15597.ch09.148-17410/30/0518:03Page149 EQA 9 Further Reactions of Alcohols and the Chemistry of Ethers sis might look like this. C-C=0 H C=C (Breaks 3) is also esented.Because these lack an oxygen hvdrogen bond.the two reactions of alcohols that involve the O-HE d are nc eactions turn out to be important in th e in their usefulness as solvents for a wide variety of reactions in organic chemistry. 148
148 9 Further Reactions of Alcohols and the Chemistry of Ethers In this chapter we explore the reactions of alcohols in detail. In the introduction to the previous chapter in this study guide, we briefly compared the reactions of alcohols with those of haloalkanes. A more detailed analysis might look like this: Indeed, any of five bonds may be involved in alcohol chemistry, and if we treat substitution and elimination separately, a total of four types of reactions are possible, as shown. A related class of compounds—ethers— is also presented. Because these lack an oxygen–hydrogen bond, the two reactions of alcohols that involve the OOH bond are not available to ethers. In fact, only substitution reactions turn out to be important in the chemistry of ethers, and those occur only under certain sets of conditions, depending on the nature of the ether. By and large, ethers, in contrast with alcohols, have been found to be very unreactive molecules, which results in their usefulness as solvents for a wide variety of reactions in organic chemistry. H H C C Y C C Substitution Elimination H H H C C O 1 2 3 5 4 H H R C C O H C C O H (Breaks bonds 4 and 5; “doubles” 3) (Breaks 1 and 3; “doubles” 2) (Breaks 4) (Breaks 3) Oxidation Replacement of hydroxy H 1559T_ch09_148-174 10/30/05 18:03 Page 148
1559T_ch09_148-17410/30/0518:03Pa9e149 ⊕ EQA Keys to the Chopler·149 Outline of the Chapter 9-1 Preparation of Alkoxides A selection of methods for deprotonation of alcohols. 92AhoanmbeistbsmieaadnoicnaecinetnodAcohob 93 Carbocation Rearrangements new reaction pathway for carbocation 9-4 Esters from Alcohols A brief overview of esters and their synthetic uses. 95,96,ond97heeomdeaiemictEhe 9-8 anyway 9-9 Reactions of Oxacyclopropanes Putting a normally unreactive functional group into a strained ring 9-10 Sulfur Analogs of Alcohols and Ethers The parallels between oxygen and sulfur compounds. 9-11 Physiological Properties and Uses of Alcohols and Ethers Keys to the Chapter Preparation of Alkoxides cohols rese Her ,gAas r hydride)and re s chapter m lons:Substitution and Elimingtion Reactions of Alcohols acid-base story for alcohols is their basicity:Just like water.they can be protonated by alcohols, neutral or basic conditions.and a comparison with haloalkanes shows why:Haloalkanes already possess a good leaving group (halide ion),whereas alcohols do not.For instance.compare the following reactions. Nuc:+R-R-Nuc+:Good leaving group Nuc:+R-OH-R-Nuc HO:Bad leaving group Alcohols require improvement of their leaving group before they can become substrates in substitution ad1leaconmon e oxygen atom w Br).Thg aci processes are halide ions,which form haloalkanes,and other molecules of alcohol,which form ethers.As
Keys to the Chapter • 149 Outline of the Chapter 9-1 Preparation of Alkoxides A selection of methods for deprotonation of alcohols. 9-2 Alkyloxonium Ions: Substitution and Elimination Reactions of Alcohols A section describing the conversion of the alcohol OH into a leaving group. 9-3 Carbocation Rearrangements A new reaction pathway for carbocations. 9-4 Esters from Alcohols A brief overview of esters and their synthetic uses. 9-5, 9-6, and 9-7 Properties and Preparations of Ethers The alcohol oxygen as a nucleophile. 9-8 Reactions of Ethers What little there is, anyway. 9-9 Reactions of Oxacyclopropanes Putting a normally unreactive functional group into a strained ring. 9-10 Sulfur Analogs of Alcohols and Ethers The parallels between oxygen and sulfur compounds. 