Organic Chemistry 2 · Carbonyl chemistry
Carbonyl additions: exam review: practice problems
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- Conjugate addition of lithium dimethylcuprate to cyclohex-2-en-1-one
- Base-catalysed cyanohydrin formation from pentanal
- Ethylene ketal of butan-2-one
- Pyrrolidine enamine of cyclopentanone
- Ethylmagnesium bromide adds to propanal
- Acid-catalysed cyclic hemiacetal from 5-hydroxypentanal
- N-Methylimine from pentan-3-one
- LiAlH4 reduction of heptan-4-one
- Methyllithium adds 1,2 to cyclohex-2-en-1-one
Draw the mechanism 40
- Conjugate addition of lithium dimethylcuprate to cyclohex-2-en-1-oneDraw the 1,4-addition of a methyl group from lithium dimethylcuprate to the β-carbon of cyclohex-2-en-1-one and the protonation of the enolate at carbon, giving 3-methylcyclohexan-1-one.
- Base-catalysed cyanohydrin formation from pentanalDraw the addition of cyanide to pentanal and the proton transfer from HCN that gives 2-hydroxyhexanenitrile and regenerates cyanide.
- Ethylene ketal of butan-2-oneDraw the acid-catalysed formation of 2-ethyl-2-methyl-1,3-dioxolane from butan-2-one and ethane-1,2-diol, using TsOH and tosylate for every proton transfer, and explain why water is removed.
- Pyrrolidine enamine of cyclopentanoneDraw the acid-catalysed condensation of cyclopentanone with the secondary amine pyrrolidine through a carbinolamine and an iminium ion to the enamine, and explain why an enamine rather than an imine forms.
- Ethylmagnesium bromide adds to propanalDraw the addition of ethylmagnesium bromide to propanal and the acidic workup that gives pentan-3-ol.
- Acid-catalysed cyclic hemiacetal from 5-hydroxypentanalDraw the acid-catalysed intramolecular addition of the hydroxyl group of 5-hydroxypentanal to its aldehyde, giving the six-membered cyclic hemiacetal oxan-2-ol.
- N-Methylimine from pentan-3-oneDraw imine formation from pentan-3-one and methylamine with acetic acid and acetate as the only proton donor and acceptor, and explain why the reaction is run near pH 4–5.
- LiAlH4 reduction of heptan-4-oneDraw hydride transfer from aluminohydride to heptan-4-one in dry ether and protonation of the alkoxide by water in the workup, giving heptan-4-ol.
- Methyllithium adds 1,2 to cyclohex-2-en-1-oneDraw the direct (1,2) addition of methyllithium to cyclohex-2-en-1-one and the mild NH4Cl workup that gives 1-methylcyclohex-2-en-1-ol.
- Two equivalents of methylmagnesium bromide add to ethyl benzoateDraw the addition of methylmagnesium bromide to ethyl benzoate, the loss of ethoxide to give acetophenone, the second addition, and the acidic workup to 2-phenylpropan-2-ol.
- NaBH4 reduction of 3-methylbutanalDraw hydride transfer from borohydride to the aldehyde carbon of 3-methylbutanal and protonation of the alkoxide by ethanol, giving 3-methylbutan-1-ol.
- Phenyllithium adds to 2-methylpropanalDraw the addition of phenyllithium to 2-methylpropanal and the separate acidic workup that gives 2-methyl-1-phenylpropan-1-ol, and explain why the addition must be run dry.
- Base-catalysed conjugate addition of ethanethiol to cyclopent-2-en-1-oneDraw the conjugate addition of ethanethiolate to cyclopent-2-en-1-one and the protonation of the enolate at carbon by ethanethiol, which regenerates the thiolate.
- Wittig methylenation of benzophenoneDraw the formation of the oxaphosphetane from methylenetriphenylphosphorane and benzophenone and its collapse to 1,1-diphenylethene and triphenylphosphine oxide.
