CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS SOLUTIONS TO TEXT PROBLEMS 10.1 As noted in the sample solution to part (a), a pair of electrons is moved from the double bond toward the positively charged carbon. (b) (c) 10.2 For two isomeric halides to yield the same carbocation on ionization, they must have the same carbon skeleton. They may have their leaving group at a different location, but the carbocations must become equivalent by allylic resonance. CH 3 Br CH 3 H11001 4-Bromo-1- methylcyclohexene Not an allylic carbocation CH 3 H11001 H11001 CH 3 CH 3 Cl CH 3 Br 3-Bromo-1- methylcyclohexene 3-Chloro-3- methylcyclohexene H11001 H11001 C(CH 3 ) 2 C(CH 3 ) 2 H 2 CCH 2 CH 3 C H11001 H 2 CCH 2 CH 3 C H11001 230 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.3 The allylic hydrogens are the ones shown in the structural formulas. (b) (c) (d) 10.4 The statement of the problem specifies that in allylic brominations using N-bromosuccinimide the active reagent is Br 2 . Thus, the equation for the overall reaction is The propagation steps are analogous to those of other free-radical brominations. An allylic hydrogen is removed by a bromine atom in the first step. H11001 H H Cyclohexene Bromine atom H11001H 2-Cyclohexenyl radical Hydrogen bromide Br H Br H11001 Br 2 H H Cyclohexene Bromine H11001 Br H 3-Bromocyclohexene Hydrogen bromide HBr 1-Octene HH 2,3,3-Trimethyl-1-butene CH 3 H H H H CH 3 1-Methylcyclohexene CH 3 H11001 1-Bromo-3- methylcyclohexene Not an allylic carbocation CH 3 Br CH 3 H11001 5-Chloro-1- methylcyclohexene Not an allylic carbocation CH 3 Cl CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 231 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 232 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS The allylic radical formed in the first step abstracts a bromine atom from Br 2 in the second propa- gation step. 10.5 Write both resonance forms of the allylic radicals produced by hydrogen atom abstraction from the alkene. Both resonance forms are equivalent, and so 2,3,3-trimethyl-1-butene gives a single bromide on treatment with N-bromosuccinimide (NBS). Hydrogen atom abstraction from 1-octene gives a radical in which the unpaired electron is delocalized between two nonequivalent positions. Allylic bromination of 1-octene gives a mixture of products 10.6 (b) All the double bonds in humulene are isolated, because they are separated from each other by one or more sp 3 carbon atoms. Humulene CH 3 CH 3 H 3 C CH 3 CH 2 CHCH 2 (CH 2 ) 4 CH 3 H11001 BrCH 2 CH CH(CH 2 ) 4 CH 3 CH 2 CHCH(CH 2 ) 4 CH 3 Br NBS 1-Octene 3-Bromo-1-octene 1-Bromo-2-octene (cis and trans) CH 2 CHCH 2 (CH 2 ) 4 CH 3 1-Octene CH 2 CHCH(CH 2 ) 4 CH 3 CH 2 CH CH(CH 2 ) 4 CH 3 NBS (CH 3 ) 3 CC CH 2 CH 3 2,3,3-Trimethyl-1- butene (CH 3 ) 3 CC CH 2 CH 2 Br 2-(Bromomethyl)-3,3- dimethyl-1-butene (CH 3 ) 3 CC CH 2 CH 3 2,3,3-Trimethyl-1-butene (CH 3 ) 3 CC CH 2 CH 2 (CH 3 ) 3 CC CH 2 CH 2 H11001 3-Bromocyclohexene Bromine atom H11001 2-Cyclohexenyl radical Bromine H H Br Br Br Br Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (c) The C-1 and C-3 double bonds of cembrene are conjugated with each other. The double bonds at C-6 and C-10 are isolated from each other and from the conjugated diene system. (d) The sex attractant of the dried-bean beetle has a cumulated diene system involving C-4, C-5, and C-6. This allenic system is conjugated with the C-2 double bond. 10.7 The more stable the isomer, the lower its heat of combustion. The conjugated diene is the most stable and has the lowest heat of combustion. The cumulated diene is the least stable and has the highest heat of combustion. 