2 The next several chapters deal with the chemistry of various oxygen-containing functional groups. The interplay of these important classes of compounds alcohols, ethers, aldehydes, ketones, carboxylic acids, and derivatives of carboxylic acids is fundamental to organic chemistry and biochemistry.
3 10.1 ALCOHOLS SOURCES OF ALCOHOLS Until the 1920s, the major source of methanol was as a byproduct in the production of charcoal from wood hence, the name wood alcohol. Now most of methanol used annually is synthetic, prepared by reduction of carbon monoxide with hydrogen. Starting material; solvent; A convenient clean-burning liquid fuel; A colorless, poisonous liquid.
4 When vegetable matter ferments ( 发酵 ), its carbohydrates are converted to ethanol and carbon dioxide by enzymes present in yeast ( 酵母 ). Fermentation of barley produces beer; grapes give wine. Synthetic ethanol is derived from petroleum by hydration of ethylene. Our bodies are reasonably well equipped to metabolize ( 代谢 ) ethanol, making it less dangerous than methanol. Isopropyl alcohol is prepared from petroleum by hydration of propene.
5 All alcohols of four carbons or fewer, as well as most of the five- and six-carbon alcohols and many higher alcohols, are commercially available at low cost.
6 CLASSIFICATION, NOMENCLATURE AND BONDING IN ALOCOHOLS Classification: primary, secondary, tertiary Nomenclature: enol Ex: CH 3 CH 3 HOH 2 CH 2 C CHCH 2 CH 2 CH 2 CH 2 OH H 3 C C C CH 3 CH 2 OH OH OH 3-Hydroxymethyl-1,7-heptanediol 2,3-Dimethyl-2,3-butanediol H 3 C H C CH 2 OH COOH Cl OH OH 2-(3-Hydroxyphenyl)-propanol 4-Chloro-3-hydroxycyclohexanecarboxylic acid
7 Bonding hetrolysis R H C H δ+ δ- C O δ+ H H H weaker acid protonated alcohol oxidation dehydration
8 PHYSICAL PROPERTIES OF ALCOHOLS Boiling point The most striking aspect of the data is the much higher boiling point of ethanol compared with propane. This suggests that the attractive forces in ethanol must be unusually strong. FIGURE Hydrogen bonding in ethanol involves the oxygen of one molecule and the proton of an -OH group of another.
9 Solubility in water Low-molecular weight alcohols (methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all proportions. Their ability to participate in intermolecular hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. FIGURE Hydrogen bonding between molecules of ethanol and water.
10 Higher alcohols become more hydrocarbonlike and less water-soluble. 1-Octanol, for example, dissolves to the extent of only 1 ml in 2000 ml of water. As the alkyl chain gets longer, the hydrophobic effect ( 憎水性 ) becomes more important, to the point that it, more than hydrogen bonding, governs the solubility of alcohols.
12 SPECTROSCOPY IR ν O-H : cm -1 A broad peak at 3300 cm -1 ascribable to O-H stretching of hydrogen-bonded alcohol groups. In dilute solution, where hydrogen bonding is less and individual alcohol molecules are present as well as hydrogen-bonded aggregates, an additional peak appears at approximately 3600 cm -1. ν C-O : cm -1 (primary alcohols: 1060,secondary alcohols: 1100,tertiary alcohols: 1140 cm -1 )
13 1 H NMR The most helpful signals in the NMR spectrum of alcohols result from the hydroxyl proton and the proton in the H-C-O unit of primary and secondary alcohols. The chemical shift of the hydroxyl proton is variable, with a range of ppm, depending on the solvent, the temperature at which the spectrum is recorded, and the concentration of the solution. The alcohol proton shifts to lower field strength in more concentrated solutions.
14 δ4.5 FIGURE The 200-MHz 1 H NMR spectrum of 2-phenylethanol (C 6 H 5 CH 2 CH 2 OH).
15 The reason that splitting of the hydroxyl proton of an alcohol is not observed is that it is involved in rapid exchange reactions with other alcohol molecules. Transfer of a proton from an oxygen of one alcohol molecule to the oxygen of another is quite fast and effectively decouples it from other protons in the molecule. Factors that slow down this exchange of OH protons, such as diluting the solution, lowering the temperature, or increasing the crowding around the OH group, can cause splitting of hydroxyl resonances.
16 (M-18) peak Mass Spectroscopy The molecular ion peak is usually quite small in the mass spectrum of an alcohol. A peak corresponding to loss of water is often evident. Alcohols also fragment readily by a pathway in which the molecular ion loses an alkyl group from the hydroxyl-bearing carbon to form a stable cation. Thus, the mass spectra of most primary alcohols exhibit a prominent peak at m/z 31.
17 CHEMICAL PROPERTIES Acidity and Basicity C 2 H 5 OH + Na C 2 H 5 ONa + H 2 Sodium ethanolate The electronic effects of R groups to acidity of alcohols: Ex: CH 3 OH The inductive effects to acidity of alcohols: Ex: CH 3 pka=15 C 2 H 5 OH pka=16 H 3 C C OH pka 19 CH 2 ClCH 2 OH > CH 3 CH 2 OH CCl 3 CH 2 OH > CHCl 2 CH 2 OH > CH 2 ClCH 2 OH CH 3 Superposition
18 Basicity: Ex 1: C 2 H 5 OH + HBr C 2 H 5 Br + H 2 O + C 2 H 5 OH 2 Accept a proton Ex 2: R OH ZnCl 2 / (BF 3 ) Donate the unshared electron pair to the unoccupied orbital of the central metal
19 Sodium alcoholate are stronger base than sodium hydrate (NaOH), which can introduce RO - groups to the products act as the strong nucleophilic reagents. Synthetic method for sodium methylate: CH 3 OH + NaOH( 固 ) pka=15 benzene CH 3 ONa + H 2 O pka=14 Methanol-bezene-water ternary azeotropic temperature: 64 C (CH 3 ) 3 COK is more basic than (CH 3 ) 3 CONa, which is often used as strongly base other than nucleophilic reagent.
