Organic Chemistry 1 · Reasoning about reactions
Carbocation rearrangements and ring expansions: practice problems
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- Ring expansion of cyclobutylmethanol with HBr
- Pinacol to pinacolone
- E1 dehydration of 2,2-dimethylcyclohexanol with a methyl shift
- E1 dehydration of 3-methyl-2-butanol with a hydride shift
- Friedel–Crafts alkylation of benzene with 1-chloropropane
- HBr converts 1-cyclobutylethanol into 1-bromo-1-methylcyclopentane
- HBr adds to 2-methyl-2-butene without rearrangement
- HBr adds to 3-methyl-1-butene with a 1,2-hydride shift
- HBr converts 3-methyl-2-butanol into a rearranged tertiary bromide
Draw the mechanism 69
- E1 dehydration of 2,2-dimethylcyclohexanol with a methyl shiftDraw the E1 dehydration of 2,2-dimethylcyclohexan-1-ol: protonation, loss of water, the methyl shift to a tertiary cation, and loss of a proton giving 1,2-dimethylcyclohexene.
- E1 dehydration of 3-methyl-2-butanol with a hydride shiftDraw the acid-catalysed E1 dehydration of 3-methylbutan-2-ol, including the 1,2-hydride shift before loss of a β-hydrogen, giving 2-methylbut-2-ene.
- Friedel–Crafts alkylation of benzene with 1-chloropropaneDraw activation of 1-chloropropane by AlCl3 with the hydride shift that gives the isopropyl cation, attack of benzene, and deprotonation of the arenium ion giving isopropylbenzene.
- HBr converts 1-cyclobutylethanol into 1-bromo-1-methylcyclopentaneDraw the reaction of 1-cyclobutylethanol with HBr: protonation, loss of water, ring expansion to a cyclopentyl cation, a hydride shift to the tertiary cation and bromide capture giving 1-bromo-1-methylcyclopentane.
- HBr adds to 2-methyl-2-butene without rearrangementDraw the Markovnikov addition of HBr to 2-methylbut-2-ene through the tertiary cation, with no shift, giving 2-bromo-2-methylbutane.
- HBr adds to 3-methyl-1-butene with a 1,2-hydride shiftDraw protonation of 3-methyl-1-butene by HBr, the 1,2-hydride shift that converts the secondary cation into a tertiary one, and bromide capture giving 2-bromo-2-methylbutane.
- HBr converts 3-methyl-2-butanol into a rearranged tertiary bromideDraw the SN1 reaction of 3-methylbutan-2-ol with HBr: protonation of the OH, loss of water, the 1,2-hydride shift and bromide capture giving 2-bromo-2-methylbutane.
- Ring expansion of cyclobutylmethanol with HBrDraw protonation of cyclobutylmethanol by HBr, loss of water concerted with ring-bond migration to the cyclopentyl cation, and bromide capture giving bromocyclopentane.
- HCl adds to 3,3-dimethyl-1-butene with a 1,2-methyl shiftDraw protonation of 3,3-dimethyl-1-butene, the 1,2-methyl shift from the quaternary carbon to the secondary cation, and chloride capture giving 2-chloro-2,3-dimethylbutane.
- Acid-catalysed hydration of 3-methyl-1-butene with a hydride shiftDraw the acid-catalysed hydration of 3-methylbut-1-ene: protonation by hydronium, the 1,2-hydride shift, water addition and deprotonation giving 2-methylbutan-2-ol.
- Methanolysis of 1-(1-chloroethyl)-1-methylcyclobutane with ring expansionDraw the SN1 methanolysis of 1-(1-chloroethyl)-1-methylcyclobutane, including the ring-bond shift that expands the four-membered ring to a five-membered tertiary cation, giving 1-methoxy-1,2-dimethylcyclopentane.
- Ethanolysis of neopentyl bromide with methyl migrationDraw the solvolysis of 1-bromo-2,2-dimethylpropane in ethanol: ionisation with concerted methyl migration to a tertiary cation, capture by ethanol and deprotonation by ethanol, giving 2-ethoxy-2-methylbutane.
- Pinacol rearrangement of 2-methyl-1,1-diphenylpropane-1,2-diolDraw the pinacol rearrangement of 2-methyl-1,1-diphenylpropane-1,2-diol: ionisation of the diphenyl carbinol, methyl migration to that cation, and deprotonation of the oxocarbenium ion giving 3,3-diphenylbutan-2-one.
- Pinacol rearrangement of 1-methylcyclohexane-1,2-diolDraw the acid-catalysed pinacol rearrangement of 1-methylcyclohexane-1,2-diol: loss of the tertiary OH as water, hydride migration from the neighbouring carbinol carbon, and deprotonation giving 2-methylcyclohexanone.
