Organic Chemistry 1 · Unsaturated molecules and radicals
Radical reactions and selectivity: practice problems
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- Brominate isobutane at its single tertiary hydrogen
- Chloromethane from methane: initiation, propagation, termination
- 1-Bromohexane from hex-1-ene: anti-Markovnikov HBr
- Split one bond evenly
- Pass the radical to carbon
- Complete one chlorination chain turn
- Allylic bromination — radical at the far end
- Kharasch peroxide effect — radical HBr propagation
- Kharasch peroxide effect — HBr chain initiation
Draw the mechanism 94
- Split one bond evenlyRepresent initiation using two single-electron arrows.
- Pass the radical to carbonAccount for all three electrons involved in radical H abstraction.
- Complete one chlorination chain turnConnect hydrogen abstraction to halogen transfer and identify the regenerated chain carrier using three-electron accounting.
- Allylic bromination — radical at the far endRemove an allylic H, draw the radical resonance forms, then add Br at the other end.
- Kharasch peroxide effect — radical HBr propagationComplete both HBr radical-addition propagation steps and locate radical stability before hydrogen transfer.
- Kharasch peroxide effect — HBr chain initiationGenerate the halogen chain carrier from an oxygen-centered radical while retaining the unused initiator fragment.
- Terminate by combining unlike radicalsDistinguish radical combination from halogen-transfer propagation even when both yield an alkyl halide.
- Terminate without joining the carbon skeletonsAccount for radical disproportionation as H transfer coupled to alkene formation, consuming two radicals.
- Grow and then terminate a short carbon chainTrack a radical through two alkene additions and complete a specified combination termination without leaving an unbounded polymer placeholder.
- Account for nitrogen extrusion from an azo initiatorUse a bounded four-fishhook fragmentation model to generate two carbon radicals and neutral nitrogen without mistaking N=N cleavage for initiation.
- Move a radical by carbon–carbon fragmentationCouple alkoxy-radical beta scission to carbonyl formation and conserve the odd electron on the departing carbon fragment.
- Use sulfur as the radical chain carrierTransfer the addition/abstraction logic to a heteroatom-centered radical and close a thiol–ene propagation cycle.
- Brominate the benzylic carbon of toluene with NBSTrace NBS-mediated benzylic bromination through a resonance-delocalized benzyl radical and explain why bromine ends on the side chain rather than the ring.
- Brominate cyclohexene at the allylic positionExplain how NBS keeps Br₂ scarce so allylic substitution outcompetes addition, and show that either terminus of the symmetric cyclohexenyl radical gives the same 3-bromocyclohexene.
- Brominate isobutane at its single tertiary hydrogenDraw a complete photobromination sequence from Br–Br homolysis through both propagation steps and connect the tertiary radical to the nearly exclusive tert-butyl bromide product.
- Out-select the statistics in propane brominationWeigh hydrogen count against secondary-radical stability and a late transition state to explain why bromination of propane gives mainly 2-bromopropane.
- Grow peroxides in an old bottle of etherClose an autoxidation chain in which a peroxyl radical abstracts an α-hydrogen and the resulting carbon radical captures O₂, regenerating the same kind of peroxyl carrier.
- Stop two chains with one BHT moleculeShow how a hindered phenol donates its O–H hydrogen to a peroxyl radical, delocalize the phenoxyl radical to the para carbon, and trap a second peroxyl radical there.
- Replace C–Br with C–H using tributyltin hydrideRun the Bu₃SnH radical chain: generate Bu₃Sn• from an initiator radical, abstract bromine from carbon, and deliver hydrogen from tin hydride to regenerate the tin radical.
- Brominate ethylbenzene and predict its stereochemistrySelect the benzylic CH₂ over the terminal CH₃ in photobromination and deduce a racemic product from a planar radical intermediate.
- AIBN radical adding to acrylonitrileDraw the three fishhook arrows by which the 2-cyano-2-propyl radical from AIBN adds to the CH₂ end of acrylonitrile, giving the nitrile-stabilized first chain radical of polyacrylonitrile.
- AIBN initiation of methyl methacrylate polymerizationDraw AIBN fragmentation to two 2-cyano-2-propyl radicals and N₂, then the addition of one radical to the CH₂ end of methyl methacrylate to give the first chain radical of PMMA.
- 2-Bromo-2,3,3-trimethylbutane by tertiary brominationDraw the radical chain that brominates 2,2,3-trimethylbutane at its only tertiary C–H, giving 2-bromo-2,3,3-trimethylbutane.