20 REACTIONS OF ALCOHOLS WITH HYDROGEN HALIDES The order of reactivity of the hydrogen halides parallels their acidity: HI > HBr > HCl >> HF. Hydrogen iodide is used infrequently, however, and the reaction of alcohols with hydrogen fluoride is not a useful method for the preparation of alkyl fluorides.
21 Ex: OH + HI Δ I + H 2 O OH + HBr 浓 H 2 SO 4 Br + H 2 O OH 无水 ZnCl 2 + HCl + H 2 O Δ Cl Among the various classes of alcohols, tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive.
22 Tertiary alcohols are converted to alkyl chlorides in high yield within minutes on reaction with hydrogen chloride at room temperature and below.
23 Secondary and primary alcohols do not react with hydrogen chloride at rates fast enough to make the preparation of the corresponding alkyl chlorides a method of practical value. Therefore, the more reactive hydrogen halide HBr is used; even then, elevated temperatures are required in order to increase the rate of reaction.
24 Lucas Reagent: con. HCl and anhydrous ZnCl 2 Ex: H 3 C CH 2 OH CHOH (+) delaminate after a while CH 2 CH 2 OH 浓 HCl / ZnCl 2 r.t. (+) delaminate immediately (-) no reaction 鉴别 : 低级一元醇 (C 6 以下 )
25 Mechanism of the reaction of tertiary alcohols with hydrogen halides: S N 1 Rate-determining step
26 Mechanism of the reaction of primary alcohols with hydrogen halides: S N 2 This alternative mechanism is believed to be one in which the carbon halogen bond begins to form before the carbon oxygen bond of the alkyloxonium ion is completely broken. The halide nucleophile helps to push off a water molecule from the alkyloxonium ion.
27 Mechanism of the reaction of secondary alcohols with hydrogen halides: S N 1, accompany with carbocation rearrangement. Ex: CH 3 CH 3 CH CH CH 3 CH 3 + HBr CH 3 C CH2 CH3 + CH 3 CH CH CH3 CH 3 OH Br major product Br by product
28 Mechanism: CH 3 CH CH CH 3 CH 3 OH CH 3 H + CH 3 C CH H OH 2 + CH3 CH 3 CH 3 C C H H CH 3 Br CH 3 rearrangement CH CH CH 3 CH 3 Br CH 3 CH 3 C CH 2 CH 3 Br CH 3 CH 3 C CH2 CH3 Br 瓦格涅尔 - 麦尔外因重排 : 当伯醇或仲醇的 β-c 具有两个或三个烷基或芳基时, 在酸作用下都能发生分子重排, 即瓦格涅尔 - 麦尔外因重排
29 REACTIONS OF ALCOHOLS WITH PHOSPHORUS HALIDES Commonly used reagents: PCl 3 PCl 5 ; PBr 3 PBr 5 ; I 2 + P Ex: CH 3 OH + I 2 + P CH 3 I b.p.63 Δ PI 3 36
30 Thionyl chloride reacts with alcohols to give alkyl chlorides. The inorganic byproducts in the reaction, sulfur dioxide and hydrogen chloride, are both gases at room temperature and are easily removed, making it an easy matter to isolate the alkyl chloride. 该反应的特点是 1 卤素易上,2 构型保持 但缺点是 SOCl 2 具腐蚀性 Ex: CH 3 SOCl 2 CH 3 C 6 H 13H OH C 6 H 13H Cl
31 ESTERS OF INORGANIC ACIDS Dialkyl sulfates are esters of sulfuric acid, trialkyl phosphites are esters of phosphorous acid (H 3 PO 3 ), and trialkyl phosphates are esters of phosphoric acid (H 3 PO 4 ). O CH 3 OH CH 3 OH + H 2 SO 4 CH 3 O S OH (CH 3 O) 2 SO 2 O Acute dermal toxicity!
32 Alkyl nitrates are esters formed by the reaction of alcohols with nitric acid. Ex: CH 2 OH CH 2 ONO 2 CHOH + 3HO NO 2 浓 H 2 SO 4 CHONO 2 CH 2 OH CH 2 ONO 2 三硝酸甘油酯 / 硝化甘油 : 炸药 心脏病急救
33 Dehydration 1 98% C 2 H 5 OH + H 2 SO 4 CH 2 CH % 2 CH 3 CH 2 CH 2 CHCH 3 62% H 2 SO 4 OH 95 C 2 H 5 65%~80% 3 CH 3 CH 2 CCH 2 CH 3 OH 46% H 2 SO 4 81 This is a frequently used procedure for the preparation of alkenes. The order of alcohol reactivity parallels the order of carbocation stability: R 3 C + > R 2 CH + > RCH 2+. Benzylic alcohols react readily.
34 The dehydration of alcohols and the conversion of alcohols to alkyl halides by treatment with hydrogen halides are similar in two important ways: 1. Both reactions are promoted by acids. Why? 2. The relative reactivity of alcohols decreases in the order tertiary > secondary > primary. These common features suggest that carbocations are key intermediates in alcohol dehydration, just as they are in the conversion of alcohols to alkyl halides.
