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Alcohols

Alcohols. 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°. Classification of Alcohols. Primary: carbon with — OH is bonded to one other carbon.

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Alcohols

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  1. Alcohols

  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°.

  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.

  4. 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 Examples of Classifications Secondary alcohol Primary alcohol Tertiary alcohol

  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.

  6. 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 Examples of Nomenclature 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

  7. O H C H C H C H C H C H 2 2 3 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 5 4 3 2 1 4-penten-2-ol pent-4-ene-2-ol

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

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

  10. 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 Common Names • Alcohol can be named as alkyl alcohol. • Useful only for small alkyl groups. isobutyl alcohol sec-butyl alcohol

  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

  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

  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. 4-methylphenol (para-cresol) 3-chlorophenol (meta-chlorophenol)

  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.

  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.

  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.

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

  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

  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

  20. 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.

  21. 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.

  22. Synthesis of Alcohols (Review) • Alcohols can be synthesized by nucleophilic substitution of alkyl halide. • Hydration of alkenes also produce alcohols: • Water in acid solution (suffers from rearragements) • Oxymercuration–demercuration • Hydroboration–oxidation

  23. Synthesis of Alcohols (Review) • Alcohols can be synthesized by nucleophilic substitution of alkyl halide. • Hydration of alkenes also produce alcohols: Chapter 10 23

  24. 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 24

  25. 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 25

  26. 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 26

  27. 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 27

  28. 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 28

  29. Alcohol Reactions • Dehydration to alkene • Oxidation to aldehyde, ketone • Substitution to form alkyl halide • Reduction to alkane • Esterification • Williamson synthesis of ether Chapter 11 29

  30. Summary Table Chapter 11 30

  31. 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 31

  32. 1º, 2º, 3º Carbons Chapter 11 32

  33. 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 33

  34. 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 34

  35. 3° Alcohols Don’t Oxidize • Cannot lose 2 H’s • Basis for chromic acid test Chapter 11 35

  36. Reduction of Alcohols • Dehydrate with conc. H2SO4, then add H2 Chapter 11 36

  37. 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 37

  38. 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 38

  39. 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 39

  40. Esterification • Fischer: alcohol + carboxylic acid • Nitrate esters • Phosphate esters Chapter 11 40

  41. Fischer Esterification • Acid + Alcohol yields Ester + Water • Sulfuric acid is a catalyst. • Each step is reversible. Chapter 11 41

  42. End Alcohols

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