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Chapter 9 Alcohols, Ethers and Epoxides

Chapter 9 Alcohols, Ethers and Epoxides. Alcohols, Ethers and Epoxides. Introduction—Structure and Bonding. Alcohols contain a hydroxy group (OH) bonded to an sp 3 hybridized carbon. Alcohols, Ethers and Epoxides. Introduction—Structure and Bonding. enols and phenols

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Chapter 9 Alcohols, Ethers and Epoxides

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  1. Chapter 9 Alcohols, Ethers and Epoxides

  2. Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • Alcohols contain a hydroxy group (OH) bonded to an sp3 hybridized carbon.

  3. Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • enols and phenols • Compounds having a hydroxy group on a sp2 hybridized carbon • undergo different reactions than alcohols. • Will be discussed in chapter 11, 19

  4. Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • Ethers have two alkyl groups bonded to an oxygen atom.

  5. Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • Epoxides are ethers having the oxygen atom in a three-membered ring. • Epoxides are also called oxiranes. • The C—O—C bond angle for an epoxide must be 60°, a considerable deviation from the tetrahedral bond angle of 109.5°. Thus, epoxides have angle strain, making them much more reactive than other ethers.

  6. Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • The oxygen atom in alcohols, ethers and epoxides is sp3hybridized. Alcohols and ethers have a bent shape like that in H2O. • The bond angle around the O atom in an alcohol or ether is similar to the tetrahedral bond angle of 109.5°. • Because the O atom is much more electronegative than carbon or hydrogen, the C—O and O—H bonds are all polar.

  7. Alcohols, Ethers and Epoxides Nomenclature of Alcohols : -- ol

  8. Alcohols, Ethers and Epoxides For cyclic compounds • When an OH group is bonded to a ring, the ring is numbered beginning with the OH group. • Because the functional group is at C1, the 1 is usually omitted from the name. • The ring is then numbered in a clockwise or counterclockwise fashion to give the next substituent the lowest number.

  9. Alcohols, Ethers and Epoxides Nomenclature of Alcohols • Common names are often used for simple alcohols. • Name all the carbon atoms of the molecule as a single alkyl group. • Add the word alcohol, separating the words with a space.

  10. Alcohols, Ethers and Epoxides Nomenclature of Alcohols • Compounds with two hydroxy groups : diols or glycols. • Compounds with three hydroxy groups : triols • and so forth.

  11. Alcohols, Ethers and Epoxides Nomenclature of Ethers • Simple ethers are usually assigned common names. To do so: • Name both alkyl groups bonded to the oxygen, arrange these names alphabetically, and add the word ether. • For symmetrical ethers, name the alkyl group and add the prefix “di-”.

  12. Alcohols, Ethers and Epoxides • More complex ethers are named using the IUPAC system. • One alkyl group is named as a hydrocarbon chain, and the other is named as part of a substituent bonded to that chain: • Name the simpler alkyl group as an alkoxy substituent by changing the –yl ending of the alkyl group to –oxy. • Name the remaining alkyl group as an alkane, with the alkoxy group as a substituent bonded to this chain.

  13. Alcohols, Ethers and Epoxides • Cyclic ethers have an O atom in the ring. A common example is tetrahydrofuran (THF). This name has a different origin

  14. Nomenclature of Epoxides • Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides. • To name an epoxide as an epoxyalkane, first name the alkane chain or ring to which the O atom is attached, • and use the prefix “epoxy” to name the epoxide as a substituent. • Use two numbers to designate the location of the atoms to which the O’s are bonded.

  15. Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides. • Epoxides bonded to a chain of carbon atoms can also be named as derivatives of oxirane, the simplest epoxide having two carbons and one oxygen atom in a ring. • The oxirane ring is numbered to put the O atom at position one, and the first substituent at position two. • No number is used for a substituent in a monosubstituted oxirane.

  16. Epoxides can be named in three different ways—As epoxyalkanes, oxiranes, or alkene oxides. • Epoxides are also named as alkene oxides, since they are often prepared by adding an O atom to an alkene. To name an epoxide in this way: • Mentally replace the epoxide oxygen with a double bond. • Name the alkene. • Add the word oxide.

  17. Alcohols, Ethers and Epoxides Physical Properties • Alcohols, ethers and epoxides exhibit dipole-dipole interactions because they have a bent structure with two polar bonds. • Alcohols are capable of intermolecular hydrogen bonding. Thus, alcohols are more polar than ethers and epoxides. • Steric factors affect hydrogen bonding.

  18. Alcohols, Ethers and Epoxides

  19. Alcohols, Ethers and Epoxides Interesting Alcohols glycerol Sugars, cellulose, starch etc.

  20. Alcohols, Ethers and Epoxides Interesting Ethers Diethyl etheris the best-known ether, first used as a general anesthetic in the 1830s. Because of its high flammability and side-effects on some patients, its place as an anesthetic has been taken by other substances. Cyclic polyether : crown ether

  21. Alcohols, Ethers and Epoxides Interesting Ethers • The ability of crown ethers to complex cations can be exploited in nucleophilic substitution reactions, as shown in Figure 9.5.