- Acetolysis of 3-chloro-2-methylpentane with a hydride shiftDraw the acetolysis of 3-chloro-2-methylpentane: ionization, the 1,2-hydride shift, capture by acetic acid and deprotonation to 2-methylpentan-2-yl acetate.
- Acid-catalysed isomerization of 3-methylbut-1-eneDraw the acid-catalysed isomerization of 3-methylbut-1-ene: protonation, 1,2-hydride shift and loss of a β-proton to 2-methylbut-2-ene.
- Dehydration of 1-cyclobutylethanol with ring expansionDraw the dehydration of 1-cyclobutylethanol: protonation, loss of water, four-to-five ring expansion, a hydride shift to the tertiary cation and loss of a proton to 1-methylcyclopentene.
- Dehydration of 1-cyclohexylethanol with a hydride shiftDraw the dehydration of 1-cyclohexylethanol: protonation, loss of water, a hydride shift from the ring CH to the secondary cation and loss of a ring proton to 1-ethylcyclohexene.
- Dehydration of (1-methylcyclopentyl)methanol with ring expansionDraw the dehydration of (1-methylcyclopentyl)methanol: protonation, ring expansion concerted with loss of water to the 1-methylcyclohexyl cation, and E1 to 1-methylcyclohexene.
- HBr on 2,2-dimethylcyclohexanol with a methyl shiftDraw HBr on 2,2-dimethylcyclohexanol: protonation, loss of water, 1,2-methyl shift and bromide capture to 1-bromo-1,2-dimethylcyclohexane.
- Dehydration of 2,3,3-trimethylbutan-2-ol without a shiftDraw the E1 dehydration of 2,3,3-trimethylbutan-2-ol: protonation, loss of water to a tertiary cation that does not rearrange, and loss of a methyl proton to 2,3,3-trimethylbut-1-ene.
- Dehydration of 2-cyclobutylpropan-2-ol with ring expansionDraw the dehydration of 2-cyclobutylpropan-2-ol: ionization, cyclobutane-to-cyclopentane expansion, a 1,2-methyl shift and E1 to 1,2-dimethylcyclopentene.
- Dehydration of 2-cyclohexylpropan-2-ol without a shiftDraw the E1 dehydration of 2-cyclohexylpropan-2-ol: protonation, loss of water to a tertiary cation that does not rearrange, and loss of the ring CH proton to isopropylidenecyclohexane.
- E1 of 2-methyl-1-phenylpropan-1-ol without a shiftDraw the E1 dehydration of 2-methyl-1-phenylpropan-1-ol: protonation, loss of water to a benzylic cation that does not rearrange, and deprotonation to 2-methyl-1-phenylprop-1-ene.
- Dehydration of 2-methylbutan-1-ol to 2-methylbut-2-eneDraw the dehydration of 2-methylbutan-1-ol: protonation, loss of water concerted with a 1,2-hydride shift to the tertiary cation, and E1 deprotonation to 2-methylbut-2-ene.
- Dehydration of 2-methylpentan-3-ol with a hydride shiftDraw the acid-catalysed dehydration of 2-methylpentan-3-ol: protonation, loss of water, a 1,2-hydride shift to the tertiary cation and E1 deprotonation to 2-methylpent-2-ene.
- Dehydration of 3,3-dimethylbutan-2-ol with a methyl shiftDraw the E1 dehydration of 3,3-dimethylbutan-2-ol: protonation, loss of water, 1,2-methyl shift and loss of a β-proton to 2,3-dimethylbut-2-ene.
- Dehydration of 3-ethylpentan-2-ol with a hydride shiftDraw the phosphoric acid dehydration of 3-ethylpentan-2-ol: protonation, loss of water, a 1,2-hydride shift to the tertiary cation and deprotonation to 3-ethylpent-2-ene.
- Dehydration of cyclohexylmethanol to 1-methylcyclohexeneDraw the dehydration of cyclohexylmethanol: protonation, loss of water concerted with a hydride shift from the ring CH, and loss of a ring proton to 1-methylcyclohexene.
- Dehydration of neopentyl alcoholDraw the dehydration of 2,2-dimethylpropan-1-ol: protonation, loss of water concerted with a methyl shift, and E1 to 2-methylbut-2-ene.
- Acid-catalysed ethanol addition to 3-methylbut-1-eneDraw acid-catalysed addition of ethanol to 3-methylbut-1-ene: protonation, the 1,2-hydride shift, ethanol capture and deprotonation to 2-ethoxy-2-methylbutane.