35 C + 有三种反应取向 : 1 与 Nu - 结合 亲核取代 ; 2 消除 β-h 成烯烃 ; 3 重排以后再取代或消除 C + 历程就是单分子历程 (E1), 往往伴随着重排反应的发生
36 Rearrangements are sometimes observed. Ex: H 2 C H 2 C H 2 C CH CH CH 3 CH 2 OH H 2 C CH 2 CH H C CH 3 + H + H 2 C rearrangement -H + H 2 C CH CH CH 3 CH + 2 OH 2 + H -H + CH 3 H 2 C H 2 C -H 2 O CH 2 C CH CH 3 Major product
37 What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers? Zaitsev s rule as applied to the acid-catalyzed dehydration of alcohols is now more often expressed in this way: β-elimination reactions of alcohols yield the most highly substituted alkene as the major product.
38 Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by β- elimination. Ex: CH 2 CHCH 2 CH 3 OH H 2 SO 4 Δ Ph H C C H C 2 H 5 CH 3 CH 3 C CH CH 3 CH 3 OH H 2 SO 4 Δ H 3 C H 3 C C C CH 3 CH 3
39 Intermolecular dehydration: primary alcohols are converted to ethers on heating in the presence of an acid catalyst, usually sulfuric acid. 2C 2 H 5 OH 浓 H 2 SO C 2 H 5 OC 2 H 5 Mechanism of intermolecular dehydration: S N 2 CH 3 CH 2 OH H+ + HOC 2 H 5 + CH 3 CH 2 OH 2 CH 3 CH 2 OCH 2 CH 3 -H 2 O H -H + C 2 H 5 OC 2 H 5
40 When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. Conclusion:Substitution and elimination are competitive reactions.
41 Oxidation Oxidation of an alcohol yields a carbonyl compound. Whether the resulting carbonyl compound is an aldehyde, a ketone, or a carboxylic acid depends on the alcohol and on the oxidizing agent. Ex: R + 2- H Cr 2 O 2 SO CH 4 2 OH 7 R C H O O K 2 CrO 7 R C OH + Cr 3+ R R' CH 2- Cr 2 O 7 OH or Cr 2 O 3, glacial HAc R R' C O
42 Primary alcohols may be oxidized either to an aldehyde or to a carboxylic acid: Sarrett reagent: CrO 3 -pyridine / CH 2 Cl 2
43 Secondary alcohols are oxidized to ketones by the same reagents that oxidize primary alcohols: Tertiary alcohols have no hydrogen on their hydroxyl-bearing carbon and do not undergo oxidation readily.
44 Jones reagent: CrO 3 -H 2 SO 4 / CH 3 COCH 3 Ex: HO CrO 3, H 2 SO 4 Acetone O MnO 2 (newly prepared): MnO Ex: CH 2 2 CHCH 2 OH CH 2 CHCHO petroleum ether 99% 双键保留!
45 Reactions of polyols a. Chelation (Use: Identification) CH 2 OH CH 2 O Cu CHOH +Cu(OH) + 2 CHO CH CH 2 OH 2 OH Blue, soluble 2H 2 O b. Oxidative cleavage of vicinal diols A reaction characteristic of vicinal diols is their oxidative cleavage on treatment with periodic acid (HIO 4 ). The carbon carbon bond of the vicinal diol unit is broken and two carbonyl groups result. Periodic acid is reduced to iodic acid (HIO 3 ).
46 Ex: CH 2 OH CH 2 OH + +7 HIO 4 2HCHO OH OH O H 3 C C C CH 3 HIO 4 2 CCH3
48 c. Pinacol Rearrangement The reactions of pinacols with H 2 SO 4 always give pinacolones as the rearrangement products. CH 3 CH 3 H 3 C C C CH 3 OH OH H + CH 3 CH 3 H 3 C C C CH 3 OH + OH 2 (ⅰ) H 2 O - CH 3 CH 3 CH 3 CH 3 H 3 C C C + CH 3 OH (ⅱ) Rearrangement H 3 C C CH 3 C (ⅲ) OH + - H + Dynamic force? H 3 C CH 3 C O C CH3 CH 3
49 The dynamic force for pinacol rearrangement: (ii) 中带正电荷的碳为 6 电子, 而 (iii) 中质子化酮的氧为 8 电子, 故 (iii) 比 (ii) 稳定 这是促使发生重排反应的原因
50 Q1: 该重排反应第一步酸化形成质子化醇后即脱水成 C + 当四个 R 基不同时, 在哪个位置更易形成 C +? A: 原则是 C + 越稳定越易形成 p-π 共轭 Ex: more stable C 6 H 5 CH 3 OH C 6 H 5 C C CH 3 C 6 H 5 C C CH 3 OH OH H + C 6 H 5 CH 3 C 6 H 5 CH 3 C 6 H 5 C C CH 3 OH C 6 H 5 O C 6 H 5 C C CH 3 CH 3 σ-p 超共轭
51 Q2: 当形成的 C + 相邻碳上基团不同时, 哪个基团优先迁移? A: 通常是能提供电子 稳定正电荷较多的基团优先迁移 The order of migrating group: RO > H 3 C > ph > > Cl > R > H CH 3 CH 3 CH 3 H + O C 6 H 5 C 6 H 5 C 6 H 5 C C CH 3 Ex: C C OH OH C 6 H 5
52 Ex: 1. (C 6 H 5 ) 2 C CHC 6 H 5 H + (C 6 H 5 ) 3 C CHO OH OH OH OH 2. H 3 CO C C OCH 3 H + O H 3 CO C C 3. H + OCH 3 OH OH O Mechanism?