- Wittig synthesis of propylidenecyclopentane from 1-bromopropaneDraw the SN2 formation of propyltriphenylphosphonium bromide, its deprotonation by butyllithium to the ylide, and the Wittig reaction with cyclopentanone that gives propylidenecyclopentane and triphenylphosphine oxide.
- Aza-Michael addition: methyl 3-(diethylamino)propanoateDraw diethylamine adding 1,4 to methyl acrylate, protonation of the enolate at carbon by methanol and loss of the N–H proton, giving methyl 3-(diethylamino)propanoate.
- Dibutylcuprate 1,4-addition: 4-methyloctan-2-oneDraw the 1,4-addition of a butyl group from lithium dibutylcuprate to the β-carbon of (E)-pent-3-en-2-one and the C-protonation of the enolate that gives 4-methyloctan-2-one.
- Conjugate addition of cyanide to chalconeDraw cyanide adding to the β-carbon of chalcone and protonation of the enolate at carbon to give 4-oxo-2,4-diphenylbutanenitrile.
- Cyanohydrin of cyclohexanoneDraw cyanide adding to cyclohexanone and HCN protonating the alkoxide to give 1-hydroxycyclohexane-1-carbonitrile.
- Cyclopentanone cyanohydrin: 1-hydroxycyclopentane-1-carbonitrileDraw cyanide adding to cyclopentanone and HCN protonating the alkoxide to 1-hydroxycyclopentane-1-carbonitrile, regenerating the cyanide catalyst.
- DIBAL-H reduction of ethyl benzoate to benzaldehydeDraw DIBAL-H coordinating to ethyl benzoate, hydride delivery to the stable tetrahedral intermediate, and the acidic workup that releases benzaldehyde.
- Dimethyl ketal of cyclohexanoneDraw the acid-catalysed formation of 1,1-dimethoxycyclohexane from cyclohexanone through the hemiketal and the oxocarbenium ion.
- Cyclic acetal from benzaldehyde: 2-phenyl-1,3-dioxaneDraw the TsOH-catalysed condensation of benzaldehyde with propane-1,3-diol through the hemiacetal and oxocarbenium ion to the cyclic acetal 2-phenyl-1,3-dioxane.
- Morpholine enamine: 4-(cyclohex-1-en-1-yl)morpholineDraw the TsOH-catalysed condensation of cyclohexanone with the secondary amine morpholine through a carbinolamine and an iminium ion to 4-(cyclohex-1-en-1-yl)morpholine.
- Two EtMgBr open γ-butyrolactone: 4-ethylhexane-1,4-diolDraw ethylmagnesium bromide adding to γ-butyrolactone, the ring opening to a keto alkoxide, the second addition to the ketone, and the NH4Cl workup to 4-ethylhexane-1,4-diol.
- Butylmagnesium bromide opens ethylene oxide: hexan-1-olDraw butylmagnesium bromide opening ethylene oxide by SN2 and the acidic workup to hexan-1-ol.
- Hydration of hexafluoroacetone to hexafluoropropane-2,2-diolDraw water adding to hexafluoroacetone and the proton transfers through water that give the gem-diol hexafluoropropane-2,2-diol, and explain why this hydrate is favoured.
- Imine from cyclohexanone and anilineDraw the acid-catalysed addition of aniline to cyclohexanone, proton transfers to the carbinolamine, loss of water and deprotonation to N-cyclohexylideneaniline.
- LiAlH4 reduction of cyclopentanecarbaldehyde to cyclopentylmethanolDraw hydride from LiAlH4 adding to cyclopentanecarbaldehyde and the acidic workup that protonates the alkoxide to cyclopentylmethanol.
- Luche reduction: 3-methylcyclohex-2-en-1-ol from the enoneDraw the 1,2-hydride addition of a methoxyborohydride to 3-methylcyclohex-2-en-1-one under Luche conditions and the protonation by methanol that gives the allylic alcohol 3-methylcyclohex-2-en-1-ol.
- Methyllithium and cyclohexanecarbaldehyde: 1-cyclohexylethan-1-olDraw methyllithium adding its methyl group to cyclohexanecarbaldehyde and the NH4Cl workup that gives the secondary alcohol 1-cyclohexylethan-1-ol.
