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Chapter 18
Introduction• Ethers      are compounds with two organic groups              (alkyl, aryl, or vinyl) bonded to the same   ...
•   Thiols (R–S–H) and sulfides (R–S–R’) are sulfur            R–S–H                R–S–R’    analogs of alcohols and ethe...
1.      Naming Ethers•    Ethers are named according to IUPAC rules:         – Simple ethers with no other functional grou...
– Simple ethers with no other functional groups are  named by identifying the two organic substituents  and adding the wor...
– If other functional groups are present, the ether part  is considered an alkoxy substituent
Practice Problem: Name the following ethers according to IUPAC                  rules (a) Diisopropyl ether               ...
2.   Structure and Properties of Ethers• The geometry around the O atom of an ether  (ROR) is similar to that of water (HO...
•   The oxygen atom gives ethers a slight dipole moment
•   Ethers have higher boiling points than alkanes with    similar MW
3.    Synthesis of Ethers•    Ethers can be synthesized by:      i.   Acid-catalyzed dehydration of alcohols      ii. Will...
i. Acid-catalyzed dehydration of alcohols•   Symmetrical ethers can be prepared by acid-catalyzed    dehydration of primar...
Practice Problem: Why do you suppose only symmetrical ethers                  are prepared by the sulfuric acid-catalyzed ...
ii. The Williamson Ether Synthesis•   Symmetrical and unsymmetrical ethers can be    prepared via the Williamson ether syn...
– Alkoxides are prepared by reaction of an alcohol  with a strong base such as sodium hydride, NaH   Acid      Base       ...
•   Silver Oxide-Catalyzed Ether Formation is a variation    of the Williamson ether synthesis     –   Direct reaction of ...
Mechanism of the Williamson Ether Synthesis•   The Williamson ether synthesis involves SN2 reaction    of an alkoxide ion ...
–   Unsymmetrical ethers should be synthesized by    reaction between the more hindered alkoxide ion and    less hindered ...
Practice Problem: How would you prepare the following ethers                  using Williamson synthesis?        a. Methyl...
iii. Alkoxymercuration of Alkenes•   Ethers can be prepared via Alkoxymercuration of    Alkenes followed by demercuration ...
Mechanism of Alkoxymercuration/Demercuration •   The mechanism involves:      –   Electrophilic addition of Hg2+ to an alk...
Practice Problem: How would you prepare the ethyl phenyl                  ether? Use whichever method you think is        ...
Practice Problem: Write the mechanism of the alkoxymercuration                  reaction of 1-methylcyclopentene with etha...
Practice Problem: How would you prepare the following ethers?                  Use whichever method you think is more     ...
3.    Reactions of Ethers•    Ethers undergo:      i.   Acidic Cleavage      ii. Claisen Rearrangement
i. Acidic Cleavage•   Ethers are generally unreactive to most reagents but    react with strong acids (HI and HBr) at high...
Mechanism of the Acidic Cleavage•   The acidic cleavage reaction takes place:     – via SN2 mechanism at the less highly s...
•   Ethers with primary and secondary alkyl groups react    with HI or HBr via SN2 mechanism     –   I- or Br- attacks the...
•   Ethers with a tertiary, benzylic, or allylic group react                  tertiary benzylic    with HI or HBr via SN1 ...
Practice Problem: Predict the products of the following reaction: •    Ethers with primary and secondary alkyl groups – vi...
Practice Problem: Predict the products of each of the following                  reactions:
Practice Problem: Write the mechanism of the acid-catalyzed                  cleavage of tert-butyl cyclohexyl ether to yi...
Practice Problem: Why are HI and HBr more effective than HCl                  in cleaving ethers?     Nucleophilicity usua...
ii. Claisen Rearrangement•   Claisen rearrangement is specific to allyl aryl ethers,                                      ...
Mechanism of the Claisen Rearrangement•   The reaction proceeds via a pericyclic mechanism:     –   a concerted reorganiza...
Practice Problem: What product would you expect from Claisen                  rearrangement of 2-butenyl phenyl ether?