10.8 Compare the mirror-image forms of each compound for superposability. For 2-methyl-2,3- pentadiene, Rotation of the mirror image 180° around an axis passing through the three carbons of the C?C?C unit demonstrates that the reference structure and its mirror image are superposable. 2-Methyl-2,3-pentadiene is an achiral allene. Rotate 180H11034C Mirror image C CH 3 H H 3 C CH 3 C Reoriented mirror image C C HH 3 C H 3 C CH 3 C 2-methyl-2,3-pentadiene and Reference structure C C HH 3 C H 3 C CH 3 C Mirror image C C CH 3 H H 3 C CH 3 C H 2 C CHCH 2 CH CH 2 H 2 C C CHCH 2 CH 3 (E)-1,3-Pentadiene Most stable 3186 kJ/mol (761.6 kcal/mol) 1,4-Pentadiene 3217 kJ/mol (768.9 kcal/mol) 1,2-Pentadiene Least stable 3251 kJ/mol (777.1 kcal/mol) CC H H 3 C H CH CH 2 CH 3 (CH 2 ) 6 CH 2 CH C CHCH CHCO 2 CH 3 654321 (CH 3 ) 2 CH CH 3 CH 3 CH 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Cembrene CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 233 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website Comparison of the mirror-image forms of 2-chloro-2,3-pentadiene reveals that they are not superposable. 2-Chloro-2,3-pentadiene is a chiral allene. 10.9 Both starting materials undergo H9252-elimination to give a conjugated diene system. Two minor prod- ucts result, both of which have isolated double bonds. 10.10 The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-1 gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Pro- tonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. Under kinetically controlled conditions the carbocation is captured at the carbon that bears the great- est share of positive charge, and the product is the tertiary chloride. H 2 CCH 2 CH 3 CCH (CH 3 ) 2 CCH CH 2 Cl HCl CCHCH 2 H 3 C H 3 C H11001 CCHCH 2 H 3 C H 3 C H11001 2-Methyl-1,3-butadiene 3-Chloro-3-methyl-1- butene (major product) X H11005 OH 3-Methyl-5-hexen-3-ol X H11005 Br 4-Bromo-4-methyl-1-hexene CHCH 2 CCH 2 CH 3 X H 2 C H11001 CH 3 CHCH CCH 2 CH 3 H 2 C CH 3 CHCH 2 CH 2 C CHCH 3 CH 3 CHCH 2 CCH 2 CH 3 H 2 C CH 2 4-Methyl-1,3-hexadiene (mixture of E and Z isomers; major product) 4-Methyl-1,4-hexadiene (mixture of E and Z isomers; minor product) 2-Ethyl-1,4-pentadiene (minor product) Faster Slower Reference structure C C HH 3 C Cl CH 3 C C C HH 3 C H 3 C Cl C Mirror image C C CH 3 H Cl CH 3 C Rotate 180H11034 Reoriented mirror image and 2-Chloro-2,3-pentadiene 234 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.11 The two double bonds of 2-methyl-1,3-butadiene are not equivalent, and so two different products of direct addition are possible, along with one conjugate addition product. 10.12 The molecular formula of the product, C 10 H 9 ClO 2 , is that of a 1:1 Diels–Alder adduct between 2-chloro-1,3-butadiene and benzoquinone. 10.13 “Unravel” the Diels–Alder adduct as described in the sample solution to part (a). (b) (c) 10.14 Two stereoisomeric Diels–Alder adducts are possible from the reaction of 1,3-cyclopentadiene and methyl acrylate. In one stereoisomer the CO 2 CH 3 group is syn to the bridge, and is called the endo isomer. In the other stereoisomer the CO 2 CH 3 group is anti to the bridge and is called the exo isomer. H11001 1,3-Cyclopentadiene H11001 O OCH 3 C H Endo isomer (75%) O OCH 3 C H Exo isomer (25%) (Stereoisomeric forms of methyl bicyclo[2.2.1]hept-5-ene-2-carboxylate) Methyl acrylate H 2 C CHCOCH 3 O HC CH HC CH H11001is prepared fromO O O CH 3 Diene CH 3 Dienophile O O O H11001 DieneDiels–Alder adduct CN CN is prepared from Dienophile (cyano groups are cis) CHN CHN C C H11001 O O Benzoquinone O O H H ClCl C 10 H 9 ClO 2 2-Chloro-1,3- butadiene 3,4-Dibromo-3- methyl-1-butene (direct addition) 3,4-Dibromo-2- methyl-1-butene (direct addition) 2-Methyl-1,3- butadiene CCH CH 2 CH 3 H 2 C Br CH 3 BrCH 2 CCH CH 2 1,4-Dibromo-2- methyl-2-butene (conjugate addition) H11001 BrCH 2 C CH 3 CHCH 2 Br Br 2 H11001 Br CH 3 CCHCH 2 BrH 2 C CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 235 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.15 An electrophile is by definition an electron-seeker. When an electrophile attacks ethylene, it inter- acts with the H9266 orbital because this is the orbital that contains electrons. The H9266 * orbital of ethylene is unoccupied. 