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Chapter 5 Alcohols Thiols Ethers

Chapter 5 Alcohols Thiols Ethers. Structure of Water and Methanol. Oxygen is sp 3 hybridized and tetrahedral. The H — O — H angle in water is 104.5°. The C — O — H angle in methyl alcohol is 108.9°. Chapter 10. 2. Classification of Alcohols.

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Chapter 5 Alcohols Thiols Ethers

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  1. Chapter 5Alcohols Thiols Ethers

  2. Structure of Water and Methanol Oxygen is sp3 hybridized and tetrahedral. The H—O—H angle in water is 104.5°. The C—O—H angle in methyl alcohol is 108.9°. Chapter 10 2

  3. Classification of Alcohols Primary: carbon with —OH is bonded to one other carbon. Secondary: carbon with —OH is bonded to two other carbons. Tertiary: carbon with —OH is bonded to three other carbons. Aromatic (phenol): —OH is bonded to a benzene ring. Chapter 10 3

  4. Examples of Classifications C H C H 3 3 C H C H C H O H C H C O H * 3 2 3 * C H 3 O H * C H C H C H C H 3 2 3 Primary alcohol Secondary alcohol Tertiary alcohol Chapter 10 4

  5. IUPAC Nomenclature Find the longest carbon chain containing the carbon with the —OH group. Drop the -e from the alkane name, add -ol. Number the chain giving the —OH group the lowest number possible. Number and name all substituents and write them in alphabetical order. Chapter 10 5

  6. Examples of Nomenclature O H C H 3 C H C H C H C H C H C H C H O H 3 2 3 3 2 C H 3 C H C O H 3 C H 3 1 2 3 4 3 2 1 2-methyl-1-propanol 2-methylpropan-1-ol 2-butanol butan-2-ol 2 1 2-methyl-2-propanol 2-methylpropan-2-ol Chapter 10 6

  7. Alkenols (Enols) Hydroxyl group takes precedence. Assign the carbon with the —OH the lowest number. End the name in –ol, but also specify that there is a double bond by using the ending –ene before -ol O H C H C H C H C H C H 2 2 3 5 4 3 2 1 4-penten-2-ol pent-4-ene-2-ol Chapter 10 7

  8. Naming Priority Acids Esters Aldehydes Ketones Alcohols Amines Alkenes Alkynes Alkanes Ethers Halides Highest ranking Lowest ranking Chapter 10 8

  9. Hydroxy Substituent When —OH is part of a higher priority class of compound, it is named as hydroxy. O H C H C H C H C O O H 2 2 2 carboxylic acid 4 3 2 1 4-hydroxybutanoic acid also known as g-hydroxybutyric acid (GHB) Chapter 10 9

  10. Common Names Alcohol can be named as alkyl alcohol. Useful only for small alkyl groups. O H C H 3 C H C H C H C H C H C H C H O H 3 2 3 3 2 isobutyl alcohol sec-butyl alcohol Chapter 10 10

  11. Naming Diols Two numbers are needed to locate the two —OH groups. Use -diol as suffix instead of -ol. 1 2 3 4 5 6 hexane-1,6- diol Chapter 10 11

  12. Glycols 1, 2-diols (vicinal diols) are called glycols. Common names for glycols use the name of the alkene from which they were made. ethane-1,2- diol ethylene glycol propane-1,2- diol propylene glycol Chapter 10 12

  13. Phenol Nomenclature —OH group is assumed to be on carbon 1. For common names of disubstituted phenols, use ortho- for 1,2; meta- for 1,3; and para- for 1,4. Methyl phenols are cresols. 3-chlorophenol (meta-chlorophenol) 4-methylphenol (para-cresol) Chapter 10 13

  14. Solved Problem 1 Give the systematic (IUPAC) name for the following alcohol. Solution The longest chain contains six carbon atoms, but it does not contain the carbon bonded to the hydroxyl group. The longest chain containing the carbon bonded to the —OH group is the one outlined by the green box, containing five carbon atoms. This chain is numbered from right to left in order to give the hydroxyl-bearing carbon atom the lowest possible number. The correct name for this compound is 3-(iodomethyl)-2-isopropylpentan-1-ol. Chapter 10 14