54 PREPARATION OF ALCOHOLS BY REDUCTION OF ALDEHYDES AND KETONES O RC H O RC The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydrogenation of the carbon oxygen double bond. Like the hydrogenation of alkenes, the reaction is exothermic but exceedingly slow in the absence of a catalyst. Finely divided metals such as platinum, palladium, nickel, and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones. R'
56 For most laboratory-scale reductions of aldehydes and ketones, catalytic hydrogenation has been replaced by methods based on metal hydride reducing agents. The two most common reagents are sodium borohydride and lithium aluminum hydride.
57 Sodium borohydride is especially easy to use, needing only to be added to an aqueous or alcoholic solution of an aldehyde or a ketone:
58 Lithium aluminum hydride reacts violently with water and alcohols, so it must be used in solvents such as anhydrous diethyl ether or tetrahydrofuran. Following reduction, a separate hydrolysis step is required to liberate the alcohol product:
59 Neither sodium borohydride nor lithium aluminum hydride reduces isolated carbon carbon double bonds. This makes possible the selective reduction of a carbonyl group in a molecule that contains both carbon carbon and carbon oxygen double bonds. 双键保留!
60 PREPARATION OF ALCOHOLS BY REDUCTION OF CARBOXYLIC ACIDS AND ESTERS O RC OH RC OR' Carboxylic acids are exceedingly difficult to reduce. A very powerful reducing agent is required to convert a carboxylic acid to a primary alcohol. Lithium aluminum hydride is that reducing agent. O
61 Esters are more easily reduced than carboxylic acids. Two alcohols are formed from each ester molecule. The acyl group of the ester is cleaved, giving a primary alcohol.
62 SYNTHESIS OF ALCOHOLS USING GRIGNARD REAGENTS Organolithum reagent is more reactive than organomagnesium reagent. Lithium dialkylcuprates react with alkyl halides to produce alkanes by carbon-carbon bond formation between the alkyl group of the alkyl halide and the alkyl group of the dialkylcuprate.
63 Grignard reagents are prepared from organic halides by reaction with magnesium. The order of halide reactivity is I > Br > Cl > F, and alkyl halides are more reactive than aryl and vinyl halides. (R may be methyl or primary, secondary, or tertiary alkyl; it may also be a cycloalkyl, alkenyl, or aryl group.)
64 The main synthetic application of Grignard reagents is their reaction with certain carbonylcontaining compounds to produce alcohols. Carbon carbon bond formation is rapid and exothermic when a Grignard reagent reacts with an aldehyde or ketone. A carbonyl group is quite polar, and its carbon atom is electrophilic. Grignard reagents are nucleophilic and add to carbonyl groups, forming a new carbon carbon bond.
65 This addition step leads to an alkoxymagnesium halide, which in the second stage of the synthesis is converted to an alcohol by adding aqueous acid. Overall reaction: O H C H OMgX δ - δ + + R MgX H C H H 3 O + RCH 2 OH R
66 The type of alcohol produced depends on the carbonyl compound. RMgX RMgX RMgX O HCH H 3 O + RCH 2 OH O H 3 O + RCH 2 CH 2 OH O R'CH O HCOR' O R'CR" O R'COR" H 3 O + H 3 O + H 3 O + H 3 O + OH R CH R' OH R CH R OH R C R" R' OH R C R R' add one more carbons add two carbon atoms asymmetric symmetric two same alkyls primary alcohols secondary alcohols tertiary alcohols
67 An ability to form carbon carbon bonds is fundamental to organic synthesis. The addition of Grignard reagents to aldehydes and ketones is one of the most frequently used reactions in synthetic organic chemistry. Not only does it permit the extension of carbon chains, but since the product is an alcohol, a wide variety of subsequent functional group transformations is possible.
68 Ex:Using Grignard Reagents and any necessary organic and inorganic reagents, suggest efficient syntheses of each of the following alcohols: OH 1 2 OH C CH 3 CH 3 CH 3 O CH 3 CH 2 C CH CH 2 CH 2 CH 2 C OH 3 4 CH 2 CH 2
69 Sample solution 1: Method 1: OH O 4C Grignard reagent + HCH 5C primary alcohol Symmetric secondary alcohol O Method 2: BrMg HBr R 2 O 2 5C aldehyde Br O 1 HCOEt 2 H 3 O + 4C Grignard reagent Mg Et 2 O T.M. T.M.
70 Sample solution 2: OH Method a: (CH 3 CO) 2 O anhydrous AlCl 3 COCH 3 C CH 3 a b c CH 3 1 CH 3 MgI 2H 3 O + OH C CH 3 CH 3 Method b: O MgBr + CH 3 CCH 3 H 3 O + T.M. Method c: O C OEt +2CH 3 MgI H 3 O + T.M.
71 Sample solution 3: Retrosynthetic analysis : CH 3 a c b CH 3 CH 2 C CH OH O a CH 3 CH 2 MgBr + CH 3 C CH CH 2 CH 2 CH C CH CH 2 CH CH b O CH 3 CH 2 CCH 3 + CH 2 CHLi c O CH 3 CH 2 CCH CH 2 + CH 3 MgI need to be prepared
72 Method a: 2 HC HCl-NH CH 4 Cl-CuCl H CH C CH CH 2 O 2 Hg 2+ O CH 3 C CH CH 2 2 HC CH lindlar Pd HBr Mg CH H 2 CH 2 CH 2 Et 2 O 3 CH 2 MgBr O CH 3 C CH CH 2 + CH 3 CH 2 MgBr Et 2 O H 3 O + T.M.