- Ethanolysis of 2-bromo-1,1-dimethylcyclohexane: Me shiftDraw the ethanolysis of 2-bromo-1,1-dimethylcyclohexane: ionization, the 1,2-methyl shift to a tertiary ring cation, ethanol capture and deprotonation to 1-ethoxy-1,2-dimethylcyclohexane.
- HBr on 1-cyclopentylethanol: ring expansion then hydride shiftDraw HBr on 1-cyclopentylethanol: protonation, loss of water, ring expansion to a cyclohexyl cation, a 1,2-hydride shift and bromide capture to 1-bromo-1-methylcyclohexane.
- HBr adds to 2,3,3-trimethylbut-1-ene without a net shiftDraw HBr addition to 2,3,3-trimethylbut-1-ene: Markovnikov protonation straight to a tertiary cation and bromide capture, recognising that a methyl shift gains nothing.
- HBr adds to 3,3-dimethylbut-1-ene with a methyl shiftDraw HBr addition to 3,3-dimethylbut-1-ene: Markovnikov protonation, the 1,2-methyl shift to a tertiary cation and bromide capture giving 2-bromo-2,3-dimethylbutane.
- HBr adds to 3-ethylpent-1-ene with a hydride shiftDraw HBr addition to 3-ethylpent-1-ene: Markovnikov protonation, the 1,2-hydride shift to the triethyl-substituted cation and bromide capture to 3-bromo-3-ethylpentane.
- HBr adds to 3-methylcyclohexene with a hydride shiftDraw HBr addition to 3-methylcyclohexene: protonation at C1, the 1,2-hydride shift to the tertiary ring cation and bromide capture giving 1-bromo-1-methylcyclohexane.
- HBr adds to allylcyclohexane without a shiftDraw HBr addition to allylcyclohexane: Markovnikov protonation to the secondary cation and direct bromide capture, recognising that no 1,2-shift improves the cation.
- Ring expansion of cyclopentylmethanol with HBrDraw protonation of cyclopentylmethanol, loss of water concerted with migration of a ring bond to the cyclohexyl cation, and bromide capture to bromocyclohexane.
- HBr on isopropenylcyclobutane: expansion and methyl shiftDraw HBr addition to isopropenylcyclobutane: protonation, ring expansion to the 2,2-dimethylcyclopentyl cation, the 1,2-methyl shift and bromide capture to 1-bromo-1,2-dimethylcyclopentane.
- HBr adds to methylenecyclobutane without ring expansionDraw HBr addition to methylenecyclobutane: protonation of the exocyclic CH2 to a tertiary ring cation and bromide capture, and see why the strained ring does not expand.
- HBr on vinylcyclobutane: ring expansion to a cyclopentaneDraw HBr addition to vinylcyclobutane: protonation, migration of a ring bond that expands the four-membered ring, the hydride shift to a tertiary cation and bromide capture.
- HCl adds to 3-methylbut-1-ene with a hydride shiftDraw HCl addition to 3-methylbut-1-ene: Markovnikov protonation, the 1,2-hydride shift to a tertiary cation and chloride capture giving 2-chloro-2-methylbutane.
- HCl adds to 3-methylcyclopentene with a hydride shiftDraw HCl addition to 3-methylcyclopentene: protonation at C1, the 1,2-hydride shift to the 1-methylcyclopentyl cation and chloride capture to 1-chloro-1-methylcyclopentane.
- HCl adds to 3-methylpent-1-ene with a hydride shiftDraw HCl addition to 3-methylpent-1-ene: protonation of the terminal carbon, the 1,2-hydride shift to the tertiary 3-methylpentan-3-yl cation and chloride capture.
- HCl adds to isopropenylcyclopentane without ring expansionDraw HCl addition to isopropenylcyclopentane: Markovnikov protonation to a tertiary cation and chloride capture, explaining why neither ring expansion nor a hydride shift occurs.
- HCl adds to vinylcyclopentane with a hydride shiftDraw HCl addition to vinylcyclopentane: protonation of the vinyl CH2, the ring-to-chain 1,2-hydride shift to the 1-ethylcyclopentyl cation and chloride capture.
- Hydration of 3,3-dimethylbut-1-ene with a methyl shiftDraw acid-catalysed hydration of 3,3-dimethylbut-1-ene: Markovnikov protonation, the 1,2-methyl shift, water capture and deprotonation to 2,3-dimethylbutan-2-ol.
- Hydration of 3,3-dimethylcyclohexene with a methyl shiftDraw acid-catalysed hydration of 3,3-dimethylcyclohexene: protonation at C1, the 1,2-methyl shift to a tertiary ring cation, water capture and deprotonation to 1,2-dimethylcyclohexan-1-ol.