10.16 Analyze the reaction of two butadiene molecules by the Woodward–Hoffmann rules by examining the symmetry properties of the highest occupied molecular orbital (HOMO) of one diene and the lowest unoccupied molecular orbital (LUMO) of the other. This reaction is forbidden by the Woodward–Hoffmann rules. Both interactions involving the ends of the dienes need to be bonding for concerted cycloaddition to take place. Here, one is bonding and the other is antibonding. 10.17 Dienes and trienes are named according to the IUPAC convention by replacing the -ane ending of the alkane with -adiene or -atriene and locating the positions of the double bonds by number. The stereoisomers are identified as E or Z according to the rules established in Chapter 5. (a) 3,4-Octadiene: (b)(E,E)-3,5-Octadiene: (c)(Z,Z)-1,3-Cyclooctadiene: (d)(Z,Z)-1,4-Cyclooctadiene: (e)(E,E)-1,5-Cyclooctadiene: ( f )(2E,4Z,6E)-2,4,6-Octatriene: (g) 5-Allyl-1,3-cyclopentadiene: (h) trans-1,2-Divinylcyclopropane: (i) 2,4-Dimethyl-1,3-pentadiene: H 2 C CCH 3 CH 3 CCH CH 3 H HH 2 CCH CH 2 CH H CH 2 CH CH 2 HH HH H CH 3 H 3 C H CC H CH 3 CH 2 C H C H CH 2 CH 3 H CH 3 CH 2 CH CHCH 2 CH 2 CH 3 C HOMO LUMO Antibonding Bonding 236 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.18 (a) (b) (c) (d) (e) ( f ) (g) (h) 10.19 (a) Since the product is 2,3-dimethylbutane we know that the carbon skeleton of the starting ma- terial must be Since 2,3-dimethylbutane is C 6 H 14 and the starting material is C 6 H 10 , two molecules of H 2 must have been taken up and the starting material must have two double bonds. The starting material can only be 2,3-dimethyl-1,3-butadiene. H 2 C C CH 3 CH 2 C H11001 CH 3 2H 2 (CH 3 ) 2 CHCH(CH 3 ) 2 Pt C C C C C C (E)-3-Ethyl-4-methyl-1,3-hexadiene CC CH 3 CH 2 H 3 C CH 2 CH 3 CH CH 2 (1E,5E,9E)-1,5,9-Cyclododecatriene H 2 C CHCHC CHCH 3 1,2,4-Hexatriene H ClHH H Cl H H (1Z,3E,5Z)-1,6-Dichloro-1,3,5-hexatriene CH 2 CH 3 H 3-Isopropenyl-1,4-cyclohexadiene CH 2 CH 2 CH CH CH CH 2 CH 3-Vinyl-1,4-pentadiene (CH 3 ) 2 C C(CH 3 ) 2 CH 3 CH 3 CC 2,3,4,5-Tetramethyl-2,4-hexadiene H 2 CCH 2 CH(CH 2 ) 5 CH 1,8-Nonadiene CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 237 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (b) Write the carbon skeleton corresponding to 2,2,6,6-tetramethylheptane. Compounds of molecular formula C 11 H 20 have two double bonds or one triple bond. The only compounds with the proper carbon skeleton are the alkyne and the allene shown. 10.20 The dienes that give 2,4-dimethylpentane on catalytic hydrogenation must have the same carbon skeleton as that alkane. 10.21 The important piece of information that allows us to complete the structure properly is that the ant repellent is an allenic substance. The allenic unit cannot be incorporated into the ring, because the three carbons must be collinear. The only possible constitution is therefore 10.22 (a) Allylic halogenation of propene with N-bromosuccinimide gives allyl bromide. (b) Electrophilic addition of bromine to the double bond of propene gives 1,2-dibromopropane. (c) 1,3-Dibromopropane is made from allyl bromide from part (a) by free-radical addition of hydrogen bromide. H 2 C CHCH 2 Br Allyl bromide 1,3-Dibromopropane BrCH 2 CH 2 CH 2 Br HBr peroxides H 2 C CHCH 3 Propene 1,2-Dibromopropane BrCH 2 CHCH 3 Br 2 Br H 2 C CHCH 3 Propene Allyl bromide H 2 C CHCH 2 Br CCl 