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Organic Chemistry

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Title: Organic Chemistry


1
Organic Chemistry
William H. Brown Christopher S. Foote
2
Alcoholsand Thiols
  • Chapter 9

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 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
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

5
Nomenclature-Alcohols
6
Nomenclature of Alcohols
  • Problem Write the IUPAC name for each alcohol.

7
Nomenclature of Alcohols
  • Compounds containing more than one -OH group are
    named diols, triols, etc.

8
Nomenclature of 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

9
Physical Properties
  • Alcohols are polar compounds
  • They interact with themselves and with other
    polar compounds by dipole-dipole interactions
  • Dipole-dipole interaction the attraction between
    the positive end of one dipole and the negative
    end of another

10
Physical Properties
  • Hydrogen bonding when the positive end of one
    dipole is an H bonded to F, O, or N (atoms of
    high electronegativity) and the other end is F,
    O, or N
  • the strength of hydrogen bonding in water is
    approximately 21 kJ (5 kcal)/mol
  • hydrogen bonds are considerably weaker than
    covalent bonds
  • nonetheless, they can have a significant effect
    on physical properties

11
Hydrogen Bonding
12
Physical Properties
  • Ethanol and dimethyl ether are constitutional
    isomers.
  • Their boiling points are dramatically different
  • ethanol forms intermolecular hydrogen bonds which
    increase attractive forces between its molecules,
    which result in a higher boiling point

13
Physical Properties
  • 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 solubility in water
    and boiling point

14
Physical Properties
15
Acidity of Alcohols
  • In dilute aqueous solution, alcohols are weakly
    acidic

16
Acidity of Alcohols
17
Acidity of Alcohols
  • Acidity depends primarily on the degree of
    stabilization and solvation of the alkoxide ion
  • the negatively charged oxygens of methanol and
    ethanol are about as accessible as hydroxide ion
    for solvation these alcohol are about as acidic
    as water.
  • as the bulk of the alkyl group increases, the
    ability of water to solvate the alkoxide
    decreases, the acidity of the alcohol decreases,
    and the basicity of the alkoxide ion increases.

18
Reaction with Metals
  • Alcohols react with Li, Na, K, and other active
    metals to liberate hydrogen gas and form metal
    alkoxides

19
Reaction with NaH
  • Alcohols are also converted to metal salts by
    reaction with bases stronger than the alkoxide
    ion
  • one such base is sodium hydride

20
Reaction with HX
  • 3 alcohols react very rapidly with HCl, HBr, and
    HI
  • low-molecular-weight 1 and 2 alcohols are
    unreactive under these conditions
  • 1 and 2 alcohols require concentrated HBr and
    HI to form alkyl bromides and iodides

21
Reaction with HX
  • with HBr and HI, 2 alcohols generally give some
    rearrangement
  • 1 alcohols with extensive ?-branching give large
    amounts of rearranged product

22
Reaction with HX
  • Based on
  • the relative ease of reaction of alcohols with HX
    (3 gt 2 gt 1) and
  • the occurrence of rearrangements,
  • Chemists propose that reaction of 2 and 3
    alcohols with HX
  • occurs by an SN1 mechanism, and
  • involves a carbocation intermediate

23
Reaction with HX - SN1
  • Step 1 proton transfer to the OH group gives an
    oxonium ion
  • Step 2 loss of H2O gives a carbocation
    intermediate

24
Reaction with HX - SN1
  • Step 3 reaction of the carbocation intermediate
    (a Lewis acid) with halide ion (a Lewis base)
    gives the product

25
Reaction with HX - SN2
  • 1 alcohols react with HX by an SN2 mechanism
  • Step 1 rapid and reversible proton transfer
  • Step 2 displacement of HOH by halide ion

26
Reaction with HX
  • For 1 alcohols with extensive ?-branching
  • SN1 not possible because this pathway would
    require a 1 carbocation
  • SN2 not possible because of steric hindrance
    created by the ?-branching
  • These alcohols react by a concerted loss of HOH
    and migration of an alkyl group

27

Reaction with HX
  • Step 1 proton transfer gives an oxonium ion
  • Step 2 concerted elimination of HOH and
    migration of a methyl group gives a 3 carbocation

28
Reaction with HX
  • Step 3 reaction of the carbocation intermediate
    (a Lewis acid) with halide ion (a Lewis base)
    gives the product

29
Reaction with PBr3
  • An alternative method for the synthesis of 1 and
    2 alkyl bromides is reaction of an alcohol with
    phosphorus tribromide
  • this method gives less rearrangement than with HBr

