ringDraw the charge-separated contributors of anisole that place negative charge on the ortho and para ring carbons and positive charge on oxygen.
- Benzaldehyde: formyl group pulls charge from paraPull a ring π pair into the formyl group of benzaldehyde to draw the alkoxide contributor with a positive charge on the para carbon.
- Benzonitrile: cyano group withdraws from paraPull a ring π pair into the cyano group of benzonitrile and shift the charge to draw the contributor with a positive para carbon and N⁻.
- Benzyl anionPush the benzylic lone pair into the ring to place the negative charge on an ortho carbon of the benzyl anion.
- Benzyl cation: four contributorsMove ring π bonds toward the empty orbital of the benzyl cation to draw its three ring contributors, placing the positive charge on both ortho carbons and the para carbon.
- Benzyl radical: fishhook arrows into the ringDraw the benzyl radical contributor with the unpaired electron on an ortho carbon using three fishhook arrows.
- But-2-enyl radical: unequal allylic radical endsDraw the second contributor of the 1-methylallyl (but-3-en-2-yl) radical with three fishhook arrows, moving the unpaired electron from the secondary carbon to the terminal CH2.
- Butadiene charge-separated minor formsDraw the charge-separated contributor of buta-1,3-diene and judge why it contributes only a little.
- Carbonate: three equivalent contributorsDraw the other two contributors of the carbonate ion by moving an oxygen lone pair into a C=O bond while the existing π pair moves onto its oxygen.
- Acetic acid charge-separated contributorDraw the charge-separated contributor of neutral acetic acid and use it to explain the shortened C–OH bond.
- Chlorobenzene: weak chlorine lone-pair donationPush a chlorine lone pair of chlorobenzene into the ring to draw the chloronium contributor with a negative ortho carbon, and judge how much it matters.
- Cyanomethyl anion: carbanion and ketenimine formsUse two curved arrows to move the negative charge of the cyanomethyl anion from carbon onto the nitrile nitrogen.
- Cyclopentadienyl anion: equivalent contributorsMove the cyclopentadienyl anion's lone pair into the ring with two curved arrows to draw an equivalent contributor with the charge on a different carbon.
- Diazomethane C⁻/N⁻ formsMove the negative charge of diazomethane from terminal nitrogen to carbon and explain why carbon is the nucleophilic site.
- Diethyl malonate enolate: carbanion to ester enolateUse two curved arrows to move the negative charge of the diethyl malonate enolate from the central carbon onto a carbonyl oxygen.
- N,N-Dimethylvinylamine: enamine β-carbanion formPush the nitrogen lone pair of N,N-dimethylvinylamine into the C=C to draw the iminium contributor with a CH2 carbanion.
- N,N-Dimethylaniline: strong donation to para carbonPush the N,N-dimethylaniline nitrogen lone pair into the ring and on to the para carbon to draw the para carbanion contributor.
- DMF: dimethylamino lone pair into the formyl C=ODraw the charge-separated contributor of N,N-dimethylformamide by pushing the nitrogen lone pair toward carbon and the C=O π pair onto oxygen.
- Enaminone: nitrogen donation through a C=C to C=ODraw the charge-separated contributor of 4-(dimethylamino)but-3-en-2-one with three arrows that relay the nitrogen lone pair through the C=C bond onto the carbonyl oxygen.
- Ethyl acetate enolate: O⁻ and C⁻ contributorsDraw the carbanion contributor of the ethyl acetate enolate by moving the oxygen lone pair into the C–O bond and the C=C π pair onto the α-carbon.
- Furan: ring oxygen lone pair donation to C3Draw the charge-separated contributor of furan by pushing a ring oxygen lone pair into the ring and the C2=C3 π pair onto C3.
- Guanidinium: three equivalentRotate the C=N⁺ double bond around all three nitrogens of guanidinium to show three equivalent contributors.
- Imidazole: N–H nitrogen donating to the other NDraw the charge-separated contributor of 1H-imidazole in which the N1 lone pair forms an N1=C2 bond and the C2=N3 π pair moves onto N3.
- Iminium ion: C=N⁺ and carbocation contributorsMove the C=N π pair of the N,N-dimethylmethaniminium ion onto nitrogen to draw its carbocation contributor.
- Indole: why C3 is the nucleophilic carbonDraw the charge-separated contributor of indole that puts the negative charge on C3 by pushing the nitrogen lone pair into the five-membered ring.
- Methyl acetate: alkoxy oxygen donates into C=OPush a lone pair of the methyl acetate ester oxygen into the carbonyl to draw the charge-separated oxonium–alkoxide contributor.