  15. Physical Properties Alcohols have high boiling points due to hydrogen bonding between molecules. Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases. Chapter 10 15

  16. Boiling Points of alcohols Alcohols have higher boiling points than ethers and alkanes because alcohols can form hydrogen bonds. The stronger interaction between alcohol molecules will require more energy to break them resulting in a higher boiling point. Chapter 10 16

  17. Solubility in Water Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases. Chapter 10 17

  18. Methanol “Wood alcohol” Industrial production from synthesis gas Common industrial solvent Toxic Dose: 100 mL methanol Used as fuel at Indianapolis 500 Fire can be extinguished with water High octane rating Low emissions Lower energy content Invisible flame Chapter 10 18

  19. Ethanol Fermentation of sugar and starches in grains 12–15% alcohol, then yeast cells die Distillation produces “hard” liquors Azeotrope: 95% ethanol, constant boiling Denatured alcohol used as solvent Gasahol: 10% ethanol in gasoline Toxic dose: 200 mL Chapter 10 19

  20. Acidity of Alcohols pKa range: 15.5–18.0 (water: 15.7) Acidity decreases as the number of carbons increase. Halogens and other electron withdrawing groups increase the acidity. Phenol is 100 million times more acidic than cyclohexanol! Chapter 10 20

  21. Table of Ka Values Chapter 10 21

  22. Formation of Alkoxide Ions Ethanol reacts with sodium metal to form sodium ethoxide (NaOCH2CH3), a strong base commonly used for elimination reactions. More hindered alcohols like 2-propanol or tert-butanol react faster with potassium than with sodium. Chapter 10 22

  23. Formation of Phenoxide Ion The aromatic alcohol phenol is more acidic than aliphatic alcohols due to the ability of aromatic rings to delocalize the negative charge of the oxygen within the carbons of the ring. Chapter 10 23

  24. Charge Delocalization on the Phenoxide Ion The negative charge of the oxygen can be delocalized over four atoms of the phenoxide ion. There are three other resonance structures that can localize the charge in three different carbons of the ring. The true structure is a hybrid between the four resonance forms. Chapter 10 24

  25. Synthesis of Alcohols (Review) Alcohols can be synthesized by nucleophilic substitution of alkyl halide. Hydration of alkenes also produce alcohols: Chapter 10 25

  26. Synthesis of Vicinal Diols Vicinal diols can be synthesized by two different methods: Syn hydroxylation of alkenes Cold, dilute, basic potassium permanganate Chapter 10 26

  27. Reduction of Carbonyl Reduction of aldehyde yields 1º alcohol. Reduction of ketone yields 2º alcohol. Reagents: Sodium borohydride, NaBH4 Lithium aluminum hydride, LiAlH4 Raney nickel Chapter 10 27

  28. Sodium Borohydride NaBH4 is a source of hydrides (H-) Hydride attacks the carbonyl carbon, forming an alkoxide ion. Then the alkoxide ion is protonated by dilute acid. Only reacts with carbonyl of aldehyde or ketone, not with carbonyls of esters or carboxylic acids. Chapter 10 28

  29. Lithium Aluminum Hydride LiAlH4 is source of hydrides (H-) Stronger reducing agent than sodium borohydride, but dangerous to work with. Reduces ketones and aldehydes into the corresponding alcohol. Converts esters and carboxylic acids to 1º alcohols. Chapter 10 29

  30. Reduction with LiAlH4 The LiAlH4 (or LAH) will add two hydrides to the ester to form the primary alkyl halide. The mechanism is similar to the attack of Grignards on esters. Chapter 10 30

  31. Reducing Agents NaBH4 can reduce aldehydes and ketones but not esters and carboxylic acids. LiAlH4 is a stronger reducing agent and will reduce all carbonyls. Chapter 10 31

  32. Catalytic Hydrogenation Raney nickel is a hydrogen rich nickel powder that is more reactive than Pd or Pt catalysts. This reaction is not commonly used because it will also reduce double and triple bonds that may be present in the molecule. Hydride reagents are more selective so they are used more frequently for carbonyl reductions. Chapter 10 32