74 Syntheses of 1-(2-bromoethyl)benzene: a. Br 2 Fe Br MgBr 1. O 2. H + CH 2 CH 2 OH PBr 3 CH 2 CH 2 Br HBr rearrangement T.M. b. CH 3 NBS CH 2 Br CH 2 MgBr 1. HCHO 2. H + HBr CH 2 CH 2 OH CH CH 2 CH 2 CH 2 Br R 2 O 2 Can be easily polymerized
75 10.2 ELIMINATION REACTION 卤代烃的脱卤化氢和醇脱水成烯的反应都是消除反应, 而且都是 1,2- 消除, 或称 β- 消除, 是从反应物的相邻碳原子上消除两个原子或基团, 形成 π 键的过程 β-elimination Reaction A unimolecular elimination is given the symbol E1 and the bimolecular elimination is given the symbol E2.
76 Mechanism in Elimination Reaction E1: rate = k [RL], first-order reaction
78 Rearrangement is usually observed in secondary alcohol by the E1 mechanism. Ex: Mechanism:
79 Neopentyl rearrangement: Ex: CH 3 CH 3 C CH 2 Br CH 3 C 2 H 5 OH C 2 H 5 ONa E1 - H + CH 3 C CH CH 3 CH 3 CH 3 S N 1 CH 3 C CH 2 CH 3 OC 2 H 5 所以 C + 重排可以作为 E1 反应的标志, 当然也是 S N 1 的标志 显然,E1 和 S N 1 是一对竞争反应
80 E2: rate = k [ RL] [:B], second-order reaction α CH 2 B H CH β 2 L Transition state for E2 E2 反应过渡态与 S N 2 反应的过渡态的不同之处是 :1 碱进攻 β-h E2;Nu - 进攻 α-c S N 2 2 消去反应过渡态的能量高, 因为 E2 反应有两个反应点 α-c 和 β-h, 需断两个 σ 键 ; 而 S N 2 反应都作用在 α-c 上, 所以相对内能较低
81 FIGURE Potential energy diagram for concerted E2 elimination of an alkyl halide.
82 Ex: CH 3 CH 2 Br + NaNH 2 CH 2 CH 2 CH 3 CH CHCl + NaNH 2 CH 3 CH CH E2 消除和 S N 2 反应也是一对竞争反应 Ex: CH 3 CH 2 CH 2 CH 2 Br KOH H 2 O KOH C 2 H 5 OH CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH CH 2 S N 2 E2
83 Regioselectivity in β-h Elimination Reactions can proceed in more than one direction when there are more than one kind of β-h in the alcohols or alkyl halides. The E1 mechanism is often observed in dehydration of alcohols. The regioselectivity of dehydration of alcohols follows the Zaitsev's rule; elimination predominates in the direction that leads to the more highly substituted alkene.
84 The E2 mechanism is followed whenever an alkyl halide be it primary, secondary, or tertiary undergoes elimination in the presence of a strong base, such as sodium ethoxide (NaOCH 2 CH 3 ), sodium methoxide (NaOCH 3 ), and potassium tertbutoxide [KOC(CH 3 ) 3 ]. The regioselectivity of dehydrohalogenation of most of alkyl halides follows the Zaitsev's rule. 但 1L 体积增大, 2 β-h 位阻增大,3 或进攻碱体积庞大时都会强烈影响消除反应方向 Ex: P.281 Table 10-2 ~ 10-4
85 Typically, elimination by the E1 mechanism is observed only for tertiary and some secondary alkyl halides, and then only when the base is weak or in low concentration. The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev's rule.
86 值得一提的是 : 反 Zaitsev 规则的 E1 消除反应也有, 如 β-h 位阻大, 很难被碱夺走的情况 Ex: steric hindrance CH 3 CH 3 CH 3 CH 3 CH 3 C CH 2 C CH 3 CH 3 Br CH 3 C CH 2 C CH 3 CH 3 CH 3 CH 3 CH 3 C CH 2 C CH 2 CH 3
87 Stereoselectivity in β-h Elimination Reaction Experiments: H 3 C Br H 3 C H C 6 H 5 H H C 6 H 5 C 6 H 5 H C 6 H 5 Br E2 E2 cis- trans- A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. H C 6 H 5 H C 6 H 5 C C (1) (2) C C CH 3 C 6 H 5 C 6 H 5 CH 3
88 For reaction 1: H B H 3 C Br C 6 H 5 H H H H 3 C C 6 H 5 C 6 H 5 H C 6 H 5 C C CH 3 C 6 H 5 C 6 H 5 Br (1) (Ⅰ) For reaction 2: H B H 3 C H C 6 H 5 H Br C 6 H 5 H 3 C H C 6 H 5 H C 6 H 5 C C C 6 H 5 CH 3 C 6 H 5 Br (2) (Ⅱ)
89 One experiment compares the rates of elimination of the cis and trans isomers of 4-tert-butylcyclohexyl bromide. Although both stereoisomers yield 4-tertbutylcyclohexene as the only alkene, they do so at quite different rates. The cis isomer reacts over 500 times faster than the trans.