9-11 Physiological Properties and Uses of Alcohols and Ethers Keys to the Chapter 9-1. Preparation of Alkoxides We have already seen how the acidity of alcohols resembles the acidity of water. Here two general approaches are presented for the removal of a proton from an alcohol to form an alkoxide ion: reaction with strong bases (such as [(CH3)2CH]2N or hydride) and reaction with active metals (especially alkali metals). Alkoxides are readily available species whose reactions will be explored at several places in this chapter. 9-2. Alkyloxonium Ions: Substitution and Elimination Reactions of Alcohols The other side of the acid-base story for alcohols is their basicity: Just like water, they can be protonated by strong acid, making alkyloxonium ions. These turn out to be very important in the chemistry of alcohols, because they allow reactions that involve cleavage of the carbon–oxygen bond. This bond is hard to break under neutral or basic conditions, and a comparison with haloalkanes shows why: Haloalkanes already possess a good leaving group (halide ion), whereas alcohols do not. For instance, compare the following reactions. Alcohols require improvement of their leaving group before they can become substrates in substitution reactions. The most common way to do this is protonation of the oxygen atom with strong acid. This reaction converts a bad leaving group (HO) into a good one (H2O, about as good as Br). Then, 1° alcohols can undergo SN2 reactions, and 2° and 3° alcohols can undergo SN1 reactions. Common nucleophiles in these processes are halide ions, which form haloalkanes, and other molecules of alcohol, which form ethers. As Nuc Good leaving group R X R Nuc X Nuc Bad leaving group R OH R Nuc HO 1559T_ch09_148-174 10/30/05 18:03 Page 149
1559Tch09148-17411/03/0519:36Page150 EQA 150.chapter9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS 9-3.Carbocation Rearrangements o far you have en two reaction ation with a mucleophile (the step of th After al.carbocations are ery reactive specics.and they vill do just about anything to find sources o electrons.I can even att specting atom ir ow ecule,mov atoms or g ups from one place in a molecule to another are.The most common kind of rearrangement is that shown in this text section:shift of a hydride (H:)or an alkyl group from one a ue br .Other common types of shifts tum 2 ions into new 2 ions and 3 ions into nev on e formed,r n the 1 mg6.cthouhthgdntundeompkomiaimoPioasAaliaofmptsoi 1.2°→2°via hydride shif CH,CHCHCH=CH,CHCHCH3 Readily reversible H 2a.2°→3°via hydride shif (CH3)CCHCH3 (CH3)CCHCH; 2b.2°→3 via alkyl shif (CH3)CCHCHs-(CHs)CCHCH3 interconversion:see next example 3a.3→3 via hydride shift (CHa)2CC(CHa)2-(CH3)zCC(CHa)Reversible H 36b.3°→3°via alkyl shi0 (CH3)2CC(CH3)=(CH3)2CC(CH3)2 Reversible CH: 由
150 • Chapter 9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS always, eliminations can compete with these substitutions, especially at high temperatures, and alkenes are the products of the very important acid-catalyzed dehydration of alcohols. 9-3. Carbocation Rearrangements So far you have seen two reactions of carbocations: combination with a nucleophile (the second step of the SN1 process) and loss of a proton (the second step of the E1 process). There are more, as you might expect. After all, carbocations are very reactive species, and they will do just about anything to find sources of electrons. They can even attack unsuspecting atoms or groups in their own molecule, moving the atom or group together with its bonding electrons, from its original location to the positively charged carbon. Such shifts of atoms or groups from one place in a molecule to another are called rearrangements. The most common kind of rearrangement is that shown in this text section: shift of a hydride (H: ) or an alkyl group from one atom to another, with the electrons of the breaking bond, to generate a more stable carbocation from a less stable one. The most typical example is a rearrangement that changes a 2° carbocation into a 3° one, a thermodynamically favorable process. Other common types of shifts turn 2° ions into new 2° ions and 3° ions into new 3° ions. All these are liable to occur whenever “rearrangeable” carbocations are formed, namely, in the first steps of SN1 or E1 reactions of appropriately constructed molecules. In addition, protonated 1° alcohols like 2,2-dimethyl-1-propanol can sometimes change directly to 2° or 3° carbocations via simultaneous ionization and rearrangement, even though they don’t undergo simple ionization to 1° ions. A short list of examples of the main types follows. 