- Methyl acrylate: the electron-poor β-carbon contributorDraw the contributor of methyl acrylate that puts positive charge on the β-carbon by shifting the C=C π pair toward the carbonyl and the C=O π pair onto oxygen.
- Methyl benzoate: ester withdraws from the ringPull the ipso–ortho π pair of methyl benzoate into the ester C=O to draw the contributor with O⁻ and a positive ortho carbon.
- Methyl carbamate: nitrogen beats oxygen as donorDraw the charge-separated contributor of methyl carbamate in which the NH2 lone pair, not the methoxy oxygen, donates into the C=O.
- Enol ether donationDraw the charge-separated contributor of methyl vinyl ether by moving an oxygen lone pair into the O–C bond and the C=C π pair onto the terminal carbon.
- Acetone N-methylimine: charge-separated contributorMove the C=N π pair of acetone N-methylimine onto nitrogen to draw its carbocation–amide anion contributor.
- Nitrate: three equivalent contributorsDraw the other two contributors of the nitrate ion by moving an oxygen lone pair into an N=O bond while the old N=O π pair moves onto its oxygen; keep the + charge on nitrogen.
- Nitro group withdraws from ortho/paraDraw the ring contributors of nitrobenzene in which the nitro group pulls π density out of the ring, placing positive charge on the ortho and para carbons.
- Nitro group equivalent contributorsSwap the N=O and N–O⁻ positions in nitromethane and conclude that both N–O bonds are the same.
- Nitronate anion: C=N form to carbanion formUse two curved arrows to convert the C=N form of the nitromethane nitronate anion into its carbanion contributor.
- Oxocarbenium major formUse an oxygen lone pair to give the 1-methoxyethyl cation an all-octet oxocarbenium contributor.
- Ozone two contributorsInterconvert the two equivalent contributors of ozone and deduce that both O–O bonds have order 1.5.
- Pentadienyl anion: negative charge at C3Use two curved arrows to move the negative charge of the 2,4-pentadienyl anion from C5 to the central carbon C3.
- Three contributors of the pentadienyl cationWalk the positive charge of the pentadienyl cation from C5 to C3 to C1 with one π-bond arrow per contributor.
- Pentadienyl radical: moving the radical to C3Use three fishhook arrows to move the unpaired electron of the 2,4-pentadienyl radical from C5 to the central carbon C3.
- Phenol: hydroxyl donation to the para carbonPush the phenol oxygen lone pair through the ring, first to an ortho carbon and then to the para carbon, to draw the para carbanion contributor.
- Phenoxide delocalizes into the ringDraw the three ring contributors of phenoxide, moving the negative charge from oxygen to the ortho and para carbons.
- Phenoxyl radical: fishhook arrows to the para carbonUse two rounds of three fishhook arrows to move the unpaired electron of the phenoxyl radical from oxygen to the ortho carbon and then to the para carbon.
- Protonated carbonylMove the C=O π pair of protonated acetaldehyde onto oxygen to reveal the carbocation character of the carbonyl carbon.
- N-Methylpropan-2-iminium: the carbocation contributorDraw the carbocation contributor of N-methylpropan-2-iminium by moving the C=N π pair onto nitrogen.
- Pyridine: C=N π bond puts + on C2Draw the charge-separated contributor of pyridine by moving the C2=N π pair onto the ring nitrogen.
- Pyridine: relaying the positive charge to C4Draw the pyridine contributor with positive charge on C4 by moving the C2=N π pair onto nitrogen and then shifting the C3=C4 π pair toward C2.
- Pyridine N-oxide: oxide oxygen donates to C2Draw the contributor of pyridine N-oxide that puts the negative charge on C2 by pushing an oxide lone pair toward nitrogen and the N=C2 π pair onto C2.
- Pyrrole: two arrow pushes put the charge on an α-carbonDraw the pyrrole contributor with the negative charge on an α-carbon by pushing the nitrogen lone pair into the ring and then moving the C3 lone pair on to C5.
- Pyrrole: nitrogen lone pair to the C3 carbonDraw the charge-separated contributor of pyrrole that puts the negative charge on C3 by pushing the N–H nitrogen lone pair into the ring.
- Cyclohexanone pyrrolidine enamine: nucleophilic α-carbonPush the nitrogen lone pair of 1-(cyclohex-1-en-1-yl)pyrrolidine into the C=C to draw the iminium contributor with a carbanion at the α-carbon.