90 Ex: H H CH 3 H E2 CH 3 Br Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene. 大部分 E2 反应是反式共平面消除 此时, 在形成 π 键轨道中电子云达到最大重叠, 过渡态在能量上有利 所以 E2 反应的立体化学要求是被消除两基团彼此处于反式共平面
91 Competitive Reaction: E and S N E 和 S N 可以由同一试剂的进攻而引起 进攻 α- C S N ; 进攻 β-h E 为了能够有效控制产物比例, 就需要通过人为地选择反应条件来达到合成的目的 反应条件包括反应物的结构 试剂 溶剂 温度等因素
92 (1) Structures of Reactants 一般地说, 伯卤代烷因其 α-c 位阻小, 有利于 S N 2 当伯卤代烷 α-c 和 β-h 的空间位阻达到一定程度时, 却有利于 E2 特别当 β-h 酸性大, 如 : 苄基型和烯丙型, 易被碱夺去, 容易发生 E2 Ex:P.284 叔卤代烷容易形成 3 C +, 稳定性好, 所以常常得到 S N 1 和 E1 的混合物 但是有强碱存在时, 主要发生 E2 反应 Ex:P.284
93 (2) The Basicity of Reagents 因为 S N 1 和 E1 反应都与碱的浓度无关, 所以试剂的碱性和浓度只与双分子反应有关 碱性越强, 浓度越大, 将有利于 E2 反应 ; 亲核性强将容易发生 S N 2 反应 试剂的碱性与亲核性在一般条件下是一致的, 即碱性强亲核性也强 但也有不一致的情况 如 :1NH 3 亲核力强, 而碱性弱 ; - NH 2 强碱弱亲核 2t-BuOK/ t-buona 强碱弱亲核
95 (3) Polarity of Solvents 一般来说, 增加溶剂的极性有利于电荷集中 S N 2 反应电荷相对 E2 反应而言电荷较集中, 而 E2 反应电荷较分散 Transition state for S N 2 Transition state for E2 增加溶剂极性有利于 S N 2 反应 非极性溶剂和低极性溶剂有利于电荷分散, 即有利于 E2 反应
96 (4) Temperature 温度高活化能大的反应有利 消除反应活化时需要拉长 C-β-H 键, 而取代反应无此过程, 所以消除反应活化能略高 升温有利于消除反应
97 对于单分子反应来说, 底物分子中烷基结构怎样是决定反应趋向的主要因素 取代烷基的空间体积大有助于消除反应 虽说试剂的因素与单分子反应无关, 但是实际反应也略有影响 弱碱强亲核有利于 S N 1, 强碱弱亲核有利于 E1 Ex:P.309 Ex.22
98 α-elimination Reaction Carbenes are neutral molecules in which one of the carbon atoms has six valence electrons. Such carbons are divalent. Dihalocarbenes are formed when trihalomethanes are treated with a strong base, such as potassium tert-butoxide.
99 The trihalomethyl anion produced on proton abstraction dissociates to a dihalocarbene and a halide anion. Carbenes are too reactive to be isolated and stored, but have been trapped in frozen argon for spectroscopic study at very low temperatures.
100 The process in which a dihalocarbene is formed from a trihalomethane corresponds to an elimination in which a proton and a halide are lost from the same carbon. It is an α-elimination proceeding via the organometallic intermediate K + [:CX 3 ] -. Ex: heat or light CH 2 N 2 CH 2 + N 2 HCCl 3 + (CH 3 ) 3 COK CCl 2 + (CH 3 ) 3 COH + KCl
101 10.3 PHENOLS Nomenclature of Phenols Properties of Phenols 1. Physical Properties The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules and with water. Thus, phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular weight.
102 2. Spectroscopic Analysis of Phenols Infrared: The infrared spectra of phenols combine features of those of alcohols and aromatic compounds. O-H stretching : in the 3600 cm -1 region; C-O stretching : around cm -1.
103 1 H NMR: The 1 H NMR signals for the hydroxyl protons of phenols are often broad, and their chemical shift lies between alcohols and carboxylic acids. The range is 4-12 ppm, with the exact chemical shift depending on the concentration, the solvent, and the temperature.
104 3. Chemical Properties Phenol is planar, with a C-O-H angle of 109, almost the same as the tetrahedral angle and not much different from the C-O-H angle of methanol: OH p-π Conjugation
105 (1) Acidity The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols but less acidic than carboxylic acids. OH + NaOH ONa ONa + CO 2 + H 2 O OH + NaHCO 3 Purification
106 Substituent effects, in general, are small. Alkyl substitution produces negligible changes in acidities, as do weakly electronegative groups attached to the ring. Only when the substituent is strongly electron-withdrawing, as is a nitro group, is a substantial change in acidity noted. Ex 1: OH OH OH OH OH > > > > NO 2 Cl OCH 3 CH 3 pka
107 Multiple substitution by strongly electronwithdrawing groups greatly increases the acidity of phenols. Ex 2: O 2 N OH O 2 N OH NO 2 NO 2 pka =4.0 NO 2 pka = 0.4 (2) Color-reaction H 3 [Fe(OC 6 H 5 ) 6 ] purple Complex
108 (3) Preparation of Aryl Ethers Aryl ethers are best prepared by the Williamson method. Ex: The alkyl halide must be one that reacts readily in an S N 2 process. Thus, methyl and primary alkyl halides are the most effective alkylating agents.