- 3-Bromo-2,2-dimethylbutane: secondary over primaryDraw the radical chain that brominates 2,2-dimethylbutane at its only secondary carbon, giving 3-bromo-2,2-dimethylbutane.
- 2-Bromo-2,3-dimethylbutane by tertiary brominationDraw the radical chain that brominates 2,3-dimethylbutane at a tertiary carbon to give 2-bromo-2,3-dimethylbutane.
- 2-Bromo-2,4-dimethylpentane by radical brominationDraw the radical chain that brominates 2,4-dimethylpentane at a tertiary C–H, giving 2-bromo-2,4-dimethylpentane.
- 2-Bromo-2-methylbutane from 2-methylbutaneDraw the radical chain that brominates 2-methylbutane at its tertiary carbon, and explain why bromine ignores the primary and secondary hydrogens.
- 3-Bromo-3-ethylpentane from 3-ethylpentaneDraw the radical chain that brominates 3-ethylpentane at its tertiary carbon to give 3-bromo-3-ethylpentane.
- 3-Bromo-3-methylpentane by tertiary selectivityDraw the radical chain that brominates 3-methylpentane at its single tertiary carbon, giving 3-bromo-3-methylpentane.
- 2-Bromobutane from butane: secondary selectivityDraw the radical chain that brominates butane at C2, and explain why 2-bromobutane forms almost exclusively and as a racemate.
- 1-Bromo-1-butylcyclohexane by tertiary brominationDraw the radical chain that brominates butylcyclohexane at its tertiary ring carbon, and judge how selective bromine is when sixteen secondary hydrogens compete.
- (2-Bromopropan-2-yl)benzene from cumeneDraw the radical chain that brominates cumene at the tertiary benzylic carbon, giving (2-bromopropan-2-yl)benzene.
- Bromocyclohexane from cyclohexane and bromineDraw Br–Br homolysis and both propagation steps that turn cyclohexane into bromocyclohexane under light.
- Bromocyclopentane from cyclopentane and bromineDraw the radical chain that brominates cyclopentane to bromocyclopentane with Br₂ and light.
- Benzhydryl bromide from diphenylmethaneDraw the radical chain that brominates diphenylmethane at its CH2 group to give benzhydryl bromide.
- Bromoethane from ethane by radical brominationDraw the initiation and both propagation steps that convert ethane into bromoethane with Br₂ and light.
- (1-Bromoethyl)benzene by benzylic brominationDraw the radical chain that brominates ethylbenzene at the benzylic CH2, giving (1-bromoethyl)benzene (Br2 and light on ethylbenzene).
- 1-Bromo-1-ethylcyclohexane from ethylcyclohexaneDraw the light-initiated radical chain that brominates ethylcyclohexane at the ring carbon carrying the ethyl group, giving 1-bromo-1-ethylcyclohexane.
- 2-Bromo-2-methylpropane by selective tertiary brominationDraw Br–Br homolysis and both propagation steps that convert isobutane into 2-bromo-2-methylpropane, removing the tertiary hydrogen (2-methylpropane).
- 2-Bromo-2,4,4-trimethylpentane from isooctaneDraw the radical chain that brominates isooctane (2,2,4-trimethylpentane) at its single tertiary carbon to give 2-bromo-2,4,4-trimethylpentane.
- Bromomethane from methane by radical brominationDraw Br–Br homolysis and the two propagation steps that turn methane into bromomethane, and explain why the hydrogen-abstraction step is slow.
- 1-Bromo-1-methylcyclohexane by tertiary ring brominationDraw the radical chain that brominates methylcyclohexane at the ring carbon bearing the methyl group, giving 1-bromo-1-methylcyclohexane.
- 1-Bromo-1-methylcyclopentane by tertiary brominationDraw the radical chain that brominates methylcyclopentane at its tertiary ring carbon to give 1-bromo-1-methylcyclopentane.
- Neopentyl bromide from neopentane by brominationDraw the radical chain that brominates neopentane to neopentyl bromide, and explain why the reaction is slow but gives one product.
- 4-Methylbenzyl bromide from p-xyleneDraw the radical chain that brominates one methyl group of p-xylene to give 4-methylbenzyl bromide.
- 2-Bromopropane from propane and bromineDraw the initiation and both propagation steps that brominate propane at its secondary carbon, giving 2-bromopropane.
- Benzylic bromination of (R)-sec-butylbenzene: racemateDraw the benzylic bromination of (R)-sec-butylbenzene through both faces of the planar benzylic radical, and explain why (2-bromobutan-2-yl)benzene forms racemic.
- Racemic 3-bromo-3-methylhexane from (S)-3-methylhexaneDraw the radical bromination of (S)-3-methylhexane at its tertiary stereocentre, following both faces of the planar radical, and explain why the product is racemic.
