1 / 65

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°

anson
Download Presentation

Introduction to Organic Chemistry 2 ed William H. Brown

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related