109 A hydroxyl group is a very powerful activating substituent, and electrophilic aromatic substitution in phenols occurs far faster, and under milder conditions, than in benzene. (4) Halogenation
111 (5) Nitration and Sulfonation Phenols are nitrated on treatment with a dilute solution of nitric acid in either water or acetic acid. It is not necessary to use mixtures of nitric and sulfuric acids, because of the high reactivity of phenols. Ex: OH OH OH + HNO 3 (dilute) 18 NO 2 + NO 2
112 二硝化产物一般不直接由苯酚硝化 ( 亲电取代 ), 而是采用如下方法 : Nucleophilic Substitution Cl NO 2 Na 2 CO 3,H 2 O 100 H + OH NO 2 NO 2 NO 2 这样做既可以避免高温反应, 而且得率也提高
113 三硝基苯酚也不能由苯酚直接硝化, 因为三硝化需要高温, 而三硝基苯酚可在高温时爆炸, 所以为了安全应避免直接硝化, 而改用磺化 硝化 OH con. H 2 SO OH SO 3 H HNO3 O 2 N OH NO 2 SO 3 H 降低电子云密度, 提高产率, 减少氧化 NO 2 90%
114 Heating a phenol with concentrated sulfuric acid causes sulfonation of the ring.
115 (6) Nitrosation On acidification of aqueous solutions of sodium nitrite, the nitrosonium ion is formed, which is a weak electrophile and attacks the strongly activated ring of a phenol. The product is a nitroso phenol. OH OH OH NaNO 2, H 2 SO 4 7~8 NO dilute HNO 3 NO 2
116 (7) Friedel-Crafts Alkylation Alcohols in combination with acids serve as sources of carbocations. Attack of a carbocation on the electron-rich ring of a phenol brings about its alkylation. OH CH 3 + H 3 C C OH CH 3 H 3 PO 4 OH C(CH 3 ) 3
117 (8) Friedel-Crafts Acylation In the presence of aluminum chloride, acyl chlorides and carboxylic acid anhydrides acylate the aromatic ring of phenols.
118 (9) Condensation O HO HO OH HCH + CH 2 phenol-formaldehyde resin n
119 (10) Oxidation Phenols are more easily oxidized than alcohols, and a large number of inorganic oxidizing agents have been used for this purpose.
120 Important Phenols Phenol Mp. 43 C, colorless, Soluble in hot water, ethanol; Toxicant.
121 (1) 苯磺酸盐碱熔法 SO 3 H SO 3 Na ONa OH NaOH (Sol.) H + + Na 2 SO 3 fused SO 2 Na 2 SO 3 SO 2 这种方法较成熟, 但设备易腐蚀 Na 2 SO 3 SO 2 + H 2 O H 2 SO 3 可循环利用 (2) Hydrolysis Cl + H 2 O OH - 220atm, >350 OH 工业制法
122 (3) 异丙苯氧化法生产苯酚最好的方法 原料易得, 无浪费 + H 3 C CH CH 2 H + CH CH 3 CH 3 C O OH + O 2 CH 3 CH 3 H 2 O, H + O OH + CH 3 CCH 3 过氧化氢烃重排
123 2. 1,4-Benzenediol 1 黑白照相显影剂 HO OH AgBr ( 已感光 ) Ag ( 显影 ) 使感光多的部分 Ag 多, 感光少的部分 Ag 少, 这样就产生了层次 深浅 显影完了就定影, 用 Na 2 S 2 O 3 等试剂将未感光的 AgBr 洗掉 ( 成络离子而溶解 ) 所以底片上就留下了黑色的感光部分 2 阻聚剂苯乙烯易自聚, 加对苯二酚抑制其聚合
124 3. Naphthol α- 萘酚,β- 萘酚都是染料中间体 β- 萘酚可以由相应的萘磺酸钠经碱熔而得 因为萘直接磺化很难得到 α- 萘磺酸, 所以也无从得到 α- 萘磺酸钠, 而实际工业生产上 α- 萘酚由 α- 萘胺制备 NO 2 NH 2 OH H H 2 SO 4 200,1.4MPa
125 Nomenclature of Ethers and Epoxides 10.4 ETHERS Ethers are named, in substitutive IUPAC nomenclature, as alkoxy derivatives of alkanes. Functional class IUPAC names of ethers are derived by listing the two alkyl groups in the general structure ROR' in alphabetical order as separate words, and then adding the word ether at the end.
126 Ex: CH 3 CH 2 CH 2 CHCH 2 CH 3 CH 2 CH 2 OCH 3 OH OC 2 H 5 3-Methoxyhexane 2-Ethoxyethanol Cyclic ethers: 1Epoxy compounds CH 3 CH 2 CH CH 2 Cl CH 2 C CH 2 CH CH2 O 3-Chloro-1,2-epoxypropane O 4-Methyl-4,5-epoxy-1-pentene
127 2 Cyclic ethers have their oxygen as part of a ring they are heterocyclic compounds. In each case the ring is numbered starting at the oxygen. Many substances have more than one ether linkage.
128 Physical Properties and Spectroscopy It is instructive to compare the physical properties of ethers with alkanes and alcohols. With respect to boiling point, ethers resemble alkanes more than alcohols. With respect to solubility in water the reverse is true; ethers resemble alcohols more than alkanes. Why?
129 Infrared: The infrared spectra of ethers are characterized by a strong, rather broad band due to C-O-C stretching between 1070 and 1150 cm -1. FIGURE The infrared spectrum of dipropyl ether.
130 1 H NMR: The chemical shift of the proton in the H- C-O-C unit of an ether is very similar to that of the proton in the H-C-OH unit of an alcohol. A range ppm is typical. FIGURE The 200-MHz 1 H NMR spectrum of dipropyl ether.