- Thiocyanate S⁻ ↔ N⁻Move the negative charge of thiocyanate from sulfur to nitrogen and relate the two contributors to its two nucleophilic sites.
- S-Methyl thioacetate: weak sulfur donation into C=ODraw the charge-separated contributor of S-methyl thioacetate by pushing a sulfur lone pair toward carbon and the C=O π pair onto oxygen.
- Thiophene: sulfur lone pair donation to C3Draw the charge-separated contributor of thiophene by pushing a sulfur lone pair into the ring and the C2=C3 π pair onto C3.
- Tropylium cation: equivalent contributorsShift one ring π bond toward the empty orbital of the tropylium cation to draw an equivalent contributor with the positive charge on a different carbon.
- Urea: nitrogen lone pair into the carbonylDraw a charge-separated contributor of urea by pushing one NH2 lone pair toward the carbonyl carbon and the C=O π pair onto oxygen.
Predict the product 3
- A neutral donor creates separated chargesPredict the formal-charge pattern after lone-pair donation into an adjacent alkene.
- Nucleophilic carbon of a cyclohexanone enamineUse the iminium-carbanion resonance contributor of 1-(pyrrolidin-1-yl)cyclohexene to locate its nucleophilic carbon.
- Where is the enone electrophilic?Predict which carbon of but-3-en-2-one carries partial positive charge from its charge-separated contributor.
Classify 4
- Did a nucleus move?Distinguish resonance from proton-transfer isomerism.
- Conjugated or not?Classify π systems as conjugated or isolated by checking for an sp³ carbon between them.
- Classify the imidazole nitrogen lone pairsClassify each nitrogen lone pair of imidazole as delocalized in the aromatic π system or localized in the ring plane.
- Resonance or tautomerism?Classify pairs of structures as resonance contributors or tautomers by checking whether any atom moved.
Compare 5
- Unequal carbonyl contributorsCompare valid contributors with different octet completion.
- Major contributor of a protonated amideChoose the major resonance contributor of O-protonated acetamide by comparing octets and charge location.
- Rank carbocation stabilityRank carbocations by combining resonance delocalization and hyperconjugation.
- Pyrrole vs pyridine: which nitrogen is basic?Compare the basicity of pyrrole and pyridine by deciding whether each nitrogen lone pair is part of the aromatic π system.
- Rank the formamide contributorsRank three resonance contributors of formamide by importance using octets, formal charge and electronegativity.
Calculate 4
- Benzene resonance energy from hydrogenationCalculate the resonance (stabilization) energy of benzene by comparing its heat of hydrogenation with three times that of cyclohexene.
- Count the benzyl cation contributorsCount the resonance contributors of the benzyl cation by moving ring π electrons toward the cationic carbon.
- Count the tropylium cation contributorsCount the equivalent resonance contributors of the tropylium cation by moving the positive charge around the seven-membered ring.
- Average N–O bond order in nitrateFind the average N–O bond order of the nitrate ion from its three equivalent resonance contributors.
Find the error 3
- Finding a broken conjugation pathDiagnose an sp³ interruption between a lone pair and a π system.
- Spot the invalid contributorDiagnose why a proposed resonance structure of acetone is invalid when a hydrogen changes position.
- Spot the invalid nitromethane contributorIdentify the drawing of nitromethane that breaks the octet rule and so cannot be a resonance contributor.
Interpret the evidence 3
- Reading the real carboxylateInfer equivalent bonds from equivalent contributors.
- Shorter C–N bond in aniline vs cyclohexylamineInterpret the shorter C–N bond of aniline compared with cyclohexylamine using lone-pair resonance into the benzene ring.
- Carbonate bond lengthsInterpret the measured C–O bond lengths of the carbonate ion using its three equivalent resonance contributors.
Work backward 2
- Recovering a nitro charge patternReconstruct charges in an octet-constrained nitro group.
- Name the substituent from its ring contributorsWork backward from resonance contributors with positive charge at the ortho and para carbons to identify a π-accepting substituent on benzene.
Plan a synthesis 2
- Planning a valid enolate redrawChoose paired electron moves that preserve a carbonyl carbon octet.
- Arrow plan for the 4-nitrophenoxide quinoid formChoose the curved-arrow sequence that converts 4-nitrophenoxide into its quinoid contributor with the negative charge on a nitro oxygen.
Explain why 2
- The amide C–N bondConnect lone-pair delocalization with partial double-bond character.
- Why allene's π bonds are not conjugatedExplain why the two C=C π bonds of allene cannot be delocalized into each other by resonance.
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