1. 2° n 2° via hydride shift 2a. 2° n 3° via hydride shift 2b. 2° n 3° via alkyl shift 3a. 3° n 3° via hydride shift 3b. 3° n 3° via alkyl shift (CH3)2CC(CH3)2 CH3 (CH3)2CC(CH3)2 CH3 Reversible (CH3)2CC(CH3)2 H (CH3)2CC(CH3)2 H Reversible (CH3)2CCHCH3 CH3 (CH3)2CCHCH3 CH3 Normally not reversible; but product ion can undergo 3 3 interconversion; see next example (CH3)2CCHCH3 H (CH3)2CCHCH3 H Reversible but normally favored in direction shown CH3CHCHCH3 H CH3CHCHCH3 H Readily reversible 1559T_ch09_148-174 11/03/05 19:36 Page 150
1559T_ch09_148-17410/30/0518:03Pa9e151 EQA Keys to the Chapler 151 4."1"→2°via hydride shif H CH,CHCH-TOH一CH,HC 5a.“/o"→3 via hydride shift (CH)CCH2TOH2(CHa)CCH2 5b.“1"→3°via alkyl shif ana一an Notice that in every example of carbocation rearrangement the migrating atom (or group)and the(+)charge switch places of the of changing the number of atoms in theing Heres example showing how a secondary cycloheptyl cation can rearrange to become tertiary in two ways. CH CH Migrates to give productA 一Min用 Product A B Product B Methyl migration to give A is no different from alkyl shifts you've seen already.To understand the result The bond between the CHa and the ca is B.Now if you count the number of atoms in this new.funny-lookinrin it turns out to be six So then cations that can form when the compound shown in the marin is treated with str ng acid
Keys to the Chapter • 151 4. “1°” n 2° via hydride shift 5a. “1°” n 3° via hydride shift 5b. “1°” n 3° via alkyl shift Notice that in every example of carbocation rearrangement the migrating atom (or group) and the () charge switch places. In some of the problems you will be asked to find products of rearrangement of carbocations in ring compounds. One of the hardest types of shifts to visualize at first is alkyl migration when the alkyl group is part of a ring. This ring-bond migration has the effect of changing the number of atoms in the ring. Here is an example showing how a secondary cycloheptyl cation can rearrange to become tertiary in two ways. Methyl migration to give A is no different from alkyl shifts you’ve seen already. To understand the result of migration of the ring CH2 group, follow the bonding change: The bond between the CH2 and the carbon with the methyls breaks, and the CH2 forms a new bond to the original carbocation carbon. The result is B. Now if you count the number of atoms in this new, funny-looking ring, it turns out to be six. So then redraw it properly like a normal six-membered ring (shown above). Part of the driving force for this ring-bond shift is the formation of a less strained ring. Now see if you can solve this practice problem: Write the carbocations that can form when the compound shown in the margin is treated with strong acid. Finally note that all of these rearranged carbocations can either combine with a nucleophile to give a substitution product or lose a proton to give an alkene (elimination) just like any other carbocation. CH2 CH3 CH3 CH3 CH3 CH2 CH3 CH3 Migrates to give product A Migrates to give product B (ring-bond migration) Product A A CH2 CH3 CH3 Product B B (CH3)2CCH2 CH3 (CH3)2CCH2 CH3 OH2 (CH3)2CCH2 OH2 H (CH3)2CCH2 H CH3CHCH2 H CH3CHCH2 H OH2 H3C CHCH3 OH 1559T_ch09_148-174 10/30/05 18:03 Page 151