131 Chemical Properties In contrast to alcohols with their rich chemical reactivity, ethers undergo relatively few chemical reactions. 1. Formation of Dialkyloxonium Ion Proton transfer to the oxygen of the ether to give a dialkyloxonium ion. ROR + HCl R O H R + Cl - H 2 SO 4 - HSO 4 + Lewis acids: BF 3,AlCl 3,ZnCl 2
132 2. Acid-catalyzed Cleavage of Ethers The order of hydrogen halide reactivity is HI > HBr >> HCl. Ex: HI + R O R' heat R excess I - H O R' Primary alkyl S N 2 Tertiary alkyl S N 1 ROH RI HI + R'I
133 Aryl Alkyl ethers react with HI always yield phenols and alkyl halides. O CH 57% HI 3 OH + CH 3 I Δ p-π Conjugation Application: 由于酚醚比酚稳定, 而且与 HI 作用可以分解出酚, 所以在合成上被用来保护酚羟基
134 3. Formation of Peroxides A dangerous property of ethers is the ease with which they undergo oxidation in air to form explosive peroxides. Test: Remove: peroxides + Fe 2+ Fe 3+ SCN - 3- Fe(SCN) 6 (Sanguine) peroxides + FeSO 4 (5%) Fe 3+ + oxides
135 小经验 : 鉴别醚 : 醚能溶于强酸 (HI HBr HCl H 2 SO 4 ) 干燥醚 : 由于其对还原剂的稳定性, 可以选用 : 第一步 : 无水 CaCl 2 ; 第二步 : 金属 Na
136 Preparation of Ethers 1. Dehydration of Alcohols Many simple dialkyl ethers are commercially available and are prepared by acid-catalyzed condensation of the corresponding alcohols. In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary.
137 2. The Williamson Ether Synthesis A long-standing method for the preparation of ethers is the Williamson ether synthesis. Nucleophilic substitution of an alkyl halide by an alkoxide gives the carbon oxygen bond of an ether: S N 2 substitution
138 Ex 1: a C 2 H 5 O C(CH 3 ) 3 b C 2 H 5 ONa + BrC(CH 3 ) 3 C 2 H 5 Br + C(CH 3 ) 3 ONa 为了避免烯烃的生成, 应选择叔醇钠和卤代烷 Ex 2: CH 2 CH CH 2 O CH CH 3 Sample solution: CH 2 CH CH 3 CH 2 CH CH 3 H 2 O H 2 SO 4 CH 3 CH 3 CH a OH b Cl CH 3 Na CH 2 CH CH 2 Cl CH 3 CH 3 CH ONa T.M.
139 Cyclic Ethers 1. Nucleophilic Ring-opening Reaction The most striking chemical property of epoxides is their far greater reactivity toward nucleophilic reagents compared with that of simple ethers. Ex:
140 Nucleophiles other than Grignard reagents also open epoxide rings. Nucleophilic ring-opening reactions of epoxides may also occur under conditions of acid catalysis.
141 2. Stereochemistry of Nucleophilic Ring-opening Reaction Nucleophilic ring opening of epoxides has many of the features of an S N 2 reaction. Inversion of configuration is observed at the carbon at which substitution occurs.
142 3. Regiochemistry of Nucleophilic Ring-opening Reaction Unsymmetrical epoxides are attacked at the less substituted, less sterically hindered carbon of the ring:
143 Mechanism: Ex:
144 Other nucleophilic ring-openings of epoxides likewise give 2-substituted derivatives of ethanol but either involve an acid as a reactant or occur under conditions of acid catalysis:
146 Because carbocation character develops at the transition state, substitution is favored at the carbon that can better support a developing positive charge. Acid catalysis promotes substitution at the position that bears the greater number of alkyl groups: Inversion of configuration
147 Conclusion: Regiochemistry of ring-opening reactions of epoxides: Unsymmetrically substituted epoxides tend to react with anionic nucleophiles at the less hindered carbon of the ring. Under conditions of acid catalysis, however, the more highly substituted carbon is attacked.
148 Ex:Synthesize the compound by using the starting materials with less than two carbons. CH 3 CH CHCH 3 Sample solution: CH 2 CH 2 O + 1/2O PdCl 2 -CuCl 2 2 CH 3 CHO CH 2 CH 2 HBr CH 3 CH 2 Br Mg Et 2 O CH 3 CH 2 MgBr H + H + CH 3 CH 2 CHCH 3 Δ CH 3 CH CHCH 3 Cl 2, H 2 O OH CH 3 CH 2 CHCH 3 Ca(OH) 2 O CH 3 CH CHCH 3 OH Cl
149 Crown Ethers Macrocyclic polyethers: cyclic compounds containing four or more oxygens in a ring of 12 or more atoms. A shorthand description is devised for naming crown ethers: the word crown is preceded by the total number of atoms in the ring and is followed by the number of oxygen atoms.
150 The metal ion complexing properties of crown ethers are clearly evident in their effects on the solubility and reactivity of ionic compounds in nonpolar media. FIGURE (a) An electrostatic potential map of 18-crown-6. (b) A spacefilling model of the complex formed between 18- crown-6 and potassium ion (K + ).
151 Application: Phase-transfer catalyst When KF is added to a solution of 18-crown-6 in benzene, potassium ion (K + ) interacts with the oxygens of the crown ether to form a Lewis acid- Lewis base complex.
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