1559Tch12220-24611/02/0521:55Pag0225 Keysto the Chapler·225 The text describes a brief history of the story entitled"What's the Matter with HBrIn the radical addition of HBr to an alkene,the reactive species is:Br instead of the Hin ionic additions.So the radical addition alkenes.Notice that both Tonic Addition of HB :Br: CHCH-cH CH;CHCH, ·CH,CHCH CH.CHCH. H.Br radica of synthetic transformations that follows includes the reactions of Functional Group Interconversions Alkanes Halogenation Haloalkanes Reduction Lots of other things Ereactions Alkenes Alcohols Carbonyl compounds Lots of other things 12-14and12-15. Dimeri ahorO5eamerteation,and These are addition reactions of carbocations,carbanions,or radicals with alkenes.In each case the addition thas pro or Ro恤 can go on
Keys to the Chapter • 225 The text describes a brief history of the story entitled “What’s the Matter with HBr?” In the radical addition of HBr to an alkene, the reactive species is instead of the H in ionic additions. So the radical addition appears to “turn around” the direction of reaction to unsymmetrically substituted alkenes. Notice that both the radical and the ionic additions regiospecifically give the most stable intermediate. Ionic Addition of HBr For radical chain reactions to be kinetically feasible, both propagation steps must have relatively low activation barriers. Such is the case for addition of HBr, but not for HCl or HI. Only HBr addition “turns around” in the presence of peroxides. HCl and HI additions remain ionic, with Markovnikov orientation, whether peroxides are present or not! As promised earlier, the updated chart of synthetic transformations that follows includes the reactions of alkenes you have just seen. Functional Group Interconversions 12-14 and 12-15. Dimerization, Oligomerization, and Polymerization of Alkenes These are addition reactions of carbocations, carbanions, or radicals with alkenes. In each case the addition gives a new carbocation, carbanion, or radical (as the case may be), which can add to another alkene molecule. This process can go on and on, making the molecule bigger and bigger: a polymer. Polymers can contain units that are all identical (like Teflon, which is a polymer of CF2PCF2) or can contain two or more different monomer units (the original Saran Wrap was a copolymer of CH2PCHCl and CH2PCCl2). SN reactions Hydrogenation reactions Addition Halogenation Reduction E reactions Addition reactions E reactions Addition reactions SN reactions SN reactions Alkanes Haloalkanes Alkenes Alcohols Lots of other things Lots of other things Carbonyl compounds reactions Oxidation reactions Reduction Radical addition of HBr (Requires presence of initiators such as peroxides or UV radiation.) CH3CH CH3CHCH3 CH2 H 2 carbocation CH3CHCH3 Br Only product (Markovnikov) Br 2 radical CH3CH CH CH2 3CHCH2Br Br CH3CH2CH2Br Only product (anti-Markovnikov) H Br Br 1559T_ch12_220-246 11/02/05 21:55 Page 225
1559T_ch12_220-24611/02/0521:55Pa9e226 ⊕ EQA 226.Chapter 12 REACTIONS OF ALKENES Solutions to Problems 27.Careful!Use DHP for CH:CH-X.not CH-X.from Table 3-1. 45 kcal mol- b)CH4+F→1-CH,CH2-F:△P°=65+67-(56+11l)=-35 keal mol- (e)C2H IBr-I-CH2CH2-Br.AH=65+43-(56+70)=-18 kcal mol-1 (d)C2H+HF-H-CHCH2-F;AF =65 135-(101 111)=-12 keal mol- (e)CH4+H→H-CH,CH2-士△r°=65+71-(101+56)=-21 kcal mol-1 (f)C2H HOCI. HO-CH2CH2-Cl:AH =65+60-(94+84)=-53 kcal mol- (g)C2H+BrCN-Br-CHaCHa-CN:AF=65+83-(70+124)=-46 kcal mol-1 h)CH4+CHSH→CHS-CH,CH-H△H°=65+88-(60+10l)=-8 kcal mol ows that the process to complex o the caofemm (blow. othe ith d H.c HC CHs 29.In all cases identify the face of the double bond that can complex to the catalyst surface with the least steric interference.Add H to that side of the molecule. a)@ H (circled)adds from the side opposite the bulky (CH)CH group (CH3)CH H (b) Hydrogenation occurs opposite the methyl group. H的