1559r.ah09.148-17410/30/0518:03Page152 EQA 152.chapter9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS 9-4.Esters from Alcohols The re their chemistry will be explored in detail in several places during the last third of the course. be often superior to the more"assical"method involving acid-catalyzed substitution.Whereas the latter is rast. With these reactions and the other reactions so far de interconversions first presented in Chapter 8 has grown to look like this: Functional Group Interconversions Reduction Haloalkanes Ss reactions Lots of other things Alkenes Alcohols Carbonyl compounds 9-6 and 9-7. Synthesis of Ethe strongly basic and unhindered-and give excellent results in S2 reactions with both methyl and I°halides(fourth olumn,rows I and 2).Thes are the prototypical Willia nson ether syntheses, g(CH at th or(CH (refer to the this study guid Obviously.3 halides are worthl ics and stereochemistr plyocoetotheesbsiionandehonceg ske water (sce s column in"Majo r reac conditions with 2 and 3 halides.Typical examples of cach are presented
152 • Chapter 9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS 9-4. Esters from Alcohols The reversible reaction of alcohols and carboxylic acids to make organic esters is presented here only to alert you to the major connection alcohols have with esters. Esters are the most common and most important carboxylic acid derivatives, and their chemistry will be explored in detail in several places during the last third of the course. Inorganic esters serve useful purposes as synthetic intermediates for certain functional group interconversions. Here, alternative ways to transform alcohols into haloalkanes using these compounds are shown to be often superior to the more “classical” method involving acid-catalyzed substitution. Whereas the latter is frequently susceptible to rearrangement, the phosphorus and sulfur reagents presented here can often allow substitutions to occur without having rearrangements interfere with the course of the reaction. This is most noticeable with 2° alcohols. Upon protonation of a 2° alcohol, SN1 reactivity (i.e., carbocation chemistry) predominates. In contrast, the leaving groups of inorganic esters derived from 2° alcohols exhibit a more moderate and well-behaved SN2 reactivity. This can be very useful. With these reactions and the other reactions so far described in this chapter, the chart of functional group interconversions first presented in Chapter 8 has grown to look like this: 9-6 and 9-7. Synthesis of Ethers In Chapters 6 and 7 we saw examples of both substitution and elimination reactions involving alcohols and alkoxides in reactions with haloalkanes and related compounds. For general calibration purposes, please refer now to the Summary Chart in the last “Keys to the Chapter” section of Chapter 7 of this study guide (“Major reactions of haloalkanes with nucleophiles”). Alkoxides derived from smaller alcohols are comparable to hydroxide—strongly basic and unhindered—and give excellent results in SN2 reactions with both methyl and 1° halides (fourth column, rows 1 and 2). These are the prototypical Williamson ether syntheses, and several are illustrated in the text. Increased bulk in either the alkoxide [e.g., (CH3)2CHO or (CH3)3CO] or the haloalkane (branched 1°, 2°, etc.) tends to increase the amount of E2 reaction at the expense of SN2 chemistry (refer to the “three questions” for favoring elimination or substitution, also in Chapter 7 of the text and also this study guide). Obviously, 3° halides are worthless in the Williamson ether synthesis because they give only elimination products upon reaction with the strongly basic alkoxide reagent. Normal considerations of kinetics and stereochemistry apply, of course, to these substitution and elimination processes. In contrast with alkoxides, alcohols are poor nucleophiles, like water (see second column in “Major reactions of haloalkanes with nucleophiles” chart). However, alcohols can act as nucleophiles to make ethers in either of two ways: strongly acidic conditions when no other nucleophiles are present, and solvolytic (SN1) conditions with 2° and 3° halides. Typical examples of each are presented. Alkanes Haloalkanes Alkenes Alcohols Carbonyl compounds Lots of other things Functional Group Interconversions Halogenation Reduction E reactions E reactions SN reactions SN reactions SN reactions Reduction reactions Oxidation reactions 1559T_ch09_148-174 10/30/05 18:03 Page 152
