1559T_ch12_220-24611/02/0521:55Pa9e220 ⊕ EQA 12 Reactions of Alkenes Alkenes s in the t are electrons in bonds.As a result,they are"available"in a"Lewis base" sense lone pai 黑义of teve adi中 known,and they allov es,inciu ing ones we've seen b an updated Functional Group Interconversions chart is presented in this chapter of the study guide. Outline of the Chapter 12-1 Thermodynamic Feasibility of Addition Reactions 12-2 Hydrogenation of Alkenes 12-3 through 12-7 12-8 ReacandSereosdle s by Hydroboration oup inter ving a 12.9 Diazomethane,Carbenes,and Cyclopropane Synthesis ng u 12-10,12-11,12-12 Oxidation of Alkenes of oxygen-containing electrophiles 12-13 Radical Additions to Alkenes d of mechanism for alkene reactions:radical additions 12-14,12-15 Dimerization,Oligomerization,and Polymerization of Alkenes 12-16 Ethene in Industry 12-17 Alkenes in Nature Keys to the Chapter 12-1.Thermodynamic Feasibility of Addition Reactions bond).but on reactions to are generally exothermi 22 use one
12 Reactions of Alkenes Alkenes are reactive and synthetically useful molecules. Their reactivity originates from the electrons in the bond. They are on the average further away from (and therefore electrostatically less tightly held to) the carbon nuclei than are electrons in bonds. As a result, they are “available” in a “Lewis base” sense, somewhat like the lone pair electrons on the oxygen of water or the nitrogen of ammonia. As you will see, attachment of an electrophile to these basic electrons has the effect of breaking the bond and is the first step in many reactions of alkenes, additions. Many of these addition reactions are known, and they allow conversion of alkenes to other organic molecules, including ones we’ve seen before (haloalkanes and alcohols) as well as new ones. These additions expand the scope of our synthetic possibilities significantly. Therefore, an updated Functional Group Interconversions chart is presented in this chapter of the study guide. Outline of the Chapter 12-1 Thermodynamic Feasibility of Addition Reactions 12-2 Hydrogenation of Alkenes 12-3 through 12-7 Electrophilic Additions to Alkenes The largest group of reactions characteristic of alkenes. 12-8 Regio- and Stereoselective Hydration of Alkenes by Hydroboration Another functional group interconversion involving addition. 12-9 Diazomethane, Carbenes, and Cyclopropane Synthesis Preparing three-membered rings. 12-10, 12-11, 12-12 Oxidation of Alkenes Additions of oxygen-containing electrophiles. 12-13 Radical Additions to Alkenes A new kind of mechanism for alkene reactions: radical additions. 12-14, 12-15 Dimerization, Oligomerization, and Polymerization of Alkenes Reactions of, mainly, industrial use. Or, “where everything in your kitchen that you don’t eat comes from.” 12-16 Ethene in Industry 12-17 Alkenes in Nature Keys to the Chapter 12-1. Thermodynamic Feasibility of Addition Reactions Carbon–carbon bonds are weaker than bonds. Addition reactions to alkenes are generally exothermic because one of the bonds broken is weak (the bond), but both new bonds formed are strong. 220 1559T_ch12_220-246 11/02/05 21:55 Page 220
15597ch12220-24611/02/0521:55Pag0221 EQA Keys to the Chapler·221 C=C+A-B一 △Hr°usualIy negative Bonds broken Bonds formed 12-2.Hydrogenation of Alkenes The simplest reaction of alkenes is the addition of hydrogento form alkanes.Hydro isa non process,unlik (Adams's catalyst),palladium on carbon (Pd-C).or Raney nickel (RaNi). 