P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 7 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS. ELIMINATION REACTIONS OF ALKYL HALIDES SOLUTIONS TO PROBLEMS 7.1 (a) (E )-1-Bromo-1-chloro-1-pentene or (E )-1-Bromo-1-chloropent-1-ene (b) (E )-2-Bromo-1-chloro-1-iodo-1-butene or (E )-2-Bromo-1-chloro-1-iodobut-1-ene (c) (Z )-3,5-Dimethyl-2-hexene or (Z )-3,5-Dimethylhex-2-ene (d) (Z )-1-Chloro-1-iodo-2-methyl-1-butene or (Z )-1-Chloro-1-iodo-2-methylbut-1-ene (e) (Z,4S )-3,4-Dimethyl-2-hexene or (Z,4S )-3,4-Dimethylhex-2-ene (f) (Z,3S )-1-Bromo-2-chloro-3-methyl-1-hexene or (Z,3S )-1-Bromo-2-chloro-3-methylhex-1-ene 7.2 <<Order of increasing stability 7.3 (a), (b) H 2 Δ H° = − 119 kJ mol−1 Pt 2-Methyl-1-butene pressure (disubstituted) H 2 Δ H° = − 127 kJ mol−1 Pt 3-Methyl-1-butene pressure (monosubstituted) H2 − Δ H° = − 113 kJ mol 1 Pt 2-Methyl-2-butene pressure (trisubstituted) (c) Yes, because hydrogenation converts each alkene into the same product. 106 CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS 107 H H (d) >> H H H (trisubstituted) (disubstituted) (monosubstituted) Notice that this predicted order of stability is confirmed by the heats of hydro- genation. 2-Methyl-2-butene evolves the least heat; therefore, it is the most stable. 3-Methyl-1-butene evolves the most heat; therefore, it is the least stable. H H (e) H H H H H 1-Pentene cis-2-Pentene trans-2-Pentene (f) Order of stability: trans-2-pentene > cis-2-pentene >1-pentene 7.4 (a) 2,3-Dimethyl-2-butene would be the more stable because the double bond is tetra- substituted. 2-Methyl-2-pentene has a trisubstituted double bond. (b) trans-3-Hexene would be the more stable because alkenes with trans double bonds are more stable than those with cis double bonds. (c) cis-3-Hexene would be more stable because its double bond is disubstituted. The double bond of 1-hexene is monosubstituted. (d) 2-Methyl-2-pentene would be the more stable because its double bond is trisubstituted. The double bond of trans-2-hexene is disubstituted. 7.5 The location of IR absorptions between 600 cm−1 and 1000 cm−1 due to out-of-plane bending of alkene C—H bonds can be the basis of differentiation. (a) 2-Methyl-2-pentene, ∼800 cm−1 2,3-Dimethyl-2-butene, no alkene C—H bonds (b) cis-3-Hexene, 650–750 cm−1 trans-3-Hexene, ∼960 cm−1 (c) 1-Hexene, ∼900 cm−1 and ∼1000 cm−1 cis-3-Hexene, 650–750 cm−1 (d) trans-2-Hexene, ∼960 cm−1 2-Methyl-2-pentene, ∼800 cm−1 7.6 Br NaOEt most EtOH, 55°C less least CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 108 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS Br OK + 7.7 (a) OH heat (trisubstituted, (monosubstituted, more stable) less stable) Major product Minor product Br OK (b) + OH heat (tetrasubstituted, (disubstituted, more stable) less stable) Major product Minor product 7.8 t-BuOK in t-BuOH 7.9 An anti coplanar transition state allows the molecule to assume the more stable staggered conformation, H H H H H Br whereas a syn coplanar transition state requires the molecule to assume the less stable eclipsed conformation. H Br 7.10 cis-1-Bromo-4-tert-butylcyclohexane can assume an anti coplanar transition state in which the bulky tert-butyl group is equatorial. Br H H H H • B • The conformation (above), because it is relatively stable, is assumed by most of the molecules present, and, therefore, the reaction is rapid. CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS 109 On the other hand, for trans-1-bromo-4-tert-butylcyclohexane to assume an anti copla- nar transition state, the molecule must assume a conformation in which the large tert-butyl group is axial: H Br Br H H H H H • B • Such a conformation is of high energy; therefore, very few molecules assume this confor- mation. The reaction, consequently, is very slow. 7.11 (a) Anti coplanar elimination can occur in two ways with the cis isomer. Br (a) H CH3 H CH3 H (a) H CH (major product) • 3 B • (b) (b) cis-1-Bromo-2-methylcyclohexane (b) Anti coplanar elimination can occur in only one way with the trans isomer. H Br H H CH3 H • B • CH3 trans-1-Bromo-2-methylcyclohexane O 7.12 (a) (1) CH3 CH OH HHO S O CH3 O H O CH3 CH O H O S O H CH3 O H (2) CH CH O H CH CH 3 3 H2O CH3 CH3 (3) CH3 CH CH2 H OSO3H CH3 CH CH2 HOSO3H CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 110 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS (b) By donating a proton to the OH group of the alcohol in step (1), the acid allows the loss of a relatively stable, weakly basic, leaving group (H2O) in step (2). In the absence of an acid, the leaving group would have to be the strongly basic OH− ion, and such steps almost never occur. OH 7.13 >> OH OH 1° 2° 3° Order of increasing case of dehydration CH3 CH3 + − 7.14 + + (1) CH3CCH2OH H A CH3CCH2OH2 A CH3 CH3 CH3 CH3 + + + (2) CH3CCH2 OH2 CH3CCH2 H2O CH3 CH3 1° Carbocation + + CH3 CH3 CH3 + (3) CH CCH CH CCH CH CCH CH 3 2 3 2 3 + 2 3 + CH3 CH3 1° Carbocation Transition state 3° Carbocation [Steps (2) and (3), ionization and rearrangement, may occur simultaneously.] CH3 CH3 H (4) CH C CH CH CC + HA 3 + 3 − CH H A 3 CH3 2-Methyl-2-butene CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS 111 (−H O) (−H O) •• •• + 2 + 2 7.15 CH CH CHCH OH+ HHO CH CH CHCH OH 3 2 2 •• + 3 2 2 •• 2 + ( H2O) ( H2O) CH3 H CH3 2-Methyl-1-butanol H H OH + 1,2-hydride + 2 CH CH C CH CH CH C CH 3 2 2 shift* 3 3 CH3 CH3 1° Cation 3° Cation + + CH3CHC CH3 H3O CH3 2-Methyl-2-butene (−H O) (−H O) •• •• + 2 + 2 CH CHCH CH OH+ HHO CH CHCH CH OH 3 2 2 •• + 3 2 2 2 + ( H2O) ( H2O) CH3 H CH3 3-Methyl-1-butanol H H + 1,2-hydride + OH2 CH CHCH CH CH C CH CH 3 2 shift* 3 3 CH3 CH3 + + CH3CCHCH 3 H3O * The hydride shift may occur simultaneously with the preceding step. CH3 2-Methyl-2-butene CH CH 7.16 HO CH3 3 CH3 3 H O+ = CH 3 CH HO 3 − 3 ( H2O) + Isoborneol H OH2 CH CH3 CH2 3 + CH2 = CH2 CH3 + + CH3 CH2 CH3 H3O + CH CH3 3 CH2 Camphene CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 112 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS + 7.17 (a) CH3CH CH2 NaNH2 No reaction +++ − − + (b) CH3C C H Na NH2 CH3CCNa NH3 Stronger Stronger Weaker Weaker acid base base acid + (c) CH3CH2CH3 NaNH2 No reaction − ++− (d) CH3C C H OCH2CH3 CH3CCH OCH2CH3 Stronger Stronger Weaker Weaker base acid acid base + − ++ (e) CH3C C H NH3 CH3CCH NH3 Stronger Stronger acid Weaker Weaker base acid base O Cl Cl (1) 3 equiv. PCl NaNH2 7.18 5 0°C mineral oil, heat (2) HA O PCl5 (1) 3 NaNH2 7.19 (a) CH CCH CH CCl CH CH C CH 3 3 0°C 3 2 3 mineral oil, heat 3 + (2) NH4 (1) 3 NaNH2 (b) CH CH CHBr CH C CH 3 2 2 mineral oil, heat 3 + (2) NH4 [same as (b)] (c) CH3CHBrCH2Br CH3C CH Br2 [same as (b)] (d) CH3CH CH2 CH3CHCH2Br CH3C CH CCl4 Br CH3 CH3 − − CH C CHC + Na+ CH C CCNa+ 7.20 3 NH2 − 3 ( NH3) CH3 CH3 (Starting the synthesis with 1-propyne and CH3 I attempting to alkylate with a tert-butyl substrate would not work because elimination would occur CH3 instead of substitution.) CH3 C CCCH3 CH3 CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS 113 O 7.21 Compound A − ° (1) Li, C2H5NH2, 78 C 7.22 (2) NH4Cl 2-Nonyne (E)-2-Nonene 7.23 Route 1 CH3 CH3 NaNH2 + − CH3 Br HC CCH2CHCH3 − Na C CCH2CHCH3 − ( NH3) ( NaBr) CH3 CH3 H2 CH C CCH CHCH CH CH CH CH CHCH 3 2 3 Pd, Pt, or Ni 3 2 2 2 3 pressure Route 2 CH3 NaNH − + Br CH CH CHCH 2 Na 2 2 3 HC CH − HC C − ( NH3) ( NaBr) CH3 CH3 H2 HC C CH CH CHCH CH CH CH CH CHCH 2 2 3 Pd, Pt, or Ni 3 2 2 2 3 pressure Route 3 CH3 CH3 NaNH2 + − CH3CH2Br HC CCHCH3 − Na C CCHCH3 − ( NH3) ( NaBr) CH3 CH3 H2 CH CH C CCHCH CH CH CH CH CHCH 3 2 3 Pd, Pt, or Ni 3 2 2 2 3 CONFIRMING PAGES P1: PBU/OVY P2: PBU/OVY QC: PBU/OVY T1: PBU Printer: Bind Rite JWCL234-07 JWCL234-Solomons-v1 December 8, 2009 21:37 114 ALKENES AND ALKYNES I: PROPERTIES AND SYNTHESIS Etc.