- Hydration of 3-methylpent-1-ene with a hydride shiftDraw acid-catalysed hydration of 3-methylpent-1-ene: protonation by hydronium, the 1,2-hydride shift, water capture and deprotonation to 3-methylpentan-3-ol.
- Hydration of 3-phenylbut-1-ene: shift to a benzylic cationDraw acid-catalysed hydration of 3-phenylbut-1-ene: protonation, the 1,2-hydride shift to a tertiary benzylic cation, water capture and deprotonation to 2-phenylbutan-2-ol.
- Hydration of vinylcyclohexane with a hydride shiftDraw acid-catalysed hydration of vinylcyclohexane: protonation, a 1,2-hydride shift from the ring CH, water capture and deprotonation to 1-ethylcyclohexan-1-ol.
- Hydration of vinylcyclopentane with a hydride shiftDraw acid-catalysed hydration of vinylcyclopentane: protonation by hydronium, the 1,2-hydride shift from the ring, water capture and deprotonation to 1-ethylcyclopentan-1-ol.
- Hydrolysis of cyclobutylmethyl tosylate to cyclopentanolDraw the hydrolysis of cyclobutylmethyl tosylate: ionisation concerted with a four-to-five ring expansion, water capture of the cyclopentyl cation and deprotonation to cyclopentanol.
- Acid-catalysed methanol addition to 3,3-dimethylbut-1-eneDraw acid-catalysed addition of methanol to 3,3-dimethylbut-1-ene: protonation, the 1,2-methyl shift, methanol capture and deprotonation to 2-methoxy-2,3-dimethylbutane.
- Acid-catalysed methanol addition to vinylcyclohexaneDraw the acid-catalysed addition of methanol to vinylcyclohexane: Markovnikov protonation, a hydride shift from the ring CH, methanol capture and deprotonation to 1-ethyl-1-methoxycyclohexane.
- Methanolysis of 1-(1-chloroethyl)-1-methylcyclopentaneDraw the methanolysis of 1-(1-chloroethyl)-1-methylcyclopentane: ionisation, a five-to-six ring expansion to the tertiary cation, methanol capture and deprotonation to 1-methoxy-1,2-dimethylcyclohexane.
- Methanolysis of 2-bromo-3-methylpentane with a hydride shiftDraw the methanolysis of 2-bromo-3-methylpentane: ionization, the 1,2-hydride shift, methanol capture and deprotonation to 3-methoxy-3-methylpentane.
- Pinacol rearrangement of 1,1'-bicyclopentyl-1,1'-diolDraw the pinacol rearrangement of 1,1'-bicyclopentyl-1,1'-diol: protonation, loss of water, ring-bond migration that expands one ring to six members, and deprotonation to spiro[4.5]decan-6-one.
- Pinacol rearrangement of 2,3-diphenylbutane-2,3-diolDraw the pinacol rearrangement of 2,3-diphenylbutane-2,3-diol: protonation, loss of water to the benzylic cation, phenyl migration to the protonated ketone and deprotonation to 3,3-diphenylbutan-2-one.
- Benzopinacol to benzopinacolone: pinacol rearrangementDraw the acid-catalysed rearrangement of benzopinacol: protonation, loss of water to the diphenyl cation, phenyl migration to the protonated ketone and deprotonation to benzopinacolone.
- Pinacol to pinacoloneDraw the pinacol rearrangement of 2,3-dimethylbutane-2,3-diol: protonation, loss of water, methyl shift to the oxocarbenium ion and deprotonation to pinacolone.
- Hydrolysis of 1-bromo-2,2-dimethylcyclopentane with a methyl shiftDraw the hydrolysis of 1-bromo-2,2-dimethylcyclopentane: ionization, 1,2-methyl shift to the ring tertiary cation, water capture and deprotonation.
- Hydrolysis of 1-chloro-2-methylcyclohexane with a H shiftDraw the SN1 hydrolysis of 1-chloro-2-methylcyclohexane: ionization, the 1,2-hydride shift to the tertiary ring cation, water capture and deprotonation to 1-methylcyclohexan-1-ol.
- Hydrolysis of 2-bromo-3,3-dimethylbutane with a methyl shiftDraw the SN1 hydrolysis of 2-bromo-3,3-dimethylbutane: ionization, the 1,2-methyl shift, water capture and deprotonation to 2,3-dimethylbutan-2-ol.
- Ethanolysis of 2-bromo-3-methylbutane with a hydride shiftDraw the solvolysis of 2-bromo-3-methylbutane in ethanol: ionization, 1,2-hydride shift, ethanol capture and deprotonation to 2-ethoxy-2-methylbutane.