12-3 through 12-7.Electrophilic Additions to Alkenes othe larg hmtPpncalcdasofaditionreactions.ofalk bonded cabons.reacion forming a cationic intermediate.Combination of this cation with any avaib nucleophile gives the final addition product: From a synthetic point of view,addition reactions allow conversion of alkenes to molecules containing new functional groups at one or both of the original double-bonded carbons.These sections present many such Most of the"A-B"-typ e molecules that particinate in electrophilic addition to alkenes are str such as H Cl.Bra)fall into this category.A hough nonpolar mo if it contained an electr tion mechanism v dd double bonds.The cation then can do any of its usual reactions (e.gattach to a nucleophile or rearrange) Larger electrophiles.especially ones with lone pairs (e.g..":Br:"from Br).add to give positively charged three-mem red nngs (bro monium ior chloronium ion, 12-8.Regio-and Stereoselective Functionalization of Alkenes by Hydroboration
12-2. Hydrogenation of Alkenes The simplest reaction of alkenes is the addition of hydrogen to form alkanes. Hydrogenation is a nonpolar process, unlike many of the addition reactions to be described later, which involve electrophiles attaching to the nucleophilic electrons of the alkene. Hydrogenation reactions require catalysts, such as platinum oxide (Adams’s catalyst), palladium on carbon (Pd-C), or Raney nickel (Ra-Ni). 12-3 through 12-7. Electrophilic Additions to Alkenes In these sections we turn to the largest and most typical class of addition reactions of alkenes. These all proceed in two steps. The first step is attachment of an electrophilic atom to one of the alkene’s doublebonded carbons, a reaction forming a cationic intermediate. Combination of this cation with any available nucleophile gives the final addition product: (1) (2) From a synthetic point of view, addition reactions allow conversion of alkenes to molecules containing new functional groups at one or both of the original double-bonded carbons. These sections present many such examples. Most of the “A–B”-type molecules that participate in electrophilic addition to alkenes are strongly polarized, such as HOCl. Others are not but can still serve as sources of electrophilic atoms. Halogens (Cl2, Br2) fall into this category. Although nonpolar molecules like Br2 do not have permanent dipoles, recall that electron movement can give rise to “fleeting dipoles,” thus allowing the halogen molecule to behave as if it contained an electrophilic atom, namely, “Br”. The details of the addition mechanism vary somewhat with the nature of the electrophile. Thus, protons add regiospecifically to give the more stable of the two possible carbocations in the case of unsymmetrical double bonds. The cation then can do any of its usual reactions (e.g., attach to a nucleophile or rearrange). Larger electrophiles, especially ones with lone pairs (e.g., “)Br >)” from Br2), add to give positively charged three-membered rings (bromonium ion, chloronium ion, etc.). Nucleophiles react with these ions in much the same way that they open the rings of the cyclic alkyloxonium ions of Chapter 9. Addition occurs at the most highly substituted carbon (Markovnikov orientation) in an anti manner. 12-8. Regio- and Stereoselective Functionalization of Alkenes by Hydroboration This section describes the special characteristics and utility of the reaction between an alkene and a borane, a molecule containing BOH bonds. Boranes are electrophilic, and their reactions with alkenes are therefore very reasonable (note carefully, however, that borohydrides like BH4 are very different—they are anions, not electrophiles, and don’t react with alkenes). C A C C C A B B CC C A C A B or something ( similar ( Either as A Bor A B B C C bond (weak) C A B C H usually negative A B Bonds broken Bonds formed bond (strong) 2 bond (strong) Keys to the Chapter • 221 1559T_ch12_220-246 11/02/05 21:55 Page 221