4 , heat N-bromosuccinimide CHO CHCCH 3 O CH 3 CH 3 HO CH 3 or or 2,4-Dimethyl- 1,3-pentadiene conjugated diene (a) 2,4-Dimethyl- 1,4-pentadiene isolated diene 2,4-Dimethyl- 2,3-pentadiene cumulated diene (c) 2,4-Dimethylpentane (b) C H 2 Pt (CH 3 ) 3 CC CCH 2 C(CH 3 ) 3 (CH 3 ) 3 CCH CHC(CH 3 ) 3 C 2,2,6,6-Tetramethyl-3-heptyne 2,2,6,6-Tetramethyl-3,4-heptadiene 238 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (d) Addition of hydrogen chloride to allyl bromide proceeds in accordance with Markovnikov’s rule. (e) Addition of bromine to allyl bromide gives 1,2,3-tribromopropane. ( f ) Nucleophilic substitution by hydroxide on allyl bromide gives allyl alcohol. (g) Alkylation of sodium acetylide using allyl bromide gives the desired 1-penten-4-yne. (h) Sodium–ammonia reduction of 1-penten-4-yne reduces the triple bond but leaves the double bond intact. Hydrogenation over Lindlar palladium could also be used. 10.23 (a) The desired allylic alcohol can be prepared by hydrolysis of an allylic halide. Cyclopentene can be converted to an allylic bromide by free-radical bromination with N-bromosuccinimide (NBS). (b) Reaction of the allylic bromide from part (a) with sodium iodide in acetone converts it to the corresponding iodide. 3-Bromocyclopentene 3-Iodocyclopentene Br NaI acetone I NBS heat, peroxides H 2 O Na 2 CO 3 Br OH Cyclopentene 2-Cyclopenten-1-ol3-Bromocyclopentene Na, NH 3 1,4-Pentadiene H 2 C CHCH 2 CH CH 2 1-Penten-4-yne H 2 C CHCH 2 C CH or H 2 , Lindlar Pd H 2 C CHCH 2 Br CHNaC Allyl bromide 1-Penten-4-yne H 2 C CHCH 2 C CH H 2 C CHCH 2 Br Allyl bromide Allyl alcohol H 2 C NaOH CHCH 2 OH H 2 C CHCH 2 Br Allyl bromide 1,2,3-Tribromopropane BrCH 2 CHCH 2 Br Br 2 Br H 2 C CHCH 2 Br Allyl bromide 1-Bromo-2-chloropropane CH 3 CHCH 2 Br HCl Cl CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 239 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (c) Nucleophilic substitution by cyanide converts the allylic bromide to 3-cyanocyclopentene. (d) Reaction of the allylic bromide with a strong base will yield cyclopentadiene by an E2 elimination. (e) Cyclopentadiene formed in part (d) is needed in order to form the required Diels–Alder adduct. 10.24 The starting material in all cases is 2,3-dimethyl-1,3-butadiene. (a) Hydrogenation of both double bonds will occur to yield 2,3-dimethylbutane. (b) Direct addition of 1 mol of hydrogen chloride will give the product of Markovnikov addition to one of the double bonds, 3-chloro-2,3-dimethyl-1-butene. H 2 CCH 2 CH 3 CH 3 CC CH 2 CH 3 Cl (CH 3 ) 2 CC HCl H 2 CCH 2 CH 3 CH 3 CC (CH 3 ) 2 CHCH(CH 3 ) 2 H 2 Pt H 2 CCH 2 CH 3 CH 3 CC 2,3-Dimethyl-1,3-butadiene COCH 3 O O COCH 3 Dimethyl bicyclo[2.2.1]heptadiene- 2,3-dicarboxylate 1,3-Cyclopentadiene CH 3 OCC CCOCH 3 O O NaOCH 2 CH 3 CH 3 CH 2 OH, heat Br 3-Bromocyclopentene 1,3-Cyclopentadiene NaCN Br 3-Bromocyclopentene CN 3-Cyanocyclopentene 240 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (c) Conjugate addition will lead to double bond migration and produce 1-chloro-2,3-dimethyl- 2-butene. (d) The direct addition product is 3,4-dibromo-2,3-dimethyl-1-butene. (e) The conjugate addition product will be 1,4-dibromo-2,3-dimethyl-2-butene. ( f ) Bromination of both double bonds will lead to 1,2,3,4-tetrabromo-2,3-dimethylbutane irre- spective of whether the first addition step occurs by direct or conjugate addition. (g) The reaction of a diene with maleic anhydride is a Diels–Alder reaction. 