  33. Thiols (Mercaptans) Sulfur analogues of alcohols are called thiols. The —SH group is called a mercapto group. Named by adding the suffix -thiol to the alkane name. They are commonly made by an SN2 reaction so primary alkyl halides work better. Chapter 10 33

  34. Synthesis of Thiols The thiolate will attack the carbon displacing the halide. This is an SN2 reaction so methyl halides will react faster than primary alkyl halides. To prevent dialylation use a large excess of sodium hydrosulfide with the alkyl halide. Chapter 10 34

  35. Alcohol Reactions Dehydration to alkene Oxidation to aldehyde, ketone Substitution to form alkyl halide Reduction to alkane Esterification Williamson synthesis of ether Chapter 11 35

  36. Summary Table Chapter 11 36

  37. Oxidation States Easy for inorganic salts (reduced, organic oxidized) CrO42- reduced to Cr2O3 KMnO4 reduced to MnO2 Oxidation: loss of H2, gain of O, O2, or X2 Reduction: gain of H2 or H-, loss of O, O2, or X2 Neither: gain or loss of H+, H2O, HX Chapter 11 37

  38. Oxidation States • Easy for inorganic salts (reduced, organic oxidized) • CrO42- reduced to Cr2O3 • KMnO4 reduced to MnO2 • Oxidation: loss of H2, gain of O, O2, or X2 • Reduction: gain of H2 or H-, loss of O, O2, or X2 • Neither: gain or loss of H+, H2O, HX Chapter 11

  39. 1º, 2º, 3º Carbons Chapter 11

  40. => Oxidation of 2° Alcohols • 2° alcohol becomes a ketone • Reagent is Na2Cr2O7/H2SO4 = H2CrO4 • Active reagent probably H2CrO4 • Color change: orange to greenish-blue Chapter 11

  41. Oxidation of 1° Alcohols • 1° alcohol to aldehyde to carboxylic acid • Difficult to stop at aldehyde • Use pyridinium chlorochromate (PCC) to limit the oxidation. • PCC can also be used to oxidize 2° alcohols to ketones. Chapter 11

  42. 3° Alcohols Don’t Oxidize • Cannot lose 2 H’s • Basis for chromic acid test Chapter 11

  43. Alcohol as a Nucleophile • ROH is weak nucleophile • RO- is strong nucleophile • New O-C bond forms, O-H bond breaks. Chapter 11

  44. Alcohol as an Electrophile • OH- is not a good leaving group unless it is protonated, but most nucleophiles are strong bases which would remove H+. • Convert to tosylate (good leaving group) to react with strong nucleophile (base).  + C-Nuc bond forms, C-O bond breaks Chapter 11

  45. Reduction of Alcohols • Dehydrate with conc. H2SO4, then add H2 Chapter 11

  46. Reaction with HBr • -OH of alcohol is protonated • -OH2+ is good leaving group • 3° and 2° alcohols react with Br- via SN1 • 1° alcohols react via SN2 Chapter 11

  47. Reaction with HCl • Chloride is a weaker nucleophile than bromide. • Add ZnCl2, which bonds strongly with -OH, to promote the reaction. • The chloride product is insoluble. • Lucas test: ZnCl2 in conc. HCl • 1° alcohols react slowly or not at all. • 2 alcohols react in 1-5 minutes. • 3 alcohols react in less than 1 minute. Chapter 11

  48. Limitations of HX Reactions • Poor yields of 1° and 2° chlorides • May get alkene instead of alkyl halide Chapter 11

  49. Reactions with Phosphorus Halides • Good yields with 1° and 2° alcohols • PCl3 for alkyl chloride (but SOCl2 better) • PBr3 for alkyl bromide Chapter 11

  50. Dehydration Reactions • Conc. H2SO4 (or H3PO4) produces alkene • Carbocation intermediate • Zaitsev product • Bimolecular dehydration produces ether • Low temp, 140°C and below, favors ether • High temp, 180°C and above, favors alkene Chapter 11

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