1559T_ch12_220-24611/02/0521:55Pa9e222 EQA 222.Chapter 12 REACTIONS OF ALKENES useful because the new carbon-boron bond can be oxidized by basic HO to give an alcohol. --一-C-0m The chart that follows summarizes the new reactions of Sections 12-3 through 12-8,using 1-methylcyclo- 1.Hydroboration-oxidation(anti-Markovnikov).e.g.. -aH=at0a--a-at CH3 OH CH-C-CH-CH2-OH H02 2.Oxymercuration-demercuration (Markovnikov).e.g. CH3 CH3OH CH-C-CH-CH.CH-c-CH-CH CH3 HgOAc CHsOH :-CH-CHs 3.Acid-catalyzed hydration (Markovnikov,with carbocations that may rearrange).e.g.. CHs CHs CH;- -C-CH-CH2 H, H.O CH,- -CH-CH H CH CH;-C-CH-CHs CH;- CH-CH CH
Borane BOH bonds add across alkenes. The reaction (1) is regioselectively anti-Markovnikov, (2) is stereospecifically a syn addition, (3) goes by a concerted, one-step mechanism, and (4) is synthetically very useful because the new carbon–boron bond can be oxidized by basic H2O2 to give an alcohol. The chart that follows summarizes the new reactions of Sections 12-3 through 12-8, using 1-methylcyclohexene as a substrate. Note the three methods for adding H2O to a double bond, each with its own specific characteristics. 1. Hydroboration–oxidation (anti-Markovnikov), e.g., 2. Oxymercuration–demercuration (Markovnikov), e.g., 3. Acid-catalyzed hydration (Markovnikov, with carbocations that may rearrange), e.g., C CH3 CH3 CH2 CH3 CH H2SO2 H2O C CH CH3 CH3 CH3 CH3 C CH CH3 CH3 CH3 CH3 C CH CH3 CH3 OH CH3 CH3 C CH3 CH3 CH2 CH3 CH Hg(OAc)2 H2O THF NaBH4 NaOH H2O C CH CH3 CH3 CH3 OH HgOAc CH2 OH C CH CH3 CH3 CH3 CH3 C CH3 CH3 CH2 CH3 CH 3 ( ( BH3 THF C CH2 CH3 CH3 CH3 CH2 B NaOH H2O2 C CH2 OH CH3 CH3 CH3 CH2 C B C OH 222 • Chapter 12 REACTIONS OF ALKENES 1559T_ch12_220-246 11/02/05 21:55 Page 222
1559r.ch12.220-24611/02/0521:55Page223 EQA 88音导8 66格说销 ⊕ 223
223 CHART OF EXAMPLES Electrophilic Additions to 1-Methylcyclohexene Typical Regiochemistry Major Example reagent Electrophile Nucleophile Intermediate Stereochemistry Product Hydroboration BH3 B H None anti-Markovnikov syn addition HX addition HCl H Cl Markovnikov Hydration H2SO4, H2O H H2O Markovnikov Halogenation ICl I Cl Haloalcohol Cl2, H2O Cl H2O formation Chlorosulfenylation CH3SCl CH3S Cl Oxymercuration Hg(OCCH3)2, H2O CH3COHg H2O OH CH3 H O HgOCCH3 CH3 O HgOCCH3 Cl CH3 SCH3 H CH3 SCH3 OH CH3 Cl H CH3 Cl Cl CH3 I H CH3 I CH3 OH CH3 CH3 Cl CH3 CH3 H H B(H, R)2 O B O B Markovnikov anti addition Markovnikov anti addition Markovnikov anti addition Markovnikov anti addition 1559T_ch12_220-246 11/02/05 21:55 Page 223
1559T_ch12_220-24611/02/0521:55Pa9e22 EQA 224.Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane ,Carbe sand Cyclopropane Synthesis which have the h-energy spec cannot be synt In thand e a co ang an octet,ca stored but have【 section,look at the a vs in the mecha nism for this process.The electron pair in the bond of the alkene these two bond-forming processes happen together,making this an example of a concerted reaction.one in which multiple happen sim from alkenes these are called ca benoids they act like carbens without carbenes Like carbenes they contan a spcia supplying ar 12-10,12-11,and 12-12.Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes Petch rea bonding changes occur in one step. R808 that adds to give oxacyclopropanes. abo The ing compound 安 cleavage of carbon-carbon b ompoun R 0 R4 12-13.Radical Additions to Alkenes the the radicalatom Unlike additions of positively chard ctophiles how hcaocation but ather radical.Like the procesese studied in Chapter 3.these l chain mechanisms