4.       Cyclic Ethers: Epoxides•    Cyclic ether behaves like an acyclic ether, except     if the ring is 3-membered     ...
Epoxides (Oxiranes)•   An epoxide is a three-membered ring ether     – It is also called an oxirane (root “ir” from “tri” ...
•   Ethylene oxide (1,2-epoxyethane) is industrially                    1,2-epoxyethane    important as an intermediate   ...
•   In the laboratory, epoxides can be prepared by:     i.   Treatment of an alkene with a peroxyacid     ii. Treatment of...
i. Preparation of Epoxides Using a Peroxyacid•   An epoxide is prepared by treatment of an alkene with a    peroxyacid (RC...
•   The mechanism of epoxidation by treatment of an alkene    with a peroxyacid (RCO3H):     –   is a one-step process in ...
ii. Preparation of Epoxides from Halohydrins•   An epoxide is prepared by treatment of a halohydrin    with base     –   A...
•   The mechanism of epoxidation by treatment of a    halohydrin with a base is an intramolecular Williamson    ether synt...
Practice Problem: What product would you expect from reaction                  of cis-2-butene with m-chloroperoxybenzoic ...
Practice Problem: Reaction of trans-2-butene with m-chloropero                  -xybenzoic acid yields an epoxide differen...
5.         Ring-Opening Reactions of Epoxides•        There are two types of ring-opening reactions of         epoxides:  ...
i. Acid-Catalyzed Epoxide Opening•   Water adds to epoxides with dilute acid at room    temperature     – The product is a...
Ethylene Glycol• 1,2-ethanediol is synthesized from acid catalyzed  hydration of ethylene oxide   – Widely used as automob...
•   The mechanism of acid-catalyzed epoxide cleavage    involves:     – Protonation: Acid protonates oxygen       Protonat...
The mechanism of acid-catalyzed epoxide cleavage issimilar to the final step of alkene bromination
Halohydrins from Epoxides• Anhydrous HF, HBr, HCl, or HI also combines with  an epoxide     – This gives a trans product (...
Regiochemistry of Acid-Catalyzed Opening of Epoxides    – When both epoxide carbon atoms are either primary or      second...
• The mechanism is neither purely SN1 nor SN2.     – more stable, tertiary carbocation T.S character (SN1-like)           ...
Practice Problem: Predict the major product of the following                  reaction:
Practice Problem: Predict the major product of each of the                  following reactions:
Practice Problem: How would you prepare the following diols?
ii. Base-Catalyzed Epoxide Opening• Unlike other ethers, epoxides can be cleaved by base  as well as by acid      – Strain...
Addition of Grignards to Ethylene Oxide  • Grignard reagents cleave the ring of epoxides     – Reaction of ethylene oxide ...
• Base-catalyzed epoxide opening is SN2-like   – Attack of the nucleophile takes place at the less     hindered epoxide ca...
Practice Problem: Predict the major product of the following                  reactions:
6.       Crown Ethers•    Crown ethers are large rings consisting of repeating     (-OCH2CH2-) or similar units         – ...
•   Crown ethers are named as x-crown-y     –   x is the total number of atoms in the ring     –   y is the number of oxyg...
Uses of Crown Ethers• Complexes between crown ethers and ionic salts  are soluble in nonpolar organic solvents     – This ...
• Inorganic salts (eg. KF, KCN, and NaN3) dissolve  in organic solvents with the help of crown ethers      – This leaves t...
Practice Problem: 15-Crown-5 and 12-crown-4 ethers complex                  Na+ and Li+, respectively. Make models of     ...
7.     Thiols and Sulfides•   Thiols (RSH), also known as mercaptans, are sulfur            RSH    analogs of alcohols    ...
Sulfides• Sulfides (RSR’) are sulfur analogs of ethers            RSR’   – They are named by rules used for ethers, with s...
Thiols: Formation and Reaction• Thiols are prepared from alkyl halides by SN2  displacement with a sulfur nucleophile such...
