Organic Chemistry 1 · Reasoning about reactions
Electron-flow fundamentals: practice problems
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Draw the mechanism 42
- Follow a migrating carbon groupUse an occupied carbon–carbon bond as an electron-pair donor while tracking a changing carbon skeleton.
- Add hydroxide to a carbonylCouple bond formation to π-bond opening at an octet carbon.
- Loss of a leaving groupDraw loss of a leaving group as one arrow from a C–I bond to iodine and assign the carbocation and iodide charges it creates.
- A nucleophile meets an electrophileCombine a nucleophile with an electrophile by starting the arrow at the bromide lone pair and ending it at the carbon with the empty orbital.
- An alkene takes a protonPredict which carbon of 2-methylpropene receives a proton from hydronium, then draw the π-bond-as-base proton transfer that leaves the positive charge on the tertiary carbon.
- Hydroxide deprotonates acetic acid: two arrowsDraw the two arrows for hydroxide removing the O–H proton of acetic acid and show acetate and water.
- Hydronium protonates acetone on oxygenDraw how hydronium transfers a proton to the carbonyl oxygen of acetone and explain why this activates the carbonyl.
- Chloride binds aluminum chlorideDraw the single arrow by which chloride donates a lone pair to aluminum chloride and assign the formal charges in tetrachloroaluminate.
- Ionization of benzhydryl chlorideDraw the C–Cl heterolysis of benzhydryl chloride and explain why the diphenylmethyl cation forms so readily.
- Borohydride delivers hydride to acetoneDraw the hydride transfer from a B–H bond of borohydride to the carbonyl carbon of acetone and identify the alkoxide formed.
- Bromide captures the cyclopentyl cationDraw the one arrow for bromide combining with the cyclopentyl cation and give the formal charges of the neutral product.
- Ionization of 2-bromo-2-methylbutaneDraw the heterolysis of the C–Br bond of 2-bromo-2-methylbutane and show the tertiary cation and bromide that result.
- Ionization of 3-bromocyclohexene: allylic cationDraw the loss of bromide from 3-bromocyclohexene and explain why the allylic cation that forms is stabilized.
- An alkene attacks the tert-butyl cationDraw the C–C bond-forming step in which the π bond of 2-methylpropene attacks the tert-butyl cation, choosing the alkene carbon that gives the more stable new cation.
- Cyanide adds to cyclohexanoneDraw the attack of cyanide on the carbonyl carbon of cyclohexanone and show the tetrahedral alkoxide that forms on the ring.
- Cyanide attacks formaldehydeDraw the single elementary step in which cyanide adds to the carbonyl carbon of formaldehyde, giving a tetrahedral alkoxide.
- Collapse of a cyanohydrin alkoxideDraw the step in which the alkoxide of acetone cyanohydrin re-forms the C=O bond and expels cyanide.
- E2 of bromoethane: three arrowsDraw the one-step E2 elimination of bromoethane with tert-butoxide using three curved arrows.
- An enol takes a proton on carbonDraw the step in which the enol of acetone uses its C=C π bond, pushed by the oxygen lone pair, to take a proton on carbon.
- Amide ion deprotonates ethanolDraw the proton transfer from ethanol to the amide ion and use pKa values to say which side is favoured.
- HBr in water: heterolysisDraw the heterolytic cleavage of H–Br as HBr transfers its proton to water, and contrast it with homolysis.
- A 1,2-hydride shiftDraw the single arrow for the 1,2-hydride shift that converts the 3-methylbutan-2-yl cation into the more stable 2-methylbutan-2-yl cation.
- 1,2-Hydride Shift in the 3-Methyl-2-butyl CationDraw the one curved arrow that moves a hydride from the tertiary C–H to the secondary cation carbon of the 3-methyl-2-butyl cation.
- Hydride Shift in the 2-Methylcyclohexyl CationDraw the arrow for the ring hydride shift that turns the 2-methylcyclohexyl cation into the 1-methylcyclohexyl cation.
- Hydride Shift to a Benzylic Cation: 1-Phenyl-2-propyl CationDraw the hydride shift that moves the positive charge of the 1-phenyl-2-propyl cation onto the benzylic carbon.
- Hydroxide meets hydroniumDraw the proton transfer from hydronium to hydroxide with two arrows and show that both oxygens end up neutral.
- LDA removes an alpha proton from acetoneDraw the two arrows by which LDA removes an alpha hydrogen of acetone and show the enolate in its carbanion form.
- Methanol captures the benzyl cationDraw the single arrow by which a methanol lone pair bonds to the benzyl cation and show where the positive charge ends up.
- Methoxide adds to acetaldehydeDraw the two arrows by which methoxide adds to the carbonyl carbon of acetaldehyde and give the tetrahedral alkoxide its charge.
- 1,2-Methyl Shift in the 3,3-Dimethyl-2-butyl CationDraw the curved arrow that moves a methyl group from the quaternary carbon to the cation carbon of the 3,3-dimethyl-2-butyl cation.
- Methyl Shift in the 2,2-Dimethylcyclohexyl CationDraw the arrow for the methyl shift that converts the 2,2-dimethylcyclohexyl cation into the 1,2-dimethylcyclohexyl cation.