224 • Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane, Carbenes, and Cyclopropane Synthesis This section introduces an unusual electrophile, the carbene, which has a specific use in synthesis. Carbenes, which have the general structure R2C:, have a neutral but electron-deficient carbon capable of electrophilic additions. Lacking an octet, carbenes are high-energy species that cannot be synthesized and stored but have to be prepared in the presence of whatever substance they are intended to react with. In the presence of alkenes, carbenes react rapidly to form bonds with both alkene carbons, giving a cyclopropane as a result. In the text section, look at the arrows in the mechanism for this process. The electron pair in the bond of the alkene moves to make one bond between one of the alkene carbons and the electrophilic carbene carbon, and the electron pair on the carbene carbon itself moves to make the second bond with the other alkene carbon. Typically these two bond-forming processes happen together, making this an example of a concerted reaction, one in which multiple bonding changes happen simultaneously. Some alternative species are encountered that also make cyclopropanes from alkenes; these are called carbenoids because they act like carbenes without actually being carbenes. Like carbenes they contain a special carbon atom capable of reacting as an electrophile with the bond of the alkene to form one carbon–carbon bond and also supplying an electron pair to form a second carbon–carbon bond. 12-10, 12-11, and 12-12. Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes. Each reaction is an example of a concerted process, in which several bonding changes occur in one step. Peroxycarboxylic acids such as MCPBA contain an electrophilic oxygen O B ROCOOOO OH that adds to give oxacyclopropanes. Osmium tetroxide and ozone both participate in a process where three pairs of electrons move in a circle to simultaneously form new COO bonds at both the double-bonded carbons. The products of these concerted cycloadditions are ring compounds: Notice, for synthetic purposes, that ozonolysis is the first process you’ve seen that can lead to complete cleavage of carbon–carbon bonds. Ozonolysis followed by reduction breaks double bonds, giving two carbonyl compounds: 12-13. Radical Additions to Alkenes Although typical radicals are neutral, they are electron deficient in the sense that they are one electron short of having a full octet. Reaction between the electrons of an alkene double bond can take place, completing the octet around the radical atom. Unlike additions of positively charged electrophiles, however, the result of this addition is not a carbocation but another radical. Like the processes we studied in Chapter 3, these reactions follow radical chain mechanisms. C C R4 R1 R2 R3 1. O3, CH2Cl2 2. Zn, CH3COH O C O O R1 R2 C R4 R3 O O Os O O O C C O O C C 1559T_ch12_220-246 11/02/05 21:55 Page 224
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
15597.eh12.220-24611/0a/052:19Page229 EQA Solutions o Problems229 dCH,cH0 H CH o. 0 (mainly),via g(OCCH) All products are formed as racemic mixtures (+enantiomer) g 风 Note the anti-Markovnikov regiochemistry for hydroboration. 37.A brief analysis of possible choices is provided for each problem oH月 (a)Need Markovnikov hydration of (CH)CHCH-CH or anti-Markovnikov hydration o H OH (CH3)2C=CHCH3.Either can be done. GG-0 CHC-CCCUCCHOC CHCHCH,CICH,CHCHOCHCH)
Solutions to Problems • 229 (c) (d) (e) All products are formed as racemic mixtures. (f ) (g) Note the anti-Markovnikov regiochemistry for hydroboration. 37. A brief analysis of possible choices is provided for each problem. OH H g g (a) Need Markovnikov hydration of (CH3)2CHCHPCH2, or anti-Markovnikov hydration of (b) Need addition to propene of “Cl” and “(CH3)2CHO.” Cl2 is the source of “Cl”, and (CH3)2CHOH provides the nucleophile. Cl2, (CH3)2CHOH solvent CH2 CHCH3 ClCH2CH(CH3)OCH(CH3)2 1. Hg(OAc)2, H2O 2. NaBH4, NaOH, H2O (CH3)2C (CH3)2CHCH (CH3)2CHCHOHCH3 H OH CHCH3. Either can be done. CH2 1. BH3, THF 2. NaOH, H2O2, H2O (CH3)2C CHCH3 (CH3)2CHCHOHCH3 H HO CH3 Blocks top syn addition to bottom CH3 N3 CH3 Br ( enantiomer) H H CH3 OCH3 (mainly), via CH3 CH3 O Blocks top, so Hg attacks bottom Hg(OCCH3) OCH3 CH3CH2 O H CH3CH2 OH Trans, from anti addition Br H 1559T_ch12_220-246 11/04/05 2:19 Page 229