Using Thiourea to Form Alkylthiols• For a pure alkylthiol, thiourea (NH2(C=S)NH2) is used             alkylthiol  as the n...
Oxidation of Thiols to Disulfides• Reaction of an alkylthiol (RSH) with bromine (Br2)  or iodine (I2) gives a disulfide (R...
Sulfides: Formation and ReactionThiolate ions (RS-)   – are formed by the reaction of a thiol with a base   – react with p...
Sulfides as Nucleophiles• Sulfur compounds are more nucleophilic than  their oxygen-compound analogs      – 3p electrons v...
• Trialkylsulfonium salts are useful alkylating agents:      – A nucleophile can attack one of the groups bonded to       ...
Oxidation of Sulfides• Unlike ethers, sulfides are easily oxidized.   – Sulfides are easily oxidized with H2O2 to the     ...
• Dimethyl sulfoxide (DMSO) is often used as a  polar aprotic solvent
Practice Problem: Name the following compounds:
Practice Problem: 2-Butene-1-thiol is one component of skunk                  spray. How would you synthesize this        ...
8.     Spectroscopy of Ethers• Infrared: C–O single-bond stretching 1050 to 1150  Infrared  cm−1 overlaps many other absor...
Infrared Spectroscopy• Ethers are difficult to distinguish by IR spectroscopy   – C–O single-bond stretching 1050 to 1150 ...
Nuclear Magnetic Resonance Spectroscopy• Proton NMR: H on a C next to ether O are shifted         NMR  downfield to 3.4 δ ...
• Proton NMR: H on a C next to ether O are shifted         NMR  downfield to 3.4 δ to 4.5 δ  – In epoxides, these H’s abso...
• Carbon NMR: C’s in ethers exhibit a downfield shift           NMR  to 50 δ to 80 δ   – Example: These C’s in methyl prop...
Practice Problem: The 1H NMR spectrum shown is that of an                  ether with the formula C4H8O. Propose a        ...
Chapter 18
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  1. 1. Chapter 18
  2. 2. Introduction• Ethers are compounds with two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R–O–R’, in a ring or in an R–O–R’ open chain industrial solvent anesthetic perfume solvent
  3. 3. • Thiols (R–S–H) and sulfides (R–S–R’) are sulfur R–S–H R–S–R’ analogs of alcohols and ethers, respectively – Sulfur replaces oxygen
  4. 4. 1. Naming Ethers• Ethers are named according to IUPAC rules: – Simple ethers with no other functional groups are named by identifying the two organic substituents and adding the word ether – If other functional groups are present, the ether part is considered an alkoxy substituent
  5. 5. – Simple ethers with no other functional groups are named by identifying the two organic substituents and adding the word ether
  6. 6. – If other functional groups are present, the ether part is considered an alkoxy substituent
  7. 7. Practice Problem: Name the following ethers according to IUPAC rules (a) Diisopropyl ether (d) 1-Methoxycyclohexene (b) Cyclopentyl propyl ether (e) Ethyl isobutyl ether (c) p-Bromoanisole or (f) Allyl vinyl ether 4-Bromo-1-methoxybenzene
  8. 8. 2. Structure and Properties of Ethers• The geometry around the O atom of an ether (ROR) is similar to that of water (HOH) – R-O-R has an ~ tetrahedral bond angle (112° in dimethyl ether) – The O atom is sp3-hybridized
  9. 9. • The oxygen atom gives ethers a slight dipole moment
  10. 10. • Ethers have higher boiling points than alkanes with similar MW
  11. 11. 3. Synthesis of Ethers• Ethers can be synthesized by: i. Acid-catalyzed dehydration of alcohols ii. Williamson ether synthesis iii. Alkoxymercuration of alkenes
  12. 12. i. Acid-catalyzed dehydration of alcohols• Symmetrical ethers can be prepared by acid-catalyzed dehydration of primary alcohols (SN2) – Example: Diethyl ether is prepared industrially by Example sulfuric acid–catalyzed dehydration of ethanol – Acid-catalyzed dehydration of secondary and tertiary alcohols yield alkenes (E1)
  13. 13. Practice Problem: Why do you suppose only symmetrical ethers are prepared by the sulfuric acid-catalyzed dehydration procedure? What product(s) would you expect if ethanol and 1-propanol were allowed to react together? In what ratio would the products be formed if the two alcohols were of equal reactivity?