- NaBH4 reduction of cyclohex-2-enone to cyclohex-2-en-1-olDraw 1,2-hydride delivery from borohydride to cyclohex-2-enone and protonation of the alkoxide by methanol to give cyclohex-2-en-1-ol.
- Oxime formation: (E)-acetophenone oxime from hydroxylamineDraw the acetic acid-catalysed condensation of hydroxylamine with acetophenone through the carbinolamine to (E)-acetophenone oxime, with the acetate buffer as acid and base.
- Benzaldehyde phenylhydrazoneDraw acid-catalysed addition of phenylhydrazine to benzaldehyde, proton transfers, loss of water and deprotonation to benzaldehyde phenylhydrazone.
- Vinylmagnesium bromide and benzaldehyde: 1-phenylprop-2-en-1-olDraw vinylmagnesium bromide adding to benzaldehyde and the ammonium chloride workup that gives the allylic alcohol 1-phenylprop-2-en-1-ol.
- Wittig reaction of acetophenone: 2-phenylbut-2-eneDraw ethylidenetriphenylphosphorane adding to acetophenone, oxaphosphetane formation and collapse to 2-phenylbut-2-ene.
- Wittig on cyclopentanone: propan-2-ylidenecyclopentaneDraw isopropylidenetriphenylphosphorane adding to cyclopentanone, ring closure to the oxaphosphetane and collapse to the tetrasubstituted alkene propan-2-ylidenecyclopentane.
- Stabilised ylide and butanal: ethyl (E)-hept-2-enoateDraw the ester-stabilised ylide ethyl (triphenylphosphoranylidene)acetate adding to butanal, ring closure to the oxaphosphetane and its collapse to ethyl (E)-hept-2-enoate, and explain the E selectivity.
- Wittig synthesis of (E)-stilbene from benzaldehydeDraw benzylidenetriphenylphosphorane adding to benzaldehyde, ring closure to the oxaphosphetane and its collapse to (E)-stilbene (benzaldehyde).
- Z-selective Wittig: benzaldehyde to (Z)-1-phenylbut-1-eneDraw propylidenetriphenylphosphorane and benzaldehyde forming the oxaphosphetane and its collapse to (Z)-1-phenylbut-1-ene, and explain why the unstabilised ylide gives the Z-alkene.
Predict the product 13
- Cuprate meets mesityl oxidePredict the conjugate-addition product of a cuprate with a β,β-disubstituted enone.
- Which enamine does 2-methylcyclohexanone give?Predict the major regioisomer of the pyrrolidine enamine of an unsymmetrical cyclic ketone from allylic strain.
- Order the events of imine formationPut the elementary events of acid-catalysed imine formation from a primary amine in the order they occur.
- NaBH₄ with a keto esterPredict the chemoselectivity of NaBH₄ toward a molecule containing both a ketone and an ester.
- Phenyl Grignard and a ketonePredict the tertiary alcohol formed when an aryl Grignard reagent adds to a ketone.
- Geometry from a stabilised ylidePredict the alkene geometry formed by a stabilised Wittig reagent with an aldehyde.
- Trap the cuprate enolatePredict the product of conjugate addition followed by alkylation of the enolate formed regiospecifically in that addition.
- Methylenation of a cyclohexanonePredict the alkene formed by Wittig methylenation of a ketone and distinguish it from Grignard-type and sulfur-ylide products.
- Formate ester plus two GrignardsPredict the secondary alcohol formed when a formate ester reacts with two equivalents of a Grignard reagent.
- What survives the workup?Predict the isolated product after cuprate conjugate addition and aqueous workup.
- Hydrolysing benzaldehyde cyanohydrinPredict that acidic hydrolysis turns the nitrile of a cyanohydrin into a carboxylic acid while the OH stays, giving an α-hydroxy acid.
- Which conditions hydrolyze an imine?Choose the conditions that convert an imine back to its carbonyl compound and amine.