- Benzyl bromide from tolueneDraw Br–Br homolysis and the two propagation steps that convert toluene into benzyl bromide with Br2 and light.
- 1-Chloro-2,2,3,3-tetramethylbutane: one monochlorideDraw the radical chain that chlorinates 2,2,3,3-tetramethylbutane, and explain why it gives a single monochloride.
- Chlorination of 2,3-dimethylbutane: two monochloridesDraw primary and tertiary hydrogen abstraction from 2,3-dimethylbutane by Cl•, and predict the ratio of the two monochlorides.
- 1-Chloro-2-methylpropane and 2-chloro-2-methylpropane from isobutaneDraw primary and tertiary hydrogen abstraction in the chlorination of isobutane (2-methylpropane) and account for the mixture of 1-chloro-2-methylpropane and 2-chloro-2-methylpropane.
- 1-Chlorobutane and 2-chlorobutane from butaneDraw both branches of butane chlorination, primary and secondary hydrogen abstraction, and predict the ratio of 1-chlorobutane to 2-chlorobutane.
- Chlorocyclohexane from cyclohexane and chlorineDraw the radical chain that converts cyclohexane into chlorocyclohexane with Cl₂ and light, and explain why chlorine gives only one monochloride here.
- Chlorocyclopentane from cyclopentaneDraw the initiation and both propagation steps that turn cyclopentane into chlorocyclopentane.
- Chloroethane from ethane and chlorineDraw Cl–Cl homolysis and the two propagation steps that convert ethane into chloroethane.
- Chloromethane from methane: initiation, propagation, terminationDraw every stage of the radical chlorination of methane: Cl–Cl homolysis, hydrogen abstraction, chlorine transfer, and a termination step, all with fishhook arrows.
- 1-Chloropropane and 2-chloropropane from propaneDraw both branches of propane chlorination, primary and secondary hydrogen abstraction, and explain why chlorine gives a mixture of 1-chloropropane and 2-chloropropane.
- Benzyl chloride from toluene by side-chain chlorinationDraw the radical chain that chlorinates the methyl group of toluene to benzyl chloride, and explain why the ring is not chlorinated.
- tert-Butanol and Br• from di-tert-butyl peroxide and HBrDraw the two initiation steps that start a peroxide-promoted HBr addition: O–O homolysis of di-tert-butyl peroxide and hydrogen-atom abstraction from HBr by a tert-butoxyl radical.
- Phenyl radical and CO₂ from benzoyl peroxideDraw O–O homolysis of dibenzoyl peroxide and the fragmentation of one benzoyloxyl radical into a phenyl radical and carbon dioxide.
- Ethene and ethane by disproportionation of two ethyl radicalsDraw the four fishhook arrows by which one ethyl radical takes a β-hydrogen from another, giving ethane and ethene (termination by disproportionation).
- 1-Bromohexane from hex-1-ene: anti-Markovnikov HBrDraw peroxide initiation and the two propagation steps that add HBr to hex-1-ene with bromine on the terminal carbon, giving 1-bromohexane.
- Hex-1-yne + HBr/ROOR: (E)- and (Z)-1-bromohex-1-eneDraw peroxide initiation and the radical chain that adds one HBr to two hex-1-yne molecules, giving (E)- and (Z)-1-bromohex-1-ene.
- 1-Bromo-2-methylcyclopentane by anti-Markovnikov HBr additionDraw the radical addition of HBr to 1-methylcyclopentene in the presence of peroxide, giving 1-bromo-2-methylcyclopentane.
- 2-Bromo-3-methylbutane from 2-methylbut-2-ene with HBr/ROORDraw the peroxide-initiated radical addition of HBr to 2-methylbut-2-ene that places bromine on the CH carbon, giving 2-bromo-3-methylbutane.
- 1-Bromo-2-methylpropane from 2-methylpropene, HBr/ROORDraw the peroxide-initiated chain that adds HBr to 2-methylpropene with bromine on the CH2 carbon, giving 1-bromo-2-methylpropane.
- 1-Bromo-3-methylbutane from 3-methylbut-1-ene, HBr/ROORDraw the peroxide-initiated chain that adds HBr to 3-methylbut-1-ene with bromine on C1 and no rearrangement, giving 1-bromo-3-methylbutane.
- (Bromomethyl)cyclohexane from methylenecyclohexaneDraw the peroxide-initiated radical chain that adds HBr to methylenecyclohexane with bromine on the exocyclic CH2, giving (bromomethyl)cyclohexane.