  14. 14. ii. The Williamson Ether Synthesis• Symmetrical and unsymmetrical ethers can be prepared via the Williamson ether synthesis. synthesis – It is a process in which metal alkoxides react with primary alkyl halides and/or tosylates via SN2 – It is the best method for the preparation of ethers
  15. 15. – Alkoxides are prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH Acid Base Sodium salt of the alcohol
  16. 16. • Silver Oxide-Catalyzed Ether Formation is a variation of the Williamson ether synthesis – Direct reaction of alcohols in Ag2O with alkyl halide forms ether in one step – Example: Glucose reacts with excess iodomethane in the Example presence of Ag2O to generate a pentaether in 85% yield
  17. 17. Mechanism of the Williamson Ether Synthesis• The Williamson ether synthesis involves SN2 reaction of an alkoxide ion with a primary alkyl halide – An alkoxide nucleophile (RO-) displaces a halide ion (X-) via SN2 – Primary halides and tosylates work best for SN2 because more hindered substrates undergo competitive E2 elimination of HX
  18. 18. – Unsymmetrical ethers should be synthesized by reaction between the more hindered alkoxide ion and less hindered alkyl halide rather than vice versa Example: Synthesis of tert-butyl methyl ether Example
  19. 19. Practice Problem: How would you prepare the following ethers using Williamson synthesis? a. Methyl propyl ether b. Anisole (methyl phenyl ether) c. Benzyl isopropyl ether d. Ethyl 2,2-dimethylpropyl ether
  20. 20. iii. Alkoxymercuration of Alkenes• Ethers can be prepared via Alkoxymercuration of Alkenes followed by demercuration – Alkoxymercuration occurs when an alkene reacts with an alcohol in mercuric acetate or trifluoroacetate – Demercuration involves reduction of C-Hg by NaBH4
  21. 21. Mechanism of Alkoxymercuration/Demercuration • The mechanism involves: – Electrophilic addition of Hg2+ to an alkene, followed by reaction of intermediate cation with alcohol: Overall Markovnikov addition of alcohol to alkene – Reduction of C-Hg by NaBH4
  22. 22. Practice Problem: How would you prepare the ethyl phenyl ether? Use whichever method you think is more appropriate, the Williamson synthesis or the alkoxymercuration reaction.
  23. 23. Practice Problem: Write the mechanism of the alkoxymercuration reaction of 1-methylcyclopentene with ethanol. Use curved arrows to show the electron flowin each step.
  24. 24. Practice Problem: How would you prepare the following ethers? Use whichever method you think is more appropriate, the Williamson synthesis or the alkoxymercuration reaction. a. Butyl cyclohexyl ether b. Benzyl ethyl ether (C6H5CH2OCH2CH3) c. sec-Butyl tert-butyl ether d. Tetrahydrofuran
  25. 25. 3. Reactions of Ethers• Ethers undergo: i. Acidic Cleavage ii. Claisen Rearrangement
  26. 26. i. Acidic Cleavage• Ethers are generally unreactive to most reagents but react with strong acids (HI and HBr) at high temperature HBr – HI, HBr produce an alkyl halide from less hindered component by SN2 (tertiary ethers undergo SN1)
  27. 27. Mechanism of the Acidic Cleavage• The acidic cleavage reaction takes place: – via SN2 mechanism at the less highly substituted site if only primary and secondary alkyl are bonded to the ether O – via SN1 or E1 mechanism if one of the alkyl groups bonded to the ether O is tertiary
  28. 28. • Ethers with primary and secondary alkyl groups react with HI or HBr via SN2 mechanism – I- or Br- attacks the protonated ether at the less hindered site