- Methanolysis of 2-chloro-3,3-dimethylbutane with a methyl shiftDraw the methanolysis of 2-chloro-3,3-dimethylbutane: ionization, 1,2-methyl shift, methanol capture and deprotonation to 2-methoxy-2,3-dimethylbutane.
- Methanolysis of 2-chloro-3-methylbutane with a hydride shiftDraw the solvolysis of 2-chloro-3-methylbutane in methanol: ionization, the 1,2-hydride shift, methanol capture and deprotonation to 2-methoxy-2-methylbutane.
- Silver-assisted hydrolysis of neopentyl iodideDraw the silver-assisted hydrolysis of neopentyl iodide: iodine coordinating Ag+, loss of AgI concerted with a methyl shift, water capture and deprotonation to 2-methylbutan-2-ol.
Predict the product 4
- Hydroboration of 3-methyl-1-butenePredict the alcohol from hydroboration–oxidation of 3-methyl-1-butene and explain why no shift occurs.
- Which OH leaves in the pinacol?Predict which hydroxyl ionizes in an unsymmetrical pinacol rearrangement and the resulting ketone.
- Product of HBr with 1-cyclobutylpropan-1-olPredict the bromide from 1-cyclobutylpropan-1-ol and HBr by following a four-to-five ring expansion and a hydride shift.
- Product after a methyl shiftPredict the alcohol from acid-catalyzed hydration of 3,3-dimethylbut-1-ene, including the 1,2-methyl shift.
Classify 3
- Rank C5 carbocationsClassify and rank four carbocations by the number of alkyl groups on the cationic carbon.
- Hydride or methyl shift in 2-chloro-3,3-dimethylbutane SN1Classify the 1,2-shift a secondary cation next to a quaternary carbon undergoes during SN1 hydrolysis of 2-chloro-3,3-dimethylbutane.
- Will it rearrange?Classify alkene and alcohol reactions by whether a carbocation forms next to a better cation site.
Compare 3
- Ring expansion versus methyl shiftCompare the two shifts open to a secondary cation beside a 1-methylcyclobutyl group.
- Acid hydration vs oxymercuration of 3,3-dimethylbut-1-eneCompare acid-catalysed hydration and oxymercuration–reduction of 3,3-dimethylbut-1-ene and pick the alcohol formed only through a carbocation shift.
- Order carbocation stability: ethyl to tritylRank ethyl, isopropyl, tert-butyl and triphenylmethyl cations by stability using hyperconjugation and resonance.
Calculate 3
- Count the alkenes from E1 of 3,3-dimethylbutan-2-olCalculate how many constitutionally different alkenes E1 dehydration of 3,3-dimethylbutan-2-ol gives from the unrearranged and the methyl-shifted cations.
- Ring size after expansionCalculate the ring size formed when a ring bond migrates to an exocyclic carbocation in cyclopentylmethanol.
- Arrows in a hydride shiftCount the curved arrows needed to draw a single 1,2-hydride shift in a carbocation.
Find the error 2
- Spot an impossible shiftDiagnose why a shift drawn away from the cationic carbon is wrong.
- Find the error in a drawn hydride-shift arrowDiagnose a reversed curved arrow in a 1,2-hydride shift of the 3-methylbutan-2-yl cation and state where the arrow must start and end.
Interpret the evidence 1
- Read the hydride-shift step on an energy diagramInterpret an energy diagram for HBr addition to 3-methylbut-1-ene and identify the step that converts one carbocation intermediate into a lower-energy one.
Work backward 2
- Which diol gives pinacoloneIdentify the 1,2-diol that rearranges to 3,3-dimethylbutan-2-one under acid.
- Alkene that gives 2-chloro-2,3-dimethylbutane via a shiftWork backward from 2-chloro-2,3-dimethylbutane to the alkene whose HCl addition requires a 1,2-methyl shift.
Plan a synthesis 2
- Hydrate without rearrangingPlan a Markovnikov hydration of 3-methylbut-1-ene that avoids a carbocation shift.
- Use a methyl shift to make 2-methoxy-2,3-dimethylbutanePlan reagents that convert 3,3-dimethylbut-1-ene into 2-methoxy-2,3-dimethylbutane by exploiting a carbocation methyl shift.
Explain why 2
- How a 1,2-shift is drawnExplain what the single curved arrow of a 1,2-shift represents.
- Why neopentyl bromide still rearranges in ethanolExplain how neopentyl bromide gives a rearranged tertiary ether in ethanol without forming a free primary carbocation.
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