Chapter 17 --Alcohols and Phenols

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Chapter 17 --Alcohols and Phenols Powered By Docstoc
					     Chapter 17: Alcohols and
     Phenols


Based on McMurry’s Organic Chemistry, 7th edition
Alcohols and Phenols
 Alcohols contain an OH group connected to a a saturated C (sp 3)
 They are important solvents and synthesis intermediates
 Phenols contain an OH group connected to a carbon in a benzene ring
 Methanol, CH3OH, called methyl alcohol, is a common solvent, a fuel
  additive, produced in large quantities
 Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beverage
 Phenol, C6H5OH (―phenyl alcohol‖) has diverse uses - it gives its name
  to the general class of compounds
 OH groups bonded to vinylic, sp2-hybridized carbons are called enols




                                                                           2
Why this Chapter?
 To begin to study oxygen-containing
  functional groups

 These groups lie at the heart of biological
  chemistry




                                                3
17.1 Naming Alcohols and Phenols
 General classifications of alcohols based on
  substitution on C to which OH is attached
 Methyl (C has 3 H’s), Primary (1°) (C has two
  H’s, one R), secondary (2°) (C has one H,
  two R’s), tertiary (3°) (C has no H, 3 R’s),




                                                  4
IUPAC Rules for Naming Alcohols
 Select the longest carbon chain containing the hydroxyl
  group, and derive the parent name by replacing the -e
  ending of the corresponding alkane with -ol
 Number the chain from the end nearer the hydroxyl group
 Number substituents according to position on chain, listing
  the substituents in alphabetical order




                                                                5
Naming Phenols
 Use ―phenol‖ (the French name for benzene)
  as the parent hydrocarbon name, not
  benzene
 Name substituents on aromatic ring by their
  position from OH




                                                6
17.2 Properties of Alcohols and
Phenols
 The structure around O of the alcohol or phenol is similar to that in
    water, sp3 hybridized
   Alcohols and phenols have much higher boiling points than similar
    alkanes and alkyl halides
   A positively polarized OH hydrogen atom from one molecule is
    attracted to a lone pair of electrons on a negatively polarized oxygen
    atom of another molecule
   This produces a force that holds the two molecules together
   These intermolecular attractions are present in solution but not in the
    gas phase, thus elevating the boiling point of the solution




                                                                              7
Properties of Alcohols and
Phenols: Acidity and Basicity
 Weakly basic and weakly acidic
 Alcohols are weak Brønsted bases
 Protonated by strong acids to yield oxonium ions,
  ROH2+




                                                      8
Alcohols and Phenols are Weak
Brønsted Acids
 Can transