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Introduction to Organic Chemistry 2 ed William H. Brown

Introduction to Organic Chemistry 2 ed William H. Brown. Alcohols, Ethers, and Thiols. Chapter 8. Structure - Alcohols. The functional group of an alcohol is an -OH group bonded to an sp 3 hybridized carbon bond angles about the hydroxyl oxygen atom are approximately 109.5°

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Introduction to Organic Chemistry 2 ed William H. Brown

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  1. Introduction to Organic Chemistry2 edWilliam H. Brown

  2. Alcohols, Ethers, and Thiols Chapter 8

  3. Structure - Alcohols • The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbon • bond angles about the hydroxyl oxygen atom are approximately 109.5° • Oxygen is also sp3 hybridized • two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen • the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons

  4. Structure - Ethers • The functional group of an ether is an oxygen atom bonded to two carbon atoms • Oxygen is sp3 hybridized with bond angles of approximately 109.5°. In dimethyl ether, the C-O-C bond angle is 110.3°

  5. Structure - Thiols • The functional group of a thiol is an -SH (sulfhydryl) group bonded to an sp3 hybridized carbon • The bond angle about sulfur in methanethiol is 100.3°, which indicates that there is considerably more p character to the bonding orbitals of divalent sulfur than there is to oxygen

  6. Nomenclature-Alcohols • IUPAC names • the longest chain that contains the -OH group is taken as the parent. • the parent chain is numbered to give the -OH group the lowest possible number • the suffix -e is changed to -ol • Common names • the alkyl group bonded to oxygen is named followed by the word alcohol

  7. Nomenclature - Alcohols

  8. Nomenclature - Alcohols • Problem: write IUPAC names for these alcohols

  9. Nomenclature - Alcohols • Compounds containing more than one -OH group are named diols, triols, etc.

  10. Nomenclature - Alcohols • Unsaturated alcohols • the double bond is shown by the infix -en- • the hydroxyl group is shown by the suffix -ol • number the chain to give OH the lower number

  11. Nomenclature - Ethers • IUPAC: the longest carbon chain is the parent. Name the OR group as an alkoxy substituent • Common names: name the groups attached to oxygen followed by the word ether

  12. Nomenclature - Ethers • Although cyclic ethers have IUPAC names, their common names are more widely used

  13. Nomenclature - Thiols • IUPAC names: • the parent is the longest chain that contains the -SH group • change the suffix -e to -thiol • Common names: • name the alkyl group bonded to sulfur followed by the word mercaptan

  14. Physical Prop - Alcohols • Alcohols are polar compounds • Alcohols are associated in the liquid state by hydrogen bonding

  15. Physical Prop - Alcohols • Hydrogen bonding: the attractive force between a partial positive charge on hydrogen and a partial negative charge on a nearby oxygen, nitrogen, or fluorine atom • the strength of hydrogen bonding in water is approximately 5 kcal/mol • hydrogen bonds are considerably weaker than covalent bonds • nonetheless, they can have a significant effect on physical properties

  16. Physical Prop - Alcohols • Ethanol and dimethyl ether are constitutional isomers. • Their boiling points are dramatically different • ethanol forms intermolecular hydrogen bonds which increases attractive forces between its molecules, which results in a higher boiling point

  17. Physical Prop. - Alcohols • In relation to alkanes of comparable size and molecular weight, alcohols • have higher boiling points • are more soluble in water • The presence of additional -OH groups in a molecule further increases boiling points and solubility in water

  18. Physical Prop. - Alcohols

  19. Physical Prop. - Ethers • Ethers are polar molecules; • the difference in electronegativity between oxygen (3.5) and carbon (2.5) is 1.0 • each C-O bond is polar covalent • oxygen bears a partial negative charge and each carbon a partial positive charge

  20. Physical Prop. - Ethers • Ethers are polar molecules, but because of steric hindrance, only weak attractive forces exist between their molecules in the pure liquid state • Boiling points of ethers are • lower than alcohols of comparable MW and • close to those of hydrocarbons of comparable MW • Ethers hydrogen bond with H2O and are more soluble in H2O than are hydrocarbons

  21. Physical Prop. - Thiols • Low-molecular-weight thiols have a STENCH • the scent of skunks is due primarily to these two thiols