30
Reaction with PBr3
  • Step 1 formation of a protonated
    dibromophosphite, which converts H2O, a poor
    leaving group, to a good leaving group
  • Step 2 displacement by bromide ion

31
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 catalyze the reaction and to
    neutralize the HCl

32
Reaction with SOCl2
  • Reaction of an alcohol with SOCl2 in the presence
    of a 3 amine is stereoselective proceeds with
    inversion of configuration

33
Reaction with SOCl2
  • Step 1 nucleophilic displacement of chlorine
  • Step 2 proton transfer to the 3 amine gives an
    alkyl chlorosulfite

34
Reaction with SOCl2
  • Step 3 backside displacement by chloride ion and
    decomposition of the chlorosulfite ester gives
    the alkyl chloride

35
Alkyl Sulfonates
  • Sulfonyl chlorides are derived from sulfonic
    acids
  • sulfonic acids are strong acids like sulfuric acid

36
Alkyl Sulfonates
  • A commonly used sulfonyl chloride is
    p-toluenesulfonyl chloride (Ts-Cl)

37
Alkyl Sulfonates
  • Another commonly used sulfonyl chloride is
    methanesulfonyl chloride (Ms-Cl)

38
Alkyl Sulfonates
  • Sulfonate anions are very weak bases (the
    conjugate base of a strong acid) and are very
    good leaving groups for SN2 reactions
  • Conversion of an alcohol to a sulfonate ester
    converts HOH, a very poor leaving group, into a
    sulfonic ester, a very good leaving group

39
Alkyl Sulfonates
  • This two-step procedure converts (S)-2-octanol to
    (R)-2-octyl acetate
  • Step 1 formation of a p-toluenesulfonate (Ts)
    ester

40
Alkyl Sulfonates
  • Step 2 nucleophilic displacement of tosylate

41
Dehydration of ROH
  • An alcohol can be converted to an alkene by
    elimination of H and OH from adjacent carbons (a
    ?-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 at or
    slightly above room temperature

42
Dehydration of ROH
43
Dehydration of ROH
  • where isomeric alkenes are possible, the alkene
    having the greater number of substituents on the
    double bond usually predominates (Zaitsev rule)

44
Dehydration of ROH
  • Dehydration of 1 and 2 alcohols is often
    accompanied by rearrangement
  • acid-catalyzed dehydration of 1-butanol gives a
    mixture of three alkenes

45
Dehydration of ROH
  • Based on evidence of
  • ease of dehydration (3 gt 2 gt 1)
  • prevalence of rearrangements
  • Chemists propose a three-step mechanism for the
    dehydration of 2 and 3 alcohols
  • because this mechanism involves formation of a
    carbocation intermediate in the rate-determining
    step, it is classified as E1

46
Dehydration of ROH
  • Step 1 proton transfer to the -OH group gives an
    oxonium ion
  • Step 2 loss of H2O gives a carbocation
    intermediate

47
Dehydration of ROH
  • Step 3 proton transfer 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

48
Dehydration of ROH
  • 1 alcohols with little ?-branching give terminal
    alkenes and rearranged alkenes
  • Step 1 proton transfer to OH gives an oxonium
    ion
  • Step 2 loss of H from the ?-carbon and H2O from
    the ?-carbon gives the terminal alkene

49
Dehydration of ROH
  • Step 3 shift of a hydride ion from ?-carbon and
    loss of H2O from the ?-carbon gives a carbocation
  • Step 4 proton transfer to solvent gives the
    alkene

50
Dehydration of ROH
  • Dehydration with rearrangement occurs by a
    carbocation rearrangement

51
Dehydration of ROH
  • Acid-catalyzed alcohol dehydration and alkene
    hydration are competing processes
  • Principle of microscopic reversibility the
    sequence of transition states and reactive
    intermediates in the mechanism of a reversible
    reaction must be the same, but in reverse order,
    for the backward reaction as for the forward
    reaction

52
Pinacol Rearrangement
  • The products of acid-catalyzed dehydration of a
    glycol are different from those of alcohols

53
Pinacol Rearrangement
  • Step 1 proton transfer to OH gives an oxonium
    ion
  • Step 2 loss of water gives a carbocation
    intermediate

54
Pinacol Rearrangement
  • Step 3 a 1,2- shift of methyl gives a more
    stable carbocation
  • Step 4 proton transfer to solvent completes the
    reaction

55
Oxidation 1 ROH
  • A primary alcohol can be oxidized to an aldehyde
    or a carboxylic acid, depending on the
    experimental conditions
  • to an aldehyde is a two-electron oxidation
  • to a carboxylic acid is a four-electron oxidation