10.25 The starting material in all cases is 1,3-cyclohexadiene. (a) Cyclohexane will be the product of hydrogenation of 1,3-cyclohexadiene: H 2 Pt H 3 C H 3 C H 3 C H 3 C O O O O O O H H H11001 H 2 CCH 2 CH 3 CH 3 CC BrCH 2 C CCH 2 Br CH 3 CH 3 BrBr 2Br 2 H 2 CCH 2 CH 3 CH 3 CC BrCH 2 C CCH 2 Br CH 3 CH 3 Br 2 H 2 CCH 2 CH 3 CH 3 CC BrCH 2 CCH 2 CH 3 Br C CH 3 Br 2 H 2 CCH 2 CH 3 CH 3 CC CCH 2 Cl CH 3 (CH 3 ) 2 C HCl CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 241 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website (b) Direct addition will occur according to Markovnikov’s rule to give 3-chlorocyclohexene (c) The product of conjugate addition is 3-chlorocyclohexene also. Direct addition and conjugate addition of hydrogen chloride to 1,3-cyclohexadiene give the same product. (d) Bromine can add directly to one of the double bonds to give 3,4-dibromocyclohexene: (e) Conjugate addition of bromine will give 3,6-dibromocyclohexene: ( f ) Addition of 2 moles of bromine will yield 1,2,3,4-tetrabromocyclohexane. (g) The constitution of the Diels–Alder adduct of 1,3-cyclohexadiene and maleic anhydride will have a bicyclo [2.2.2]octyl carbon skeleton. H11001 O O O O O H H O 2Br 2 Br Br Br Br Br 2 conjugate addition Br Br 3,6-Dibromocyclohexene Br 2 direct addition Br Br 3,4-Dibromocyclohexene HCl conjugate addition Cl 3-Chlorocyclohexene HCl direct addition Cl 3-Chlorocyclohexene 242 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.26 Bond formation takes place at the end of the diene system to give a bridged bicyclic ring system. 10.27 The two Diels–Alder adducts formed in the reaction of 1,3-pentadiene with acrolein arise by the two alignments shown: 10.28 Compound B arises by way of a Diels–Alder reaction between compound A and dimethyl acetylenedicarboxylate. Compound A must therefore have a conjugated diene system. 10.29 The reaction is a nucleophilic substitution in which the nucleophile (C 6 H 5 S H11002 ) becomes attached to the carbon that bore the chloride leaving group. Allylic rearrangement is not observed; therefore, it is reasonable to conclude that an allylic carbocation is not involved. The mechanism is S N 2. 10.30 (a) Solvolysis of in ethanol proceeds by an S N 1 mechanism and involves a carbocation intermediate. This carbocation has some of the character of a tertiary carbocation. It is more stable and is therefore formed faster than allyl cation, . (b) An allylic carbocation is formed from the alcohol in the presence of an acid catalyst. H 2 SO 4 H 2 O CH 2 CHCHCH 3 OH 3-Buten-2-ol CH 2 CHCHCH 3 O H11001 HH H 2 OH11001CH 2 CHCHCH 3 H11001 CH 2 CHCH 2 H11001 (CH 3 ) 2 C CHCH 2 Cl H11002Cl H11002 (CH 3 ) 2 CCHCH 2 H11001 (CH 3 ) 2 CCHCH 2 H11001 1-Chloro-3-methyl-2-butene (CH 3 ) 2 C CHCH 2 Cl H11001CH 3 CH CHCH 2 Cl CH 3 CH CHCH 2 SSNa ethanol 1-Chloro-2-butene Sodium benzenethiolate 2-Butenyl phenyl sulfide H11001 CH 3 O 2 CC CCO 2 CH 3 H 2 C CH 2 CH 2 Compound A H 2 C CO 2 CH 3 CO 2 CH 3 Compound B CH 3 CH 3 CHO CH 3 CHO CHO CH 3 CHO H11001 H11001 and 3-Methylcyclohexene- 4-carboxaldehyde 3-Methylcyclohexene- 5-carboxaldehyde H11001 C CO 2 CH 3 CO 2 CH 3 C Dimethyl acetylenedicarboxylate 1,3-Cyclohexadiene Dimethyl bicyclo[2.2.2]octa- 2,5-diene-2,3-dicarboxylate CO 2 CH 3 CO 2 CH 3 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 243 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website This carbocation is a delocalized one and can be captured at either end of the allylic system by water acting as a nucleophile. (c) Hydrogen bromide converts the alcohol to an allylic carbocation. Bromide ion captures this carbocation at either end of the delocalized allylic system. (d) The same delocalized carbocation is formed from 3-buten-2-ol as from 2-buten-1-ol. Since this carbocation is the same as the one formed in part (c), it gives the same mixture of products when it reacts with bromide. (e) We are told that the major product is 1-bromo-2-butene, not 3-bromo-1-butene. The major product is the more stable one. It is a primary rather than a secondary halide and contains a more substituted double bond. The reaction is therefore governed by thermody- namic (equilibrium) control. 