- Methylamine takes a proton from HClDraw the two arrows by which methylamine takes the proton of HCl and assign the formal charge on nitrogen.
- Amide ion deprotonates propyneDraw the two arrows for the amide ion removing the terminal alkyne proton of propyne to give an acetylide ion.
- Protonated tert-butyl methyl ether ionizesDecide which C–O bond of protonated tert-butyl methyl ether breaks and draw the arrow that releases methanol.
- Protonated tert-butanol loses waterDraw the single arrow by which the protonated OH of tert-butanol leaves as water, forming the tert-butyl cation.
- Ring Expansion of the 1-Cyclobutylethyl CationDraw the single arrow by which a cyclobutane ring bond migrates to the cation carbon, expanding the 1-cyclobutylethyl cation to the 2-methylcyclopentyl cation.
- SN2 of (R)-2-Bromobutane With Hydroxide: InversionDraw both arrows of the one-step SN2 reaction of (R)-2-bromobutane with hydroxide and give (S)-butan-2-ol with inverted configuration.
- SN2: iodide displaces chlorideDraw the concerted SN2 displacement of chloride from chloromethane by iodide with exactly two curved arrows.
- Ionization of tert-butyl bromideDraw the single arrow for the heterolysis of the C–Br bond of tert-butyl bromide and give both fragments their formal charges.
- Tetrahedral intermediate expels methoxideDraw the two arrows by which the tetrahedral intermediate of methyl acetate saponification re-forms C=O and expels methoxide.
- Water adds to protonated acetoneDraw how water adds to the carbon of protonated acetone, not to the charged oxygen, and place the positive charge on the product.
- Water captures the tert-butyl cationDraw water adding to the tert-butyl cation and a second water removing the proton from the oxonium ion, giving tert-butyl alcohol.
Predict the product 4
- Read a heterolysis arrow backwardRecover products and charges from the direction of bond cleavage.
- Concerted or stepwise: tert-butyl bromide in waterDecide whether a substitution is concerted or stepwise and name its intermediate.
- Which Atom of the Acetone Enolate Attacks Methyl Iodide?Predict the reacting atom of an ambident enolate with iodomethane and name the product of C-alkylation.
- Electrophilic atom when cyanide meets acetoneLocate the electrophilic atom that a nucleophile's arrow must point at.
Classify 3
- Identify the donor in an adductInfer donor and acceptor roles from explicit valence shells.
- Intermediate or Transition State: SN1 vs SN2Classify a carbocation and a pentacoordinate carbon species as an intermediate or a transition state from their place on an energy diagram.
- Name the arrow pattern for ionizationClassify a mechanistic step as one of the four basic arrow-pushing patterns.
Compare 2
- Separate a contributor from a reactionPreserve nuclei and connectivity while drawing resonance.
- Rank Carbonyls Toward Cyanide AttackRank four aldehydes and ketones by the rate of cyanide addition using electron donation and steric hindrance at the carbonyl carbon.
Calculate 5
- Count a protonated oxygenCalculate formal charge from lone pairs and bond ownership.
- Arrows in an E2 stepCount the curved arrows needed for a concerted E2 elimination.
- Electrons Each Chlorine Gets in Cl2 HomolysisCount how many electrons of a breaking bond each atom keeps when Cl–Cl cleaves homolytically.
- Formal charge on oxygen after water attacks a carbocationTrack formal charge on an atom after it uses a lone pair to form a bond.
- Formal Charge on Nitrogen: Ammonia + Methyl IodideTrack the formal charge on nitrogen after ammonia uses its lone pair to bond to iodomethane.
Find the error 2
- Repair a carbonyl arrow setRecognize a missing electron displacement rather than an extra reaction step.
- Arrow Drawn From a Charge: Methoxide + BromomethaneDiagnose a curved-arrow error in the SN2 reaction of methoxide with bromomethane by checking where each arrow starts and ends.
Interpret the evidence 2
- Choose an electron-count conventionDistinguish homolytic and heterolytic cleavage by electron allocation.
- Resonance Arrows or Reaction Arrows: AcetateTell resonance arrows from reaction arrows by checking whether atoms move or new bonds to other atoms form in the acetate ion.
Work backward 3
- Reconstruct the return arrow setDerive the microscopic reverse of an addition using present electron sources.
- Microscopic Reverse of Water Loss From tert-ButyloxoniumFind the reactants and the curved arrow of the microscopic reverse of water leaving the tert-butyloxonium ion.
- What came before the cyanohydrin alkoxide?Run a nucleophilic-attack step backwards to find its reactants.
Plan a synthesis 2
- Plan a minimal acid–base stepReject gratuitous bond rearrangements in a proton transfer.
- Arrow Patterns in tert-Butanol Dehydration, in OrderOrder the elementary arrow patterns of the acid-catalysed E1 dehydration of 2-methylpropan-2-ol.
Explain why 1
- Why SN2 of Iodomethane Needs a Second ArrowExplain, using the octet rule, why the SN2 step of hydroxide with iodomethane needs a bond-breaking arrow as well as the attacking arrow.
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