  29. 29. • Ethers with a tertiary, benzylic, or allylic group react tertiary benzylic with HI or HBr via SN1 or E1 mechanism – These can produce stable intermediate carbocations – Example: tert-Butyl cyclohexyl ether reacts via E1 Example
  30. 30. Practice Problem: Predict the products of the following reaction: • Ethers with primary and secondary alkyl groups – via SN2 • Ethers with a tertiary, benzylic, or allylic group – via SN1 or E1 tertiary benzylic
  31. 31. Practice Problem: Predict the products of each of the following reactions:
  32. 32. Practice Problem: Write the mechanism of the acid-catalyzed cleavage of tert-butyl cyclohexyl ether to yield cyclohexanol and 2-methylpropene
  33. 33. Practice Problem: Why are HI and HBr more effective than HCl in cleaving ethers? Nucleophilicity usually increases going down a column of the periodic table The halide reactivity order is I- > Br- > Cl-
  34. 34. ii. Claisen Rearrangement• Claisen rearrangement is specific to allyl aryl ethers, ethers ArOCH2CH=CH2 – Heating the allyl aryl ether to 200–250°C leads to an o-allylphenol – Result is alkylation of the phenol in an ortho position
  35. 35. Mechanism of the Claisen Rearrangement• The reaction proceeds via a pericyclic mechanism: – a concerted reorganization of bonding electrons involving a 6-electron, 6-membered ring transition state leading to 6-allyl-2,4-cyclohexadienone intermediate• The mechanism is consistent with 14C labelling
  36. 36. Practice Problem: What product would you expect from Claisen rearrangement of 2-butenyl phenyl ether?
  37. 37. 4. Cyclic Ethers: Epoxides• Cyclic ether behaves like an acyclic ether, except if the ring is 3-membered – Dioxane and tetrahydrofuran are used as solvents
  38. 38. Epoxides (Oxiranes)• An epoxide is a three-membered ring ether – It is also called an oxirane (root “ir” from “tri” for 3- membered; prefix “ox” for oxygen; “ane” for saturated) – It has a unique chemical reactivity (behaves differently from other open-chain ethers) due to the strain of the 3-membered ring
  39. 39. • Ethylene oxide (1,2-epoxyethane) is industrially 1,2-epoxyethane important as an intermediate – It is the simplest epoxide (oxirane) – It is prepared by reaction of ethylene with oxygen at 300°C and silver oxide catalyst
  40. 40. • In the laboratory, epoxides can be prepared by: i. Treatment of an alkene with a peroxyacid ii. Treatment of a halohydrin with base
  41. 41. i. Preparation of Epoxides Using a Peroxyacid• An epoxide is prepared by treatment of an alkene with a peroxyacid (RCO3H) – m-chloroperoxybenzoic acid is a common peroxyacid used
  42. 42. • The mechanism of epoxidation by treatment of an alkene with a peroxyacid (RCO3H): – is a one-step process in which peroxyacids transfer oxygen to the alkene with syn stereochemistry (no intermediates)
  43. 43. ii. Preparation of Epoxides from Halohydrins• An epoxide is prepared by treatment of a halohydrin with base – Addition of HO-X to an alkene gives a halohydrin – Treatment of a halohydrin with base eliminates HX and gives an epoxide
  44. 44. • The mechanism of epoxidation by treatment of a halohydrin with a base is an intramolecular Williamson ether synthesis: – The nucleophilic alkoxide ion and the electrophilic alkyl halide are in the same molecule
  45. 45. Practice Problem: What product would you expect from reaction of cis-2-butene with m-chloroperoxybenzoic acid? Show the stereochemistry
  46. 46. Practice Problem: Reaction of trans-2-butene with m-chloropero -xybenzoic acid yields an epoxide different from that obtained by reaction of the cis isomer. Explain.