1559T_ch09_148-17411/03/0518:56Pa9e15 EQA Keys to the Chopler·153 of alcohols gs8 Rearrangements? uncommon common common common 9-8.Reactions of Ethers As mentioned in the introduction to this study guide chapter,the chemistry of ethers is very limited,showing ard nuc displacemen reactivity only under fairly 21 As is the cas a strong acid.Then reaction can occur with a good nucleophile Nue-+R-R R-Nuc +H-6-R Good leaving group 一 CH-6-cH,soCH一-CH, CH3-O-CH,+CH;CH2OH (Not CH:CH2-0-CH3) For the s.nucleophilic ether cleay es are limited to good nucleophiles that are weakly basic like Brand which can exist in the presence of strong acid.(If you look back now at the reactions of alcohols you'll see the same onsiderations applying g there.too.)Methyl and alkyl ethers reactv worse thanfor S the latter mechanism is more typical,however)
Keys to the Chapter • 153 This section concludes the coverage of alcohol chemistry for now. A summary chart concerning the various conditions for substitution and elimination reactions follows. SUMMARY CHART Substitution and elimination reactions of alcohols Substitution Strong acid Strong acid with poor nucleophile, via inorganic with good e.g., H2SO4, in alcohol as solvent ester, e.g., nucleophile, Type of RSO2Cl, e.g., conc. Lower Higher alcohol then IE HI temperatures temperatures Methyl SN2 SN2 SN2 SN2 1° SN2 SN2 SN2 E2 2° SN2 SN1 SN1 E1 3° SN1 SN1 SN1 E1 Rearrangements? uncommon common common common 9-8. Reactions of Ethers As mentioned in the introduction to this study guide chapter, the chemistry of ethers is very limited, showing a tendency toward nucleophilic displacement reactivity only under fairly special conditions. As is the case with alcohols, for any kind of nucleophilic displacement to occur to an ether (SN1 or SN2), the leaving group (alkoxide in this case) has to be improved. This improvement is again done most simply by protonation with a strong acid. Then reaction can occur with a good nucleophile. Good leaving group Notice that the nucleophile in such a reaction cannot ever be a strong base! A strong base cannot be present together with the strong acid needed to protonate the ether: They would just neutralize each other. Addition of a strongly basic nucleophile to an already protonated ether is also no good. All that would happen would be loss of the proton from the protonated ether to the base; no nucleophilic displacement would occur. For these reasons, nucleophilic ether cleavages are limited to good nucleophiles that are weakly basic like Br and I, which can exist in the presence of strong acid. (If you look back now at the reactions of alcohols, you’ll see the same considerations applying there, too.) Methyl and 1° alkyl ethers react via the SN2 mechanism, whereas 3° ethers follow an SN1 pathway. Least reactive are 2° ethers (worse than 1° for SN2, and worse than 3° for SN1; the latter mechanism is more typical, however). 9-9. Reactions of Oxacyclopropanes Strained cyclic ethers (e.g., oxacyclopropanes) react with acid like ordinary ethers do, only faster. Order of CH3 CH3 CH3 O CH3 O CH3) H Then add CH3CH2ONa H2SO4 (Basic!) O CH3 O CH3 (Acidic) CH3CH2OH (Not CH3CH2 O Nuc R O R R Nuc H R O H 1559T_ch09_148-174 11/03/05 18:56 Page 153
1559Tch09148-17410/30/0518:03Page154 154.chapter9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS reactivity is again31.At a 1 carbon,the reaction clearly takes place via an Sx2 mechanism to displace the protonated oxygen. -CH2-CH2 At 2 and 3 carbons,the reaction may be described as an "SN2-like SyI reaction."To clarify this,let's look at three ways to draw the Lewis structure of protonated trimethyloxacyclopropane. carbo H CH: H LC-0 H CH, H CH; H C ated molecule.It may look odd to e resonanc carbocations). H H-Cc Nuc -CH; Likely resonance hybrid Reaction of this protonated molecule with a nucleophile will therefore occur at the 3 carbon,which is the beca oxygccubocation-like.whichiscast partially n owe expect for an S2 reaction (see illustration above).For these reasons.the Sv1 and S2 labels really don't