  22. Physical Prop. - Thiols • The difference in electronegativity between S (2.5) and H (2.1) is 0.4. Because of the low polarity of the S-H bond, thiols • show little association by hydrogen bonding • have lower boiling points and are less soluble in water than alcohols of comparable MW

  23. Acidity of Alcohols • In dilute aqueous solution, alcohols are weakly acidic

  24. Acidity of Alcohols

  25. Basicity of Alcohols • In the presence of strong acids, the oxygen atom of an alcohol behaves as a weak base • proton transfer from the strong acid forms an oxonium ion

  26. Reaction with Metals • Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides

  27. Conversion of ROH to RX • Conversion of an alcohol to an alkyl halide involves substitution of halogen for -OH at a saturated carbon • the most common reagents for this purpose are the halogen acids, HX, and thionyl chloride, SOCl2

  28. Reaction with HX • Water-soluble 3° alcohols react very rapidly with HCl, HBr, and HI. Low-molecular-weight 1° and 2° alcohols are unreactive under these conditions

  29. Reaction with HX • Water-insoluble 3° alcohols react by bubbling gaseous HX through a solution of the alcohol dissolved in diethyl ether or THF

  30. Reaction with HX • 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides

  31. Mechanism: 3° ROH + HCl • An SN1 reaction • Step 1: rapid, reversible acid-base reaction that transfers a proton to the OH group

  32. Mechanism: 3° ROH + HCl • Step 2: loss of H2O to give a carbocation intermediate

  33. Mechanism: 3° ROH + HCl • Step 3: reaction of the carbocation intermediate (a Lewis acid) with halide ion (a Lewis base)

  34. Mechanism: 1°ROH + HBr • An SN2 reaction • Step 1: rapid and reversible proton transfer

  35. Mechanism: 1° ROH + HX • Step 2: displacement of HOH by halide ion

  36. Mechanisms: SN1 vs SN2 • Reactivities of alcohols for SN1 and SN2 are in opposite directions

  37. Reaction with SOCl2 • Thionyl chloride is the most widely used reagent for the conversion of 1° and 2° alcohols to alkyl chlorides • a base, most commonly pyridine or triethylamine, is added to neutralize the HCl

  38. Dehydration of ROH • An alcohol can be converted to an alkene by elimination of H and OH from adjacent carbons (a b-elimination) • 1° alcohols must be heated at high temperature in the presence of an acid catalyst, such as H2SO4 or H3PO4 • 2° alcohols undergo dehydration at somewhat lower temperatures • 3° alcohols often require temperatures only at or slightly above room temperature

  39. Dehydration of ROH

  40. Dehydration of ROH • Where isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond generally predominates (Zaitsev rule)

  41. Dehydration of ROH • A three-step mechanism • Step 1: proton transfer from H3O+ to the -OH group to form an oxonium ion

  42. Dehydration of ROH • Step 2: the C-O bond is broken and water is lost, giving a carbocation intermediate

  43. Dehydration of ROH • Step 3: proton transfer of H+ from a carbon adjacent to the positively charged carbon to water. The sigma electrons of the C-H bond become the pi electrons of the carbon-carbon double bond

  44. Dehydration of ROH • Acid-catalyzed alcohol dehydration and alkene hydration are competing processes • large amounts of water favor alcohol formation • scarcity of water or experimental conditions where water is removed favor alkene formation

  45. Oxidation: 1° ROH • A primary alcohol can be oxidized to an aldehyde or a carboxylic acid, depending on the oxidizing agent and experimental conditions

  46. Oxidation: chromic acid • Chromic acid is prepared by dissolving chromium(VI) oxide or potassium dichromate in aqueous sulfuric acid

  47. Oxidation: 1° ROH • Oxidation of 1-octanol by chromic acid gives octanoic acid • the aldehyde intermediate is not isolated

  48. PCC • Pyridinium chlorochromate (PCC): a form of Cr(VI) prepared by dissolving CrO3 in aqueous HCl and adding pyridine to precipitate PCC • PCC is selective for the oxidation of 1° alcohols to aldehydes; it does not oxidize aldehydes further to carboxylic acids

  49. Oxidation: 1° ROH • PCC oxidation of a 1° alcohol to an aldehyde

  50. Oxidation: 2° ROH • 2° alcohols are oxidized to ketones by both chromic acid and and PCC

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