  47. 47. 5. Ring-Opening Reactions of Epoxides• There are two types of ring-opening reactions of epoxides: i. Acid-Catalyzed Epoxide Opening ii. Base-Catalyzed Epoxide Opening
  48. 48. i. Acid-Catalyzed Epoxide Opening• Water adds to epoxides with dilute acid at room temperature – The product is a 1,2-diol, also called vicinal glycol 1,2-diol (on adjacent C’s: vicinal) – Epoxides react under milder conditions because of ring strain
  49. 49. Ethylene Glycol• 1,2-ethanediol is synthesized from acid catalyzed hydration of ethylene oxide – Widely used as automobile antifreeze (lowers freezing point of water solutions)
  50. 50. • The mechanism of acid-catalyzed epoxide cleavage involves: – Protonation: Acid protonates oxygen Protonation – Backside attack of a nucleophile: water adds to nucleophile opposite side (trans addition)
  51. 51. The mechanism of acid-catalyzed epoxide cleavage issimilar to the final step of alkene bromination
  52. 52. Halohydrins from Epoxides• Anhydrous HF, HBr, HCl, or HI also combines with an epoxide – This gives a trans product (halohydrin)
  53. 53. Regiochemistry of Acid-Catalyzed Opening of Epoxides – When both epoxide carbon atoms are either primary or secondary, the nucleophile preferably attacks the less secondary highly substituted site (less hindered site) – When one of the epoxide carbon atoms is tertiary, the tertiary nucleophile attacks the more highly substituted site
  54. 54. • The mechanism is neither purely SN1 nor SN2. – more stable, tertiary carbocation T.S character (SN1-like) 1-like – back-side displacement of leaving group (SN2-like) 2-like
  55. 55. Practice Problem: Predict the major product of the following reaction:
  56. 56. Practice Problem: Predict the major product of each of the following reactions:
  57. 57. Practice Problem: How would you prepare the following diols?
  58. 58. ii. Base-Catalyzed Epoxide Opening• Unlike other ethers, epoxides can be cleaved by base as well as by acid – Strain of the three-membered ring is relieved on ring- opening – Hydroxide ion cleaves epoxides at elevated temperatures to give trans 1,2-diols
  59. 59. Addition of Grignards to Ethylene Oxide • Grignard reagents cleave the ring of epoxides – Reaction of ethylene oxide with Grignard reagent adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react Grignard reagent
  60. 60. • Base-catalyzed epoxide opening is SN2-like – Attack of the nucleophile takes place at the less hindered epoxide carbon
  61. 61. Practice Problem: Predict the major product of the following reactions:
  62. 62. 6. Crown Ethers• Crown ethers are large rings consisting of repeating (-OCH2CH2-) or similar units – They were discovered by Charles Pedersen (Dupont; early 1960’s)
  63. 63. • Crown ethers are named as x-crown-y – x is the total number of atoms in the ring – y is the number of oxygen atoms – Example: 18-crown-6 ether: 18-membered ring Example containing 6 oxygens atoms• Central cavity is electronegative and attracts cations
  64. 64. Uses of Crown Ethers• Complexes between crown ethers and ionic salts are soluble in nonpolar organic solvents – This allows reactions to be carried out under aprotic conditions – It thus creates reagents that are free of water that have useful properties
  65. 65. • Inorganic salts (eg. KF, KCN, and NaN3) dissolve in organic solvents with the help of crown ethers – This leaves the anion dissociated, enhancing reactivity Purple benzene
  66. 66. Practice Problem: 15-Crown-5 and 12-crown-4 ethers complex Na+ and Li+, respectively. Make models of these crown ethers, compare the sizes of the cavities.