154 • Chapter 9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS reactivity is again 3° 2° 1°. At a 1° carbon, the reaction clearly takes place via an SN2 mechanism to displace the protonated oxygen. At 2° and 3° carbons, the reaction may be described as an “SN2-like SN1 reaction.” To clarify this, let’s look at three ways to draw the Lewis structure of protonated trimethyloxacyclopropane. These are actually three resonance forms of the protonated molecule. It may look odd to draw resonance forms where a whole single bond is missing, but such pictures (“no-bond structures”) are useful in some cases, provided you recognize that the individual forms are not real and that only the intermediate resonance hybrid really counts. In the case above, the resonance hybrid will probably look more like the alkyloxonium and 3° carbocation structures than the 2° carbocation structure (because 2° carbocations are worse than 3° carbocations). Reaction of this protonated molecule with a nucleophile will therefore occur at the 3° carbon, which is the most carbocation-like, as you would expect for an SN1 reaction. However, because of the position of the oxygen leaving group, which is at least partially bonded to the 3° carbon, the nucleophile can attach to the 3° carbon only from the side opposite the oxygen, resulting in inversion at that carbon atom, as you would expect for an SN2 reaction (see illustration above). For these reasons, the SN1 and SN2 labels really don’t H O H C C CH3 CH3 CH3 Likely resonance hybrid Nuc Nuc H H C C O CH3 CH3 CH3 O H Trimethyloxacyclopropane H C C CH3 CH3 CH3 Alkyloxonium ion H O H C C CH3 CH3 CH3 3 Carbocation H O H C C CH3 CH3 CH3 2 Carbocation 2 carbon 3 carbon CH2 CH2 HO H O CH2 CH2 O H CH2 CH2 Cl Cl 1559T_ch09_148-174 10/30/05 18:03 Page 154
1559T_ch09_148-17410/30/0518:03Pa9e155 ⊕ EQA Solutions to Problems.155 which C-bond breaks,but the direction of also react with basic nuc .This is an S2 process that displaces an alkoxide. nng str the energy content sucn t tively charge tucpisctioe to small-ring ctbers brea ote Small E. *0 c+CH,一CH :-CHz-CHz-O Nue-CH3 +O-CHs Reaction coordinate- 9-10.Sulfur Analogs of Alcohols and Ethers This short section expands on the obvious parallels between oxygen and sulfur that arise as a result of their relationship in the periodic table.As you saw earlier.the larger sized atoms are more nucleophilic.bu ess ba cludes a variety of oxidized species.Common examples are SO2 and HSO.New systems introduced here in- clude sulfonic acids(RSO3H).sulfoxides(RSOR).and sulfones(RSO2R). Solutions to Problems 25.Equilibrium always lies on the side with the weaker acid-base pai (a)Left;(b)left;(c)right:(d)right. 26.(a)CH,CH,CH,I (b)(CH)CHCH,CH,Br (Both by S2 mechanisms) (d)(CH CH)CCl (Both by Sxl mechanisms)
Solutions to Problems • 155 apply in a clear-cut way: SN1 considerations determine which COO bond breaks, but the direction of approach of the nucleophile (back-side attack) is characteristic of an SN2 process. Strained cyclic ethers also react with basic nucleophiles. This is an SN2 process that displaces an alkoxide, which is a very bad leaving group. The nucleophile has to be a good one because the leaving group (a negatively charged alkoxide ion) is a terrible one. The reaction follows the SN2 reactivity order of 1° 2° 3°. Normally alkoxides cannot be displaced in SN2 reactions. In oxacyclopropanes, however, ring strain raises the energy content such that suitably reactive nucleophiles can displace negatively charged oxygen leaving groups (see graph, below). The only reason this reaction occurs at all is that the displacement reaction breaks open a small, strained ring. Please note that this is a reaction unique to small-ring ethers. Unstrained ethers are unreactive toward basic nucleophiles. 9-10. Sulfur Analogs of Alcohols and Ethers This short section expands on the obvious parallels between oxygen and sulfur that arise as a result of their relationship in the periodic table. As you saw earlier, the larger sized atoms are more nucleophilic, but less basic. Thus comparisons of the chemical properties of pairs of species like HS vs. HO, H2S vs. H2O, and CH3SH vs. CH3OH are straightforward. Larger atoms are also more readily oxidized, and sulfur chemistry includes a variety of oxidized species. Common examples are SO2 and H2SO4. New systems introduced here include sulfonic acids (RSO3H), sulfoxides (RSOR), and sulfones (RSO2R). Solutions to Problems 25. Equilibrium always lies on the side with the weaker acid-base pair. (a) Left; (b) left; (c) right; (d) right. 26. (a) CH3CH2CH2I (b) (CH3)2CHCH2CH2Br (Both by SN2 mechanisms) (c) (d) I (CH3CH2)3CCl (Both by SN1 mechanisms) 1559T_ch09_148-174 10/30/05 18:03 Page 155
