有機化學課件Chapter 8 Ethers and Epoxides

1、有機化學課件有機化學課件Chapter 8 Ethers and EpoxidesChapter 8 Ethers and Epoxides Teaching Objectives and Requirements Understand: Ethers, Physical Properties, Synthesis of Epoxides Grasp: Nomenclature of Ethers, Synthesis of Ether, Reactions: Acid-Catalyzed Cleavage, Reactions of Epoxides. Key points and Main

2、 Areas of Difficulty Synthesis of Ether, Reactions: Acid-Catalyzed Cleavage, Reactions of Epoxides. 8.1 Ethers Ethers contain two groups ,which may be alkyl or aryl groups,bonded to an oxygen atom. The groups are the same in a symmetrical ether and different in an un symmetrical ether. CH3CH2OCH2CH3

3、 CH3CH2OOdiethyl ether phenyl propyl ether diphenyl ether(symmetrical ether) (unsymmetrical ether) (symmetrical ether)8.1 EthersThe oxygen atom of an ether is sp3-hybridized,and the COC bond angle is approximately the tetrahedral bond angle. 8.2 Nomenclature of Ethers Simple ethers are named as alky

4、l (or aryl) groups in alphabetical order and appending the name ether. IUPAC Names of Ethers CH3CH2CH2CH2OCH3 CH3CH OCH CH3CH3CH3butyl methyl ether diisopropyl ether8.2 Nomenclature of Ethers Ethers are named according to IUPAC nomenclature as alkoxyalkanes, where the smaller alkyl group and the oxy

5、gen atom constitute an alkoxy group. An alkoxy group is treated as a substitutent on the larger parent alkane chain or cycloalkane ring. 8.2 Nomenclature of Ethers 8.2 Nomenclature of Ethers Cyclic Ethers The three- through six-membered cyclic ethers have common names. In all ring systems, the oxyge

6、n atom is assigned the number 1,and the rings are numbered in the direction that gives the first substituent the lower number. Cyclic ethers with three-atom rings called epoxides. 8.2 Nomenclature of Ethers Because these compounds are formed from the oxidation of an alkene, the common name of an epo

7、xide is derived by adding oxide to the name of the alkene. 8.2 Nomenclature of EthersThe name of the five-membered ring ether is tetrahydrofuran (THF),based on its relationship to the aromatic compound furan.Similarly,tetrapyran (THP),a six-membered ring ether, is related to pyran,an unsaturated eth

8、er.OOOOfuran tetrafuran pyran tetrapyran8.2 Nomenclature of EthersThe IUPAC names for cyclic ethers having three-,four-,five-,and six-membered rings are oxiran, oxetane, oxolane, and oxane, respectively. The oxygen atom in each of these rings receives the number 1.The ring is numbered in the directi

9、on that gives the lowest numbers to substituents. 8.2 Nomenclature of EthersOOCH3OClOCH3CH3OCH2CH312312341 23451234562,2-dimethyloxiran 2-ethyloxetane 3-methoxyoxolane 4-chlorooxane8.3 Physical Properties of EthersEthers are more polar than alkanes and somewhat less polar than alcohols.Ethers are mo

10、re polar than alkanes and somewhat less polar than alcohols. Ethers do not have an OH bond and therefore cannot serve as hydrogen bond donors.As a consequence,ether molecules do not hydrogen bond to each other. 8.3 Physical Properties of EthersThus,ethers have boiling points substantially lower than

11、 those of alcohols of comparable molecular weight.Because ethers are polar,they are more soluble in water than alkanes of similar molecular weight. Ethers such as diethyl ether dissolve a variety of nonpolar and polar compounds. 8.4 The Grignard Reagent and Ethers Grignard reagent ,represented as RM

12、gX, which can be prepared from haloalkanes as well as from aryl halides. Victor Grignard: French chemistRXRMgXMgether Victor Grignard8.4 The Grignard Reagent and Ethers In a Grignard reagent, the R group maybe a 1,2,or 3 alkyl group as well as a vinyl or aryl group.The halogen may be Cl, Br,or I; fl

13、uorine compounds do not form Grignard reagents. 8.4 The Grignard Reagent and EthersA Grignard reagent has a very polar carbon-magnesium bond in which the carbon atom has a partial negative charge and the metal a partial positive charge. CMgX -+8.4 The Grignard Reagent and EthersGrignard reagents rea