10.31 Since both products of reaction of hydrogen chloride with vinylacetylene are chloro-substituted dienes, the first step in addition must involve the triple bond. The carbocation produced is an allylic 3-Bromo-1-butene (minor) 1-Bromo-2-butene (major) CH 3 CH CHCH 2 Br CH 3 CHCH CH 2 Br CH 3 CHCH CH 2 H11001 CH 3 CH CHCH 2 H11001 HBr CH 3 CHCH CH 2 OH 3-Buten-2-ol CH 3 CH CHCH 2 H11001 CH 3 CHCH CH 2 H11001 Br H11002 1-Bromo-2-butene 3-Bromo-1-butene CH 3 CH CHCH 2 Br CH 3 CHCH CH 2 Br 2-Buten-1-ol CH 3 CH CHCH 2 OH CH 3 CH CHCH 2 CH 3 CH CHCH 2 O HBr H H H11001 H11001 CH 2 CHCHCH 3 H11001 CH 2 CH CHCH 3 H11001 H 2 O CH 2 CHCHCH 3 O H11001 HH CH 2 CHCHCH 3 OH H11002H H11001 O H11001 CH 2 CH CHCH 3 H H HOCH 2 CH CHCH 3 H11002H H11001 3-Buten-2-ol 2-Buten-1-ol 244 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website vinyl cation for which two Lewis structures may be written. Capture of this cation gives the prod- ucts of 1,2 and 1,4 addition. The 1,2 addition product is more stable because of its conjugated sys- tem. The observations of the experiment tell us that the 1,4 addition product is formed faster, although we could not have predicted that. 10.32 (a) The two equilibria are: For (E)-1,3-pentadiene: For (Z)-1,3-pentadiene: (b) The s-cis conformation of (Z)-1,3-pentadiene is destabilized by van der Waals strain involv- ing the methyl group. The equilibrium favors the s-trans conformation of (Z)-1,3-pentadiene more than it does that of the E isomer because the s-cis conformation of the Z isomer has more van der Waals strain. H CH 3 H H CH 3 H H H s-cis conformation of (Z)-1,3-pentadiene s-cis conformation of (E)-1,3-pentadiene H20903 H20903 Methyl–hydrogen repulsion Hydrogen–hydrogen repulsion HH H H 3 C HH s-trans s-cis H 3 C HH H H H H HH H H 3 C H s-trans s-cis H 3 C H H H H H HC C CH 2 CH H 2 CC Cl CH 2 H11001CH H 2 CC CH 2 ClCH HCl H 2 CC CH 2 CH H11001 H 2 CC CH 2 CH H11001 Vinylacetylene 2-Chloro-1,3-butadiene (1,2 addition) 4-Chloro-1,2-butadiene (1,4 addition) CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 245 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.33 Compare the mirror-image forms of each compound for superposability. (a) Rotation of the mirror image 180° around an axis passing through the three carbons of the C?C?C unit demonstrates that the reference structure and its mirror image are superposable. 2-Methyl-2,3-hexadiene is an achiral allene. (b) The two mirror-image forms of 4-methyl-2,3-hexadiene are as shown: The two structures cannot be superposed. 4-Methyl-2,3-hexadiene is chiral. Rotation of either representation 180° around an axis that passes through the three carbons of the C?C?C unit leads to superposition of the groups at the “bottom” carbon but not at the “top.” (c) 2,4-Dimethyl-2,3-pentadiene is achiral. Its two mirror-image forms are superposable. The molecule has two planes of symmetry defined by the three carbons of each CH 3 CCH 3 unit. 10.34 (a) Carbons 2 and 3 of 1,2,3-butatriene are sp-hybridized, and the bonding is an extended version of that seen in allene. Allene is nonplanar; its two CH 2 units must be in perpendicular planes in order to maximize overlap with the two mutually perpendicular p orbitals at C-2. With one Reference structure C C CH 3 H 3 C H 3 C CH 3 C Mirror image C C CH 3 H 3 C H 3 C CH 3 C Reference structure C C CH 2 CH 3 H 3 C H 3 C H C Rotate 180H11034 C C H 3 C H C Mirror image CH 3 CH 2 CH 3 Reoriented mirror image C C H CH 3 C H 3 C CH 2 CH 3 Rotate 180H11034 Mirror image C C CH 2 CH 3 H H 3 C CH 3 C Reoriented mirror image C C HCH 3 CH 2 H 3 C CH 3 C 2-Methyl-2,3-hexadiene and Reference structure C C HCH 3 CH 2 H 3 C CH 3 C Mirror image C C CH 2 CH 3 H H 3 C CH 3 C 246 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website more sp-hybridized carbon, 1,2,3-butatriene has an “extra turn” in its carbon chain, making the molecule planar. (b) The planar geometry of the cumulated triene system leads to the situation where cis and trans stereoisomers are possible for 2,3,4-hexatriene (CH 3 CH?C?C?CHCH 3 ). Cis–trans stereoisomers are diastereomers of each other. 