1559r_eh09_148-17410/30/0518:03Page156 EQA 156.chapter9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS 27.In each case the species are written in an order reflecting a sequence of rearrangement steps. 一st心udrtructres to the h (a)CH,CHCHOH3.CH,CHCHs (Similar to rearrangement of 2.2-dimethyl-1-propanol in Section 9-3. (b)CH CHOH-CH,CH CHCHs (e)CH;CH2CH2CH2OH2.CH,CH2CHCH3 (d)(CH)2CHCH,OH2.(CH)C* (e)(CH)CCHCH,OH,.(CHCCHCH.(CHCCH(CH) Some mechanism arrows are included below to help you find your way CH; H C(CH3)3 CH C(CHa)2 ⊕ (h)CH3 -CH CH CH CHs CH:CH CH:- CH; CHs- CH ons to exist for a long time,because the (a),(b)CH;CH-CH2 (c)CH,CH.CH-CH2.CH,CH-CHCH3(major product) (d)(CH:C=CH2 (e)(CH3)aCCH-CH2.(CH3)2C-C(CH3)2(major product). CH(CH H2C=C CH3
156 • Chapter 9 FURTHER REACTIONS OF ALCOHOLS AND THE CHEMISTRY OF ETHERS 27. In each case the species are written in an order reflecting a sequence of rearrangement steps. Rearrangements do not occur to an equal extent under all circumstances. Structures to the right are generally most stable. (a) CH3CH2CH2 OH2, CH3 CHCH3 (Similar to rearrangement of 2,2-dimethyl-1-propanol in Section 9-3.) (b) CH3CH OH2CH3, CH3 CHCH3 (c) CH3CH2CH2CH2 OH2, CH3CH2 CHCH3 (d) (CH3)2CHCH2 OH2, (CH3)3C (e) (CH3)3CCH2CH2 OH2, (CH3)3C CHCH3, (CH3)2 CCH(CH3)2 Some mechanism arrows are included below to help you find your way. (f ) (g) (h) 28. These conditions favor rearrangements. They allow carbocations to exist for a long time, because the reaction mixtures are strongly acidic and lack decent nucleophiles. (a), (b) CH3CHPCH2 (c) CH3CH2CHPCH2, CH3CHPCHCH3 (major product) (d) (CH3)2CPCH2 (e) (CH3)3CCHPCH2, (CH3)2CPC(CH3)2 (major product), CH3 H2C C CH(CH3)2 CH3 CH3 CH3 CH3 also CH3 CH3 CH3 CH3 CH3 H CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 C(CH3)3 CH3 C(CH3)2 CH3 H CH3 H CH3 1559T_ch09_148-174 10/30/05 18:03 Page 156
1559T_ch09_148-17410/30/0518:03Pa9e157 EQA Solutions to Problems.157 Line formulas will be used for most of the cyclic struct below.Note that methyl groups are understood to be present at the ends of lines even whenCHis not written in. CH. 代七女☆ 心心88 Each product arises from loss of a proton from a carbon adjacent to the positively charged carbon of a structure in the previous problem 29.Rea nditions:The is eaker (HO'rather hero muclie Nome of h pmh (b)CH,CHBrCH3 (e)CH,CH2CH.CH2Br (d)(CH)CHCH,Br (e)(CH)CCH,CH,Br e likely.Products will result fron sent.See answers to Problem 27(f)through 27(h). Na 30.(a) HO: Br
Solutions to Problems • 157 Line formulas will be used for most of the cyclic structures below. Note that methyl groups are understood to be present at the ends of lines even when “CH3” is not written in. (f ) (g) (h) (h) Each product arises from loss of a proton from a carbon adjacent to the positively charged carbon of a structure in the previous problem. 29. Rearrangements are much less likely under these conditions: The acid is much weaker (H3O rather than H2SO4), and there is a good nucleophile around. None of the primary alcohols rearrange. (a) CH3CH2CH2Br (b) CH3CHBrCH3 (c) CH3CH2CH2CH2Br (d) (CH3)2CHCH2Br (e) (CH3)3CCH2CH2Br With secondary or tertiary alcohols, rearrangements become more likely. Products will result from attachment of Br to any positively charged carbon in the carbocations present. See answers to Problem 27(f ) through 27(h). 30. (a) (b) HO H HOH H Br Br Rearrangement from secondary to tertiary O H Na H O Na H2 CH3 CH3 CH3 CH3 C(CH3)3 CH3 CH CH3 2 CH3 CH3 CH3 CH2 (major product) 1559T_ch09_148-174 10/30/05 18:03 Page 157