Solutions to Problems 27. Careful! Use DH° for CH3CH2OX, not CH3OX, from Table 3-1. (a) C2H4 Cl2 88n ClOCH2CH2OCl Heat in: 65 58 Heat out: 2 84 H° 65 58 (2 84) 45 kcal mol1 (b) C2H4 IF 88n IOCH2CH2OF; H° 65 67 (56 111) 35 kcal mol1 (c) C2H4 IBr 88n IOCH2CH2OBr; H° 65 43 (56 70) 18 kcal mol1 (d) C2H4 HF 88n HOCH2CH2OF; H° 65 135 (101 111) 12 kcal mol1 (e) C2H4 HI 88n HOCH2CH2OI; H° 65 71 (101 56) 21 kcal mol1 (f ) C2H4 HOCl 88n HOOCH2CH2OCl; H° 65 60 (94 84) 53 kcal mol1 (g) C2H4 BrCN 88n BrOCH2CH2OCN; H° 65 83 (70 124) 46 kcal mol1 (h) C2H4 CH3SH 88n CH3SOCH2CH2OH; H° 65 88 (60 101) 8 kcal mol1 28. The structure of the product shows that the process leads to attachment of two hydrogen atoms to the face of the double bond opposite to the cyclopropane ring. Thus we can infer that the starting cyclic alkene is able to complex to the catalyst surface from only one face, the face opposite to the fused three-membered ring (below, left). The steric bulk of the cyclopropane makes approach of the other face of the alkene very difficult (below, right), interfering with complexation and leading to high stereoselectivity in the hydrogenation process. 29. In all cases identify the face of the double bond that can complex to the catalyst surface with the least steric interference. Add H2 to that side of the molecule. (a) H2 (circled) adds from the side opposite the bulky (CH3)2CH group. (b) Hydrogenation occurs opposite the methyl group. H CH3 H H H H CH3 (CH3)2CH H H CH3 H3C Complexation possible H3C catalyst surface catalyst surface CH3 CH3 H3C Complexation difficult 226 • Chapter 12 REACTIONS OF ALKENES 1559T_ch12_220-246 11/02/05 21:55 Page 226
1559r.ah12.220-24611/02/0521:55Page227 Solutions o Problems227 c nation from the more exposed (bottom)side of the folded 30.Mor 3 90° 19).The greater strain in cyclobutene tane,thus resulting in greater energy release upor 31 (i)Peroxide-free HBr (i HBr+ (Markovnikov addition) (anti-Markovnikov addition) (a)2-Bromohexane 1-Bromohexane (b)2-Bromo-2-methylpentane 1-Bromo-2-methylpentane (c)2-Bromo-2-methylpentane 3-Bromo-2-methylpentane (d)3-Bromohexane 3-Bromohexane All chiral products are formed as racemic mixtures. 32.(a)1.2-Dibromohexane (b)1.2-Dibromo-2-methylpentanc (c)2.3-Dibromo-2-methylpentanc Br H H Br Br H R一HBr Br R-= R Br Racemic mixture RC-C Meso compou (e)trans-1.2-Dibromocyclohexan All products in this problem (except for the meso compound)are chiral:all are racemic
Solutions to Problems • 227 (c) Hydrogenation occurs from the more exposed (bottom) side of the folded molecule. 30. More exothermic. There is essentially no bond angle strain in either cyclohexane or cyclohexene. Heat of hydrogenation of the latter is essentially the same as that for an acyclic cis-disubstituted alkene. Both cyclobutane and cyclobutene are strained, but bond angle compression is greater in the alkene (120° 90° 30°) compared with the alkane (109° 90° 19°). The greater strain in cyclobutene increases the energy difference between it and cyclobutane, thus resulting in greater energy release upon hydrogenation. 31. (i) Peroxide-free HBr (ii) HBr peroxides (Markovnikov addition) (anti-Markovnikov addition) (a) 2-Bromohexane 1-Bromohexane (b) 2-Bromo-2-methylpentane 1-Bromo-2-methylpentane (c) 2-Bromo-2-methylpentane 3-Bromo-2-methylpentane (d) 3-Bromohexane 3-Bromohexane (e) Bromocyclohexane Bromocyclohexane All chiral products are formed as racemic mixtures. 