- (2-Bromoethyl)benzene from styrene: HBr with peroxideDraw peroxide initiation and both propagation steps that add HBr across styrene with bromine on the terminal CH2, giving (2-bromoethyl)benzene.
- Kharasch addition of BrCCl₃ to styreneDraw peroxide initiation, addition of the trichloromethyl radical to styrene and bromine-atom transfer from BrCCl₃, giving (1-bromo-3,3,3-trichloropropyl)benzene.
- 1,1,1,3-Tetrabromoheptane from hex-1-ene and CBr₄Draw the light-initiated chain in which the tribromomethyl radical adds to hex-1-ene and the adduct radical takes Br from CBr₄, giving 1,1,1,3-tetrabromoheptane.
- 1,1,1,3-Tetrachlorononane from oct-1-ene and CCl₄Draw benzoyl peroxide initiation and the chain in which the trichloromethyl radical adds to oct-1-ene and the adduct radical takes Cl from CCl₄, giving 1,1,1,3-tetrachlorononane.
- Ethane from two methyl radicals (termination)Draw the fishhook arrows for the termination step in which two methyl radicals combine to form ethane.
- NBS on hex-1-ene: 3-bromohex-1-ene and 1-bromohex-2-eneDraw the NBS chain on two hex-1-ene molecules: trap one allylic radical at C3, draw the resonance form of the other and trap it at C1, giving both allylic bromides.
- 3-Bromo-1-methylcyclohexene: allylic bromination with a regio mixtureDraw the NBS chain that removes the allylic hydrogen at C3 of 1-methylcyclohexene and traps the radical at that carbon, giving 3-bromo-1-methylcyclohexene, and explain why other allylic bromides also form.
- NBS on 2,3-dimethylbut-2-ene: two allylic bromidesDraw the NBS chain on two 2,3-dimethylbut-2-ene molecules: trap one allylic radical at its primary CH2, redraw the other as the tertiary contributor and trap it there.
- 4-Methoxybenzyl bromide from 4-methylanisole and NBSDraw benzoyl peroxide initiation and the steps that brominate the methyl group of 4-methylanisole, giving 4-methoxybenzyl bromide.
- 4-Nitrobenzyl bromide from 4-nitrotoluene with NBS, AIBNDraw AIBN initiation and the radical chain that converts the methyl group of 4-nitrotoluene into CH2Br, giving 4-nitrobenzyl bromide.
- (E)-Cinnamyl bromide from allylbenzene with NBSDraw the NBS chain that removes the benzylic, allylic hydrogen of allylbenzene, shift the radical to the terminal carbon by resonance, and trap it to give (E)-cinnamyl bromide.
- 3-Bromocyclohexene by NBS allylic brominationDraw initiation and both propagation steps that convert cyclohexene into 3-bromocyclohexene with NBS and light, keeping the C=C intact.
- 3-Bromocyclopentene by NBS allylic brominationDraw initiation and both propagation steps that turn cyclopentene into 3-bromocyclopentene with NBS and light, leaving the C=C in place.
- 1-Bromoindane by NBS benzylic bromination of indaneDraw the NBS chain that brominates indane at a benzylic CH2 of the five-membered ring, giving 1-bromoindane.
- 3-Bromo-2-methylprop-1-ene from 2-methylpropene and NBSDraw the NBS chain that brominates 2-methylpropene on a methyl group, giving methallyl bromide (3-bromo-2-methylprop-1-ene).
- Allyl bromide (3-bromoprop-1-ene) from propene with NBSDraw the NBS radical chain that brominates propene at its methyl group, giving allyl bromide (3-bromoprop-1-ene).
- (1-Bromopropyl)benzene from propylbenzene with NBSDraw the NBS radical chain that brominates propylbenzene only at the benzylic CH2, giving (1-bromopropyl)benzene.
- 1-Bromotetralin by NBS benzylic brominationDraw the NBS chain that brominates tetralin at a benzylic CH2 of the saturated ring, giving 1-bromo-1,2,3,4-tetrahydronaphthalene.
- Benzoyl peroxide initiation of vinyl chlorideDraw O–O homolysis of benzoyl peroxide, loss of CO₂ to the phenyl radical and its addition to the CH₂ end of vinyl chloride, giving the 1-chloro-2-phenylethyl radical that starts a PVC chain.
- Growing polystyrene: initiation and two propagation stepsDraw peroxide homolysis, addition of a tert-butoxyl radical to styrene, and two propagation steps in which the benzylic radical adds to further styrene molecules, ending with the growing trimer radical.