  67. 67. 7. Thiols and Sulfides• Thiols (RSH), also known as mercaptans, are sulfur RSH analogs of alcohols – They are named with the suffix –thiol – SH group is called “mercapto group” (“capturer of group mercury”)
  68. 68. Sulfides• Sulfides (RSR’) are sulfur analogs of ethers RSR’ – They are named by rules used for ethers, with sulfide in place of ether for simple compounds and alkylthio in place of alkoxy
  69. 69. Thiols: Formation and Reaction• Thiols are prepared from alkyl halides by SN2 displacement with a sulfur nucleophile such as −SH – The alkylthiol product can undergo further reaction with the alkyl halide to give a symmetrical sulfide, giving a poorer yield of the thiol
  70. 70. Using Thiourea to Form Alkylthiols• For a pure alkylthiol, thiourea (NH2(C=S)NH2) is used alkylthiol as the nucleophile – This gives an intermediate alkylisothiourea salt, which is hydrolyzed cleanly to the alkyl thiourea – This avoids the problem of thiols undergoing further reaction with the alkyl halide to give dialkyl sulfides
  71. 71. Oxidation of Thiols to Disulfides• Reaction of an alkylthiol (RSH) with bromine (Br2) or iodine (I2) gives a disulfide (RSSR’) – The thiol is oxidized in the process and the halogen is reduced – It is reversed when the disulfide is reduced back to thiol by treatment with Zn and H+ – Disulfide “bridges” form the cross-links between protein chains (stabilize the three dimensional
  72. 72. Sulfides: Formation and ReactionThiolate ions (RS-) – are formed by the reaction of a thiol with a base – react with primary or secondary alkyl halide to give sulfides (RSR’) RSR’ – are excellent nucleophiles and react with many electrophiles via SN2 mechanism
  73. 73. Sulfides as Nucleophiles• Sulfur compounds are more nucleophilic than their oxygen-compound analogs – 3p electrons valence electrons (on S) are less tightly held than 2p electrons (on O)• Sulfides react with primary alkyl halides via SN2 to give trialkylsulfonium salts (R3S+) – This is unlike dialkyl ethers
  74. 74. • Trialkylsulfonium salts are useful alkylating agents: – A nucleophile can attack one of the groups bonded to the positively charged sulfur, displacing a neutral sulfide as leaving group
  75. 75. Oxidation of Sulfides• Unlike ethers, sulfides are easily oxidized. – Sulfides are easily oxidized with H2O2 to the sulfoxide (R2SO) – Oxidation of a sulfoxide with a peroxyacid yields a sulfone (R2SO2)
  76. 76. • Dimethyl sulfoxide (DMSO) is often used as a polar aprotic solvent
  77. 77. Practice Problem: Name the following compounds:
  78. 78. Practice Problem: 2-Butene-1-thiol is one component of skunk spray. How would you synthesize this substance from methyl 2-butenoate? From 1,3-butadiene?
  79. 79. 8. Spectroscopy of Ethers• Infrared: C–O single-bond stretching 1050 to 1150 Infrared cm−1 overlaps many other absorptions.• Proton NMR: H on a C next to ether O are shifted NMR downfield to 3.4 δ to 4.5 δ – The 1H NMR spectrum of dipropyl ether shows the these signals at 3.4 δ – In epoxides, these H’s absorb at δ 2.5 to δ 3.5 in their 1 H NMR spectra• Carbon NMR: C’s in ethers exhibit a downfield shift NMR to δ 50 to δ 80
  80. 80. Infrared Spectroscopy• Ethers are difficult to distinguish by IR spectroscopy – C–O single-bond stretching 1050 to 1150 cm−1 overlaps many other absorptions. IR spectrum: CH3CH2OCH2CH3
  81. 81. Nuclear Magnetic Resonance Spectroscopy• Proton NMR: H on a C next to ether O are shifted NMR downfield to 3.4 δ to 4.5 δ – The 1H NMR spectrum of dipropyl ether shows the these signals at 3.4 δ
  82. 82. • Proton NMR: H on a C next to ether O are shifted NMR downfield to 3.4 δ to 4.5 δ – In epoxides, these H’s absorb at 2.5 to 3.5 δ in their 1H NMR spectra – Example: 1,2-epoxypropane
  83. 83. • Carbon NMR: C’s in ethers exhibit a downfield shift NMR to 50 δ to 80 δ – Example: These C’s in methyl propyl ether absorb at 58.5 and 74.8 δ – Example: These C’s in anisole absorb at 54.8 δ
  84. 84. Practice Problem: The 1H NMR spectrum shown is that of an ether with the formula C4H8O. Propose a structure.
  85. 85. Chapter 18
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