14、ct rapidly with acidic hydrogen atoms in molecules such as alcohols and water to produce alkanes. RXRMgXMgetherH2ORH8.5 Synthesis of Ethers Ethers can be prepared by a method called the Williamson synthesis. In this reaction,a halide ion is displaced from an alkyl halide by an alkoxide ion in an SN2

15、 reaction. The alkoxide ion is prepared by the reaction of an alcohol with a strong base such as sodium hydride. 8.5 Synthesis of Ethers Because the steric hindrance,the yields are decreased in the order of primary, secondary and tertiary. OHNaHOH2+:.:.-Na+Na-.:O+ CH3Br.:OCH3+NaBr8.5 Synthesis of Et

16、hersThus, to make an unsymmetrical ether that has a primary and a tertiary alkyl group, a primary alkyl halide and a tertiary alkoxide ion must be used. CH3COCH3CH3:.-+ CH3ISN2.CH3COCH3CH3CH3+ I-.CH3O + CH2CClCH3CH3HE2CH2CCH3CH3+ CH3OH.+ Cl8.6 Reactions of Ethers Ethers are stable compounds that do

17、not react with most common reagents. They do not react with bases because they are aprotic, but do react with strong acids whose conjugate bases are good nucleophiles. RORHXROH + XRROHHXRX8.6 Reactions of Ethers In general,the less substituted alkyl halide is formed by an SN2 reaction. The halide io

18、n attacks the less hindered carbon atom, and the displaced alkoxy group has the oxygen atom bonded to the more substituted carbon atom. The cleavage reaction does not occur with a halide salt: a proton from the halogen acid is required to protonate the oxygen atom and provide an alcohol as the leavi

19、ng group. 8.6 Reactions of Ethers+HBr.O.:.OH+fast-:Br.+.+:Br.OH.+:slowSN2less hinderedmore hinderedOH8.7 Synthesis of Epoxides The synthesis and reaction of cyclic ethers containing four or more atoms are similar to those of acyclic ethers. The three-membered cyclic ethers,which are important interm

20、ediates in synthesis, can be synthesized by oxidizing an alkene with a peroxyacid(RCO3H).8.7 Synthesis of Epoxides In the epoxidation of alkenes with m-chloroperoxybenzoic acid(MCPBA), the stererochemistry of the groups in the alkene is retained. +HHClCOOHOCH2Cl225HHOOOHCCl8.7 Synthesis of Epoxides

21、That is,groups that are cis in the alkene are cis in the epoxide, and groups that are trans in the alkene remain trans in the epoxide. CH3CCCH3HHMCPBAOCH3CH3HH8.8 Reaction of EpoxidesEpoxides are highly reactive because the three-member ring has considerable bond angle strain.Acid-Catalyzed Ring Ope

22、ningThe acid-catalyzed ring opening of epoxides occurs by an SN2 process in which water is the nucleophile and the leaving group is the protonated oxygen atom of the epoxide.8.8 Reaction of Epoxides Ring Opening by Nucleophiles Ethers do not generally react with nucleophiles under neutral or basic c

23、onditions. OHHHHCCHOHHHHOHH3O+HOHOH:.+:HOCH2CH2OHH.+.HOCH2CH2OH.+ H3O+8.8 Reaction of EpoxidesHowever,epoxides are so reactive that the ring is opened even by nucleophiles such as OH-,SH-,or NH3 or the related organic species RO-.RS-,and RNH2.OHHHHCCHNH2HHHOH+NH38.8 Reaction of EpoxidesOHHHHCCHO-+Mg

24、BrHHHOH+RMgBrRBr +MgetherRMgBrHCCHO-+MgBrHHHO+H2ORCH2CH2OH + HOMgBr8.8 Reaction of Epoxides Direction of Ring Opening Unsymmetrical epoxides give different products under acid-catalyzed and base-catalyzed conditions.HCCCH3CH3HOCH3O-CH3OHH+CH3OHCH3OCH2CCH3OHCH3HOCH2CCH3CH3OCH31-methoxy-2-methyl-2-pro

25、panol2-methoxy-2-methl-1-propanol8.8 Reaction of EpoxidesIn the case of the ring opening by a nucleophile under basic conditions,the reaction is controlled by the same features as the SN2 displacement reactions we considered in Chapter 7.The nucleophile attacks the less hindered primary carbon atom instead of the tertiary carbon atom.8.8 Reaction of Epoxides The resulting alkoxide ion then exchanges a proton with the solvent,and the methoxide base is regenera