16.8 Acid-Catalyzed Cleavage of Ethers SAMPLE SOLUTION (a)In the reaction of dialkyl ethers with excess hydrogen bromide, each alkyl group of the ether function is cleaved and forms an alkyl bro mide. Since bromocyclopentane and 1-bromobutane are the products, the start ing ether must be butyl cyclopentyl ether OCH2 CH2CH2 CHa CH3 CH2CH2CH2 Br Butyl cyclopentyl ether Bromocyclopentane 1-Bromobutane A mechanism for the cleavage of diethyl ether by hydrogen bromide is outlined in Figure 16.4. The key step is an SN2-like attack on a dialkyloxonium ion by bromide (step 2) Overall reaction CH CHOCHCH2+ 2CH, CH, Br HO Ethyl bromide Mechanism Step 1: Proton transfer to the oxygen of the ether to give a dialkyloxonium ion. CH, CH, CH, CH CHCH CH,CH Diethyl ether Step 2: Nucleophilic attack of the halide anion on carbon of the dialkyloxonium ion. This step gives one molecule of an alkyl halide and one molecule of an alcohol 过 -H CHCH尊s+ CH, CH,OH Diethvloxoniu Ethyl bromide Ethanol Step 3 and Step 4: These two steps do not involve an ether at all. They correspond to those in which an alcohol is converted to an alkyl halide(Sections 4.8-4. 13) H CH, CH,OH+H—Br CH CHEO →CHCH2Br+H2O Ethanol Water bromide bromide Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
SAMPLE SOLUTION (a) In the reaction of dialkyl ethers with excess hydrogen bromide, each alkyl group of the ether function is cleaved and forms an alkyl bromide. Since bromocyclopentane and 1-bromobutane are the products, the starting ether must be butyl cyclopentyl ether. A mechanism for the cleavage of diethyl ether by hydrogen bromide is outlined in Figure 16.4. The key step is an SN2-like attack on a dialkyloxonium ion by bromide (step 2). HBr heat OCH2CH2CH2CH3 Butyl cyclopentyl ether CH3CH2CH2CH2Br 1-Bromobutane Br Bromocyclopentane 16.8 Acid-Catalyzed Cleavage of Ethers 629 Overall Reaction: CH3CH2OCH2CH3 HBr ±£ 2CH3CH2Br H2O heat Water Step 1: Proton transfer to the oxygen of the ether to give a dialkyloxonium ion. O H Br Diethyloxonium ion Bromide ion Step 2: Nucleophilic attack of the halide anion on carbon of the dialkyloxonium ion. This step gives one molecule of an alkyl halide and one molecule of an alcohol. Step 3 and Step 4: These two steps do not involve an ether at all. They correspond to those in which an alcohol is converted to an alkyl halide (Sections 4.8–4.13). Mechanism: CH3CH2 CH3CH2 CH3CH2 CH3CH2 CH3CH2 CH3CH2 CH3CH2 Hydrogen bromide Hydrogen bromide Hydrogen bromide Diethyl ether Diethyl ether O H Br Diethyloxonium ion Bromide ion slow Ethyl bromide Ethyl bromide Ethanol Ethanol slow Ethyl bromide Water fast Br O±H ±£ CH3CH2Br CH3CH2OH fast Br CH3CH2OH H Br –O ±£ CH3CH2Br H2O H H ± ± FIGURE 16.4 The mechanism for the cleavage of ethers by hydrogen halides, using the reaction of diethyl ether with hydrogen bromide as an example. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER SIXTEEN Ethers, Epoxides, and Sulfides PROBLEM 16.10 Adapt the mechanism shown in Figure 16. 4 to the reaction ICH2 CH2 CH2 CH2I Tetrahydrofuran 1, 4-Diiodobutane(65%) With mixed ethers of the type ROR, the question of which carbon-oxygen bond is broken first arises. Although some studies have been carried out on this point of mech anistic detail. it is not one that we need e at our level of study 16.9 PREPARATION OF EPOXIDES: A REVIEW AND A PREVIEW There are two main laboratory methods for the preparation of epoxides 1. Epoxidation of alkenes by reaction with peroxy aci 2. Base-promoted ring closure of vicinal halohydrins Epoxidation of alkenes was discussed in Section 6.18 and is represented by the RC=(R2+RcOo→→R2CCR2+RCOH Alkene Peroxy acid The reaction is easy to carry out, and yields are usually high. Epoxidation is a stereo- specific syn addition ChE +CH2COOH一 ch3 COh trans-2,3-Diphenyloxirane Acetic acid The following section describes the preparation of epoxides by the base-promoted ring closure of vicinal halohydrins. Since vicinal halohydrins are customarily prepared from alkenes(Section 6. 17), both methods--epoxidation using peroxy acids and ring closure of halohydrins--are based on alkenes as the starting materials for preparing epoxides 16.10 CONVERSION OF VICINAL HALOHYDRINS TO EPOXIDES The formation of vicinal halohydrins from alkenes was described in Section 6. 17. Halo- hydrins are readily converted to epoxides on treatment with base R,C=CR, R2C—CR2—>R2C Vicinal halohydrin Epoxid Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 16.10 Adapt the mechanism shown in Figure 16.4 to the reaction: With mixed ethers of the type ROR, the question of which carbon–oxygen bond is broken first arises. Although some studies have been carried out on this point of mechanistic detail, it is not one that we need examine at our level of study. 16.9 PREPARATION OF EPOXIDES: A REVIEW AND A PREVIEW There are two main laboratory methods for the preparation of epoxides: 1. Epoxidation of alkenes by reaction with peroxy acids 2. Base-promoted ring closure of vicinal halohydrins Epoxidation of alkenes was discussed in Section 6.18 and is represented by the general equation The reaction is easy to carry out, and yields are usually high. Epoxidation is a stereospecific syn addition. The following section describes the preparation of epoxides by the base-promoted ring closure of vicinal halohydrins. Since vicinal halohydrins are customarily prepared from alkenes (Section 6.17), both methods—epoxidation using peroxy acids and ring closure of halohydrins—are based on alkenes as the starting materials for preparing epoxides. 16.10 CONVERSION OF VICINAL HALOHYDRINS TO EPOXIDES The formation of vicinal halohydrins from alkenes was described in Section 6.17. Halohydrins are readily converted to epoxides on treatment with base: R2C CR2 Alkene X2 H2O HO HO X R2C CR2 Vicinal halohydrin R2C O CR2 Epoxide H C6H5 C6H5 H C C (E)-1,2-Diphenylethene CH3COOH O Peroxyacetic acid H OC6H5 C6H5 H trans-2,3-Diphenyloxirane (78–83%) CH3COH O Acetic acid R2C CR2 Alkene RCOOH O Peroxy acid R2C O CR2 Epoxide RCOH O Carboxylic acid Tetrahydrofuran O ICH2CH2CH2CH2I 1,4-Diiodobutane (65%) HI 150°C 630 CHAPTER SIXTEEN Ethers, Epoxides, and Sulfides Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website