- tert-Butoxyl radical initiation of ethyleneDraw homolysis of di-tert-butyl peroxide and the addition of a tert-butoxyl radical to ethylene, giving the 2-tert-butoxyethyl radical that starts a polyethylene chain.
- Radical thioacetic acid addition to cyclohexeneDraw AIBN initiation, formation of the acetylthiyl radical and its two-step chain that adds thioacetic acid across cyclohexene, giving S-cyclohexyl thioacetate.
- Ethyl hexyl sulfide by thiol–ene additionDraw peroxide initiation and the two propagation steps that add ethanethiol across hex-1-ene with sulfur on the terminal carbon, giving ethyl hexyl sulfide (1-(ethylsulfanyl)hexane).
- Octyl phenyl sulfide by thiol–ene addition to oct-1-eneDraw AIBN fragmentation, hydrogen abstraction from thiophenol and the two propagation steps that add PhS–H across oct-1-ene with sulfur on the terminal carbon, giving octyl phenyl sulfide.
- Head-to-tail propagation of vinyl acetateDraw the propagation step in which a 1-acetoxyethyl chain-end radical adds to the CH₂ end of vinyl acetate, regenerating an acetoxy-bearing radical and placing OAc groups on alternate carbons.
Predict the product 4
- Kharasch peroxide effect — predict HBr connectivityPredict anti-Markovnikov HBr addition from radical stability.
- NBS product from cyclohexenePredict the allylic bromination product of NBS with an alkene.
- Repeat unit of radical-polymerized vinyl chloridePredict the head-to-tail repeat pattern that radical chain growth gives from vinyl chloride.
- Product formed only by termination in ethane chlorinationIdentify the by-product of ethane chlorination that can arise only from a termination step.
Classify 3
- Classify a radical-chain stepClassify a step from its radical balance.
- Classify a polymer chain termination eventClassify a termination between two growing polymer radicals as combination or disproportionation from the chain ends it produces.
- Classify a radical chain stepClassify a radical reaction step as initiation, propagation or termination.
Compare 3
- Compare two halogen radicalsUse transition-state energetics to explain greater bromination selectivity.
- Rank C–H bond dissociation energiesOrder C–H bond dissociation energies from weakest to strongest and connect them to the stability of the radical formed.
- Rank alkyl radical stabilityRank methyl, primary, secondary and tertiary carbon radicals by stability.
Calculate 8
- Combine abundance and reactivityCalculate a product fraction using supplied per-hydrogen factors.
- Compare propagation with terminationUse supplied elementary rate laws to compare competing radical pathways.
- Enthalpy of hydrogen abstraction by chlorineCalculate the enthalpy change of a propagation step from bond dissociation energies.
- Count monochlorination products of butane, stereo includedCount all monochlorination products of butane, including the enantiomers formed at a new stereocentre.
- Enthalpy of Br• abstracting H from methaneCalculate the enthalpy change of the hydrogen-abstraction step of methane bromination from bond dissociation energies.
- Count monobromination productsCount constitutionally distinct monohalogenation products by finding nonequivalent hydrogen sets.
- Percent 2-bromopropane from propane brominationCombine hydrogen counts with the 82:1 per-hydrogen selectivity of Br• to predict the percentage of 2-bromopropane from propane.
- Percent tertiary chloride from 2-methylbutanePredict a radical chlorination product percentage from hydrogen counts and per-hydrogen reactivity.
Find the error 2
- Separate a dot from a pairDiagnose using a two-electron arrow for homolysis.
- Diagnose a radical chain that stalls in airDiagnose why dissolved oxygen stops a radical chlorination and choose the fix.
Interpret the evidence 2
- Read a radical beside a π bondRecognize allylic resonance without moving nuclei.
- Hammond postulate: late transition state in brominationUse the Hammond postulate to interpret why endothermic hydrogen abstraction by Br• is far more selective than exothermic abstraction by Cl•.
Work backward 1
- Alkene that gives (bromomethyl)cyclohexane with HBr/ROORWork backwards from a primary bromide to the alkene that gives it by radical anti-Markovnikov HBr addition.
Plan a synthesis 2
- Choose a radical initiatorChoose a reagent that starts a radical chain reaction.
- Choose NBS or Br2 for cyclohexene brominationPlan conditions that give allylic substitution rather than addition when brominating cyclohexene.
Explain why 3
- Cancel the chain carriersDistinguish the net reaction from the elementary steps carrying a radical chain.
- Why alkanes do not undergo radical iodinationExplain from bond dissociation energies why the I• hydrogen-abstraction step stops radical iodination of alkanes.
- Why peroxides do not reverse HCl additionExplain why only HBr adds anti-Markovnikov under radical conditions.
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