10.35 Reaction (a) is an electrophilic addition of bromine to an alkene; the appropriate reagent is bromine in carbon tetrachloride. Reaction (b) is an epoxidation of an alkene, for which almost any peroxy acid could be used. Peroxybenzoic acid was actually used. Reaction (c) is an elimination reaction of a vicinal dibromide to give a conjugated diene and re- quires E2 conditions. Sodium methoxide in methanol was used. Reaction (d) is a Diels–Alder reaction in which the dienophile is maleic anhydride. The dienophile adds from the side opposite that of the epoxide ring. O H11001 O O O O O O O NaOCH 3 CH 3 OH (80%) Br Br O O Br Br C 6 H 5 COOH O (69%) Br Br O Br Br CCl 4 Br 2 (74%) CC H H 3 C C CH 3 H C CC H H 3 C C CH 3 H C cis-2,3,4-Hexatriene trans-2,3,4-Hexatriene CCCC H H H H All atoms of 1,2,3-butatriene lie in same plane. Nonplanar geometry of allene CCC H H H H CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 247 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website 10.36 To predict the constitution of the Diels–Alder adducts, we can ignore the substituents and simply remember that the fundamental process is (a) (b) (c) 10.37 The carbon skeleton of dicyclopentadiene must be the same as that of its hydrogenation product, and dicyclopentadiene must contain two double bonds, since 2 mol of hydrogen are consumed in its hydrogenation . The molecular formula of dicyclopentadiene (C 10 H 12 ) is twice that of 1,3-cyclopentadiene (C 5 H 6 ), and its carbon skeleton suggests that 1,3-cyclopentadiene is undergoing a Diels–Alder reac- tion with itself. Therefore: One molecule of 1,3-cyclopentadiene acts as the diene, and the other acts as the dienophile in this Diels–Alder reaction. 10.38 (a) Since allyl cation is positively charged, examine the process in which electrons “flow” from the HOMO of ethylene to the LUMO of allyl cation. LUMO HOMO Antibonding Bonding H11001 C 5 H 6 C 5 H 6 Dicyclopentadiene C 10 H 12 C 10 H 16 2H 2 Pt (C 10 H 12 C 10 H 16 ) H11001 NO 2 H NO 2 CH 2 OCH 3 CH 3 OCH 2 H11001 C CO 2 CH 3 CO 2 CH 3 CO 2 CH 3 CO 2 CH 3 C O O OCH 3 H11001 C CO 2 CH 3 CO 2 CH 3 CO 2 CH 3 CO 2 CH 3 C (CH 3 ) 3 SiO OCH 3 (CH 3 ) 3 SiO 248 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website This reaction is forbidden. The symmetries of the orbitals are such that one interaction is bonding and the other is antibonding. The same answer is obtained if the HOMO of allyl cation and the LUMO of ethylene are examined. (b) In this part of the exercise we consider the LUMO of allyl cation and the HOMO of 1,3- butadiene. This reaction is allowed by the Woodward–Hoffmann rules. Both interactions are bonding. The same prediction would be arrived at if the HOMO of allyl cation and LUMO of 1,3-buta- diene were the orbitals considered. 