32. (a) 1,2-Dibromohexane (b) 1,2-Dibromo-2-methylpentane (c) 2,3-Dibromo-2-methylpentane (d) (R,R) and (S,S)-3,4-Dibromohexane. Anti addition to a cis compound gives a racemic mixture of chiral products; a trans substrate gives the meso isomer. (e) trans-1,2-Dibromocyclohexane All products in this problem (except for the meso compound) are chiral; all are racemic. Br Br H R R H C C Br Br Br H H Racemic mixture Meso compound R R R C C Br Br H R H R C C Br Br H H R R C C Br Br H H R R C C Br Br H H R C C R Br H H R C C H CH2 H H H 1559T_ch12_220-246 11/02/05 21:55 Page 227
1559T_ch12_220-24611/02/0521:55Pa9e228 EQA 228.Chapter 12 REACTIONS OF ALKENES 33. (a)2-Hexanol 1-Hexanol (b)2-Methy1-2-pentanol 2-Methyl-1-pentanol (c)2-Methyl-2-pentanol 2-Methyl-3-pentanol (d)3-Hexanol 3-Hexanol (e)Cyclohexanol Cyclohexanol Oxymercuration-dem theare not paricularly prone to rearrangememt roducts 34.(a)Hot,concentrated H2SO (b)Cold,aqueous H2SO (c)NaOCH,CH in CHCH,OH (d)HCI in CCl Additions [reactions(b)and(d)]are normally favored by thermodynamics(Section 12-1).For ave to b the No good nucleophiles are present:therefore.the carbocation under goes loss of a on the double bond has an equal chance of occurring from the top and bottom faces of the double bond. H Co.CH,o (CH) H。H O.CHs (CHs)CH H HO CO:CH+ (CHa)CH Br 253S 2R,3R (CH)2CH Co.c,(CH).c H H Co.CH,+(CH).CH Br cis 253 2R3 36.All chiral products are formed as racemic mixtures CH2CH3 CH2CH3 (b)H- -CI CI- -H Racemi H mixture CH-CH-CHs CH.CH2CH
33. H2SO4 H2O BH3, THF; then NaOH, H2O2 (Markovnikov hydration) (anti-Markovnikov hydration) (a) 2-Hexanol 1-Hexanol (b) 2-Methyl-2-pentanol 2-Methyl-1-pentanol (c) 2-Methyl-2-pentanol 2-Methyl-3-pentanol (d) 3-Hexanol 3-Hexanol (e) Cyclohexanol Cyclohexanol Oxymercuration–demercuration gives the same products as aqueous sulfuric acid. The carbocations formed from these substrates and H are not particularly prone to rearrangement. All chiral products are formed as racemic mixtures. 34. (a) Hot, concentrated H2SO4 (b) Cold, aqueous H2SO4 (c) NaOCH2CH3 in CH3CH2OH (d) HCl in CCl4 Additions [reactions (b) and (d)] are normally favored by thermodynamics (Section 12-1). For elimination to occur, conditions have to be established to drive the equilibria the opposite way. In (a) the water lost in the reversible E1 process is protonated by the concentrated H2SO4, removing it from the equilibrium. No good nucleophiles are present; therefore, the carbocation undergoes loss of a proton to form the alkene. In (c) the strongly basic ethoxide ion induces bimolecular elimination and neutralizes the liberated HCl, forming ethanol and NaCl. No species electrophilic enough to add to the alkene are present in the reaction mixture. 35. The transformation proceeds through anti addition. The possible products arising from addition to the trans and the cis isomers of the starting compound are shown below. Addition to the trans isomer gives the required 2S,3S isomer, but as a racemic mixture with its 2R,3R enantiomer, because initial attack by bromine on the double bond has an equal chance of occurring from the top and bottom faces of the double bond. 36. All chiral products are formed as racemic mixtures. (a) (b) CH2CH2CH3 CH2CH3 Cl H Cl H Racemic mixture CH2CH2CH3 CH2CH3 Cl Cl H H CH2CH2CH3 Cl HO H CO2CH3 CO2CH3 (CH3)2CH (CH3)2CH H H H S S Br (CH3)2CH CO2CH3 R R Br HO H H H trans 2S,3S 2R,3R Br2, H2O HO H CO2CH3 (CH3)2CH (CH3)2CH CO2CH3 H H H S R Br (CH3)2CH CO2CH3 S R Br HO H cis 2S,3R 2R,3S Br2, H2O 228 • Chapter 12 REACTIONS OF ALKENES 1559T_ch12_220-246 11/02/05 21:55 Page 228