  22. Alcohols, Ethers and Epoxides Interesting Ethers : nature’s crown ether • Brevetoxin B is a naturally occurring polyether that interferes with Na+ion transport across cell membranes. • “Red Tide”

  23. Interesting Epoxides : epoxides are reactive! • Ethylene oxide : precursor of ethylene glycol, polyethylene glycol • Natural epoxides

  24. Preparation of Alcohols, Ethers, and Epoxides • Alcohols and ethers are both common products of nucleophilic substitution of alkyl halides. Williamson ether synthesis.

  25. Preparation of Alcohols, Ethers, and Epoxides

  26. Williamson ether synthesis. • SN2 reaction • Strong Nucleophile (An alkoxide salt) is needed to make an ether. • Cyclic ethers are formed through intramolecular reaction. When n= -2, epoxides

  27. Preparation of Epoxides • Organic compounds that contain both a hydroxy group and a halogen atom on adjacent carbons are called halohydrins.

  28. Reactions of Alcohols Substitution, elimination, oxidation • The OH group in alcohols is a very poor leaving group. • For an alcohol to undergo nucleophilic substitution, OH must be converted into a better leaving group (activated). • By using acid, ¯OH can be converted into H2O, a good leaving group. • By converting to sulfonates, ¯OH can be converted into ¯OSO2R, a good leaving group.

  29. Reactions of Alcohols

  30. Reactions of Ethers and Epoxides Ethers do not have a good leaving group and are difficult to activate. Epoxides are highly reactive due to high angle strain.

  31. Reactions of Alcohols—Dehydration • Dehydration, like dehydrohalogenation, is a  elimination reaction in which the elements of OH and H are removed from the  and  carbon atoms respectively. • Dehydration is typically carried out using H2SO4 and other strong acids, or phosphorus oxychloride (POCl3) in the presence of an amine base.

  32. Reactions of Alcohols—Dehydration • Typical acids used for alcohol dehydration : H2SO4 or p-toluenesulfonic acid (TsOH). strong organic acid • More substituted alcohols dehydrate more easily, giving rise to the following order of reactivity.

  33. Alcohols, Ethers and Epoxides Reactions of Alcohols—Dehydration • Dehydration follows the Zaitsev rule. • The more substituted alkene is the major product when a mixture of constitutional isomers is possible.

  34. Alcohols, Ethers and Epoxides Reactions of Alcohols—Dehydration • Secondary and 3° alcohols react by an E1 mechanism

  35. Alcohols, Ethers and Epoxides Reactions of Alcohols—Dehydration • The E1 dehydration of 20 and 30 alcohols with acid gives clean elimination products without any by-products formed from an SN1 reaction. • Clean elimination takes place because the reaction mixture contains no good nucleophile to react with the intermediate carbocation, so no competing SN1 reaction occurs. • This makes the E1 dehydration of alcohols much more synthetically useful than the E1 dehydrohalogenation of alkyl halides.

  36. Alcohols, Ethers and Epoxides Reactions of Alcohols—Dehydration • 1° alcohols undergo dehydration following an E2 mechanism. • 1° carbocations are highly unstable, their dehydration cannot occur by an E1 mechanism involving a carbocation intermediate.

  37. Reactions of Alcohols—Dehydration Whiledehydrations have favorable entropy changes, the position of equilibrium favors reactants. Operation of the Principle of Le Châtelier in Alcohol Dehydrations • removing a product from a reaction mixture as it is formed drives the equilibrium to the right, forming more product. • Thus, the alkene, which usually has a lower boiling point than the starting alcohol, can be removed by distillation as it is formed, thus driving the equilibrium to the right to favor production of more product.

  38. Carbocation Rearrangements

  39. Alcohols, Ethers and Epoxides Carbocation Rearrangements • Often, when carbocations are intermediates, a less stable carbocation will be converted into a more stable carbocation by a shift of a hydrogen or an alkyl group. This is called a rearrangement.

  40. Carbocation Rearrangements • A 1,2-shift can convert a less stable carbocation into a more stable carbocation. • Rearrangements can occur whenever a carbocation is formed as a reactive intermediate. 1,2-hydride shift

  41. Dehydration of Alcohols Using POCl3 and Pyridine • Dehydration under basic condition • A common method uses phosphorus oxychloride (POCl3) and pyridine (an amine base) in place of H2SO4 or TsOH. Dehydration proceeds by an E2 mechanism.

  42. Dehydration of Alcohols Using POCl3 and Pyridine Mechanism : E2 mechanism

  43. Dehydration of Alcohols in the natural product synthesis

  44. Conversion of Alcohols to Alkyl Halides with HX • Substitution reactions do not occur with alcohols unless ¯OH is converted into a good leaving group. • The reaction of alcohols with HX (X = Cl, Br, I) is a general method to prepare 1°, 2°, and 3° alkyl halides.

  45. Conversion of Alcohols to Alkyl Halides with HX • More substituted alcohols usually react more rapidly with HX: • The mechanism depends on the structure of the R group.

  46. Conversion of Alcohols to Alkyl Halides with HX 1o alcohols and methanol react via an SN2 mechanism

  47. 2o and 3o alcohols react via an SN1 mechanism

  48. Alcohols, Ethers and Epoxides Conversion of Alcohols to Alkyl Halides with HX • The reactivity of hydrogen halides increases with increasing acidity.

  49. Conversion of Alcohols to Alkyl Halides with HX • 2o alcohols

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