10.39 Since oxygen has two unpaired electrons, it can abstract a hydrogen atom from the allylic position of cyclohexene to give a free-radical intermediate. The cyclohexenyl radical is resonance-stabilized. It reacts further via the following two propagation steps: 10.40–10.41 Solutions to molecular modeling exercises are not provided in this Study Guide and Solutions Man- ual. You should use Learning By Modeling for these exercises. SELF-TEST PART A A-1. Give the structures of all the constitutionally isomeric alkadienes of molecular formula C 5 H 8 . Indicate which are conjugated and which are allenes. A-2. Provide the IUPAC name for each of the conjugated dienes of the previous problem, includ- ing stereoisomers. A-3. Hydrolysis of 3-bromo-3-methylcyclohexene yields two isomeric alcohols. Draw their structures and the structure of the intermediate that leads to their formation. H H11001 O O O O H H11001 H H H11001 HOOH O O HH HH H11001 O O H H11001 HO O LUMO HOMO Bonding Bonding CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 249 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website A-4. Give the chemical structure of the reactant, reagent, or product omitted from each of the following: (a) (b) (c) (d) (e) A-5. One of the isomeric conjugated dienes having the formula C 6 H 8 is not able to react with a dienophile in a Diels–Alder reaction. Draw the structure of this compound. A-6. Draw the structure of the carbocation formed on ionization of the compound shown. A con- stitutional isomer of this compound gives the same carbocation; draw its structure. A-7. Give the structures of compounds A and B in the following reaction scheme. A-8. Give the reagents necessary to carry out the following conversion. Note that more than one reaction step is necessary. PART B B-1. 2,3-Pentadiene, , is (a) A planar substance (b) An allene (c) A conjugated diene (d) A substance capable of cis-trans isomerism CH 3 CH CHCH 3 C Br NBS CCl 4 , heat NaI acetone Compound A Compound B C CH 3 CH 3 Br H11001 CH 3 OCC CCOCH 3 ? O O ? Br Br H11001 O O O Diels–Alder ? CH 2 CHCH CH 2 HCl (1 mol) ? (two products) CH 3 CH CHCH CHCH 3 Br 2 ? (two products) 250 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website B-2. Rank the following carbocations in order of increasing stability (least A most): (a)1H11021 2 H11021 3(c)3H11021 1 H11021 2 (b)2H11021 3 H11021 1(d)2H11021 1 H11021 3 B-3. Hydrogenation of cyclohexene releases 120 kJ/mol (28.6 kcal/mol) of heat. Which of the following most likely represents the observed heat of hydrogenation of 1,3-cyclohexadiene? (a) 232 kJ/mol (55.4 kcal/mol) (b) 240 kJ/mol (57.2 kcal/mol) (c) 247 kJ/mol (59.0 kcal/mol) (d) 120 kJ/mol (28.6 kcal/mol) B-4. Which of the following compounds give the same carbocation on ionization? (a) 1 and 3 (c) 1 and 2 (b) 2 and 4 (d) 1 and 4 B-5. For the following reactions the major products are shown: These provide an example of ______ control at low temperature and ______ control at 12 higher temperature. 12 (a) kinetic thermodynamic (b) thermodynamic kinetic (c) kinetic kinetic (d) thermodynamic thermodynamic B-6. Which of the following bonds would have the smallest bond dissociation energy? CH 3 CHCH 3 H (a) (b) CH 3 CH CH H (c) (d) H 2 C CHCH 3 CH H CH 3 CH 2 CH 2 H CH CH 2 CH 2 CH CH CH 2 CHCHCH 3 Br CH 2 CH CHCH 3 Br HBr 0H11034C H1100125H11034C Br BrBr Br 123 4 CH 3 CHCH 3 H11001 CH 3 CHCH CHCH 3 H11001 (CH 3 ) 3 CCH 2 H11001 132 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 251 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website B-7. Which of the following compounds would undergo solvolysis (S N 1) most rapidly in aqueous ethanol? B-8. What is the product of 1,4-addition in the reaction shown? B-9. Which of the following compounds will undergo hydrolysis (S N 1) to give a mixture of two alcohols that are constitutional isomers? B-10. What hydrocarbon reacts with the compound shown (on heating) to give the indicated product? (a) 2-Methyl-1-butene (d) 2-Methyl-1,3-butadiene (b) 2-Methyl-2-butene (e) 1,3-Pentadiene (c) 3-Methyl-1-butyne O O O O O O H 3 C Cl Cl (a) (b) Cl Cl (c) (d) Cl Cl (a) (b) Cl Cl (c) (d) Cl (e) HCl CH 3 Br CH 3 Br (a) (b) Br CH 3 CH 3 Br (c) (d) 252 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website