56
Oxidation 1 ROH
  • A common oxidizing agent for this purpose is
    chromic acid, prepared by dissolving chromium(VI)
    oxide or potassium dichromate in aqueous sulfuric
    acid

57
Oxidation 1 ROH
  • Oxidation of 1-octanol gives octanoic acid
  • the aldehyde intermediate is not isolated

58
Oxidation 1 ROH
  • 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

59
Oxidation 1 ROH
  • PCC oxidation of a 1 alcohol to an aldehyde

60
Oxidation 2 ROH
  • 2 alcohols are oxidized to ketones by both PCC
    and chromic acid

61
Oxidation 1 2 ROH
  • The mechanism of chromic acid oxidation of an
    alcohol involves two steps
  • Step 1 formation of an alkyl chromate ester

62
Oxidation 1 2 ROH
  • Step 2 proton transfer to solvent and
    decomposition of the alkyl chromate ester gives
    the product

63
Oxidation 1 2 ROH
  • In chromic acid oxidation of a CHO group, it is
    the hydrated form that is oxidized

64
Oxidation of Glycols
  • Glycols are cleaved by oxidation with periodic
    acid, H5IO6 (or, alternatively HIO42H2O)

65
Oxidation of Glycols
  • the glycol undergoes a two-election oxidation
  • periodic acid undergoes a two-electron reduction

66
Oxidation of Glycols
  • The mechanism of periodic acid oxidation of a
    glycol is divided into two steps
  • Step 1 formation of a cyclic periodic ester
  • Step 2 redistribution of electrons within the
    five-membered ring

67
Thiols Structure
  • 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

68
Nomenclature
  • IUPAC names
  • the parent is the longest chain that contains the
    -SH group
  • change the suffix -e to -thiol
  • as a substituent, it is a sulfanyl group
  • Common names
  • name the alkyl group bonded to sulfur followed by
    the word mercaptan

69
Thiols Physical Properties
  • 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

70
Thiols Physical Properties
  • Low-molecular-weight thiols STENCH
  • the scent of skunks is due primarily to these two
    thiols

71
Thiols preparation
  • The most common preparation of thiols, RSH,
    depends on the very high nucleophilicity of
    hydrosulfide ion, HS-

72
Thiols acidity
  • Thiols are stronger acids than alcohols

73
Thiols acidity
  • When dissolved an aqueous NaOH, they are
    converted completely to alkylsulfide salts

74
Thiols oxidation
  • Thiols are oxidized to disulfides by a variety of
    oxidizing agents, including O2.
  • they are so susceptible to this oxidation that
    they must be protected from air during storage
  • the most common reaction of thiols in biological
    systems in interconversion between thiols and
    disulfides, -S-S-

75
Prob 9.22
  • From each pair of compounds, select the one
    more soluble in water.

76
Prob 9.24
  • From each pair of compounds, select the one
    more soluble in water.

77
Prob 9.25
  • Calculate the percent of each isomer present
    at equilibrium. Assume a value of DG (equatorial
    to axial) for cyclohexanol is 4.0 kJ (0.95
    kcal/mol).

78
Prob 9.26
  • Complete each acid-base reaction. Use curved
    arrows to show the flow of electrons.

79
Prob 9.26 (contd)
  • Complete each acid-base reaction. Use curved
    arrows to show the flow of electrons.

80
Prob 9.27
  • From each pair, select the stronger acid and
    write a structural formula for its conjugate base.

81
Prob 9.28
  • From each pair select the stronger base.
    Write a structural formula for its conjugate acid.

82
Prob 9.29
  • In each equilibrium, label the stronger acid
    and base, and the weaker acid and base. Estimate
    the position of equilibrium.

83
Prob 9.32
  • Complete each equation, but do not balance

84
Prob 9.32 (contd)
  • Complete each equation, but do not balance

85
Prob 9.34
  • When A or B is treated with HBr, racemic
    2,3-dibromobutane is formed. When C or D is
    treated with HBr, meso 2,3-dibromobutane is
    formed. Explain.

86
Prob 9.36
  • Show how to bring about each conversion.

87
Prob 9.36 (contd)
  • Show how to bring about each conversion.

88
Prob 9.37
  • Propose a mechanism for the following pinacol
    rearrangement.

89
Prob 9.40
  • Propose a mechanism for this reaction.

90
Prob 9.43
  • Show how to bring about this conversion.

91
Prob 9.44
  • Propose a structural formula for the product
    of this reaction and a mechanism for its
    formation.

92
Prob 9.45
  • Propose a mechanism for the formation of the
    products of this solvolysis.

93
Prob 9.46
  • Show how to convert cyclohexene to each
    compound.

94
  • Alcohols and Thiols

End of Chapter 9
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