- Stabilised ylide geometryPredict the alkene geometry from a Wittig reaction with a stabilised ylide.
Classify 6
- Spot the hemiacetalClassify carbonyl addition products as hemiacetal, acetal, hydrate or none from the groups on a single carbon.
- Find the conjugate acceptorIdentify a Michael acceptor by checking whether the C=C is conjugated with a carbonyl group.
- Spot the ketalRecognise a ketal by its carbon bearing two ether oxygens and no OH.
- Which reagents add 1,4?Sort nucleophiles into those that add conjugately (1,4) to an enone.
- Which amine gives an enamine?Identify the amine class that forms an enamine with a ketone.
- Which structure is 4-methylpentan-2-one?Turn the IUPAC name 4-methylpentan-2-one into its structure: a five-carbon chain numbered from the end nearer the C=O, with the ketone at C2 and a methyl at C4.
Compare 3
- Rank carbonyl electrophilicityRank aldehydes and ketones by how readily they undergo nucleophilic addition, using hydration as the measure.
- Which ketone resists cyanohydrin formation?Compare the position of cyanohydrin equilibria for aldehydes and ketones from steric and conjugation effects.
- Rank the hydride reagentsOrder three common hydride reagents by their strength as hydride donors toward carbonyl groups.
Calculate 6
- How much ethanal is hydrated?Calculate the percentage of an aldehyde present as its hydrate from the hydration equilibrium constant.
- Grignard equivalents for a hydroxy ketoneCount the equivalents of Grignard reagent consumed by a substrate that carries both an acidic O–H and a ketone.
- How much NaBH₄ is needed?Calculate the theoretical minimum mass of NaBH₄ for a ketone reduction from the number of hydrides per borohydride.
- Atom economy of a Wittig reactionCalculate the atom economy of a Wittig methylenation and relate it to the triphenylphosphine oxide by-product.
- Hydride equivalents for a keto esterCount the hydride equivalents LiAlH4 delivers to a molecule with a ketone and an ester.
- Count the Grignard routes to 2-methylbutan-2-olCount the distinct Grignard reagent and carbonyl partner pairs that build the tertiary alcohol 2-methylbutan-2-ol, including the ester route that adds two identical groups.
Find the error 4
- Why did the LiAlH₄ reduction fail?Diagnose the failure of a LiAlH₄ reduction run in a protic solvent.
- Fix the route to 3,3-dimethylcyclohexanoneDiagnose a synthesis that uses an organolithium where a conjugate-addition reagent is needed.
- A Wittig reaction that stalledDiagnose a failed Wittig reaction caused by an acidic proton in the substrate.
- Diagnose a failed GrignardExplain why acid must be added only after the Grignard addition is complete.
Interpret the evidence 1
- IR C=O stretch: pentan-2-one or methyl butanoate?Use the C=O stretching frequency to tell a saturated ester (about 1735 cm⁻¹) from a saturated ketone (about 1715 cm⁻¹).
Work backward 3
- Which carbonyl partner for Et₃COH?Work back from a tertiary alcohol with three identical groups to the ester that gives it with excess Grignard reagent.
- Make ethylidenecyclopentaneChoose the carbonyl compound and ylide whose Wittig reaction gives a target exocyclic alkene, and reject a dehydration route that misplaces the C=C.
- Which halide makes benzylidenetriphenylphosphorane?Work back from a Wittig ylide to the alkyl halide that forms its phosphonium salt by SN2 with triphenylphosphine.
Plan a synthesis 2
- Reduce the ester, keep the ketonePlan a protecting-group sequence that reduces an ester in the presence of a more reactive ketone.
- Reduce the C=O but keep the C=CChoose reagents that reduce an enone carbonyl while leaving the conjugated alkene intact.
Explain why 2
- Why are acetals good protecting groups?Explain why acetals survive basic and nucleophilic reagents but are hydrolysed by aqueous acid.
- Why does the conjugate adduct win?Explain why reversible nucleophiles give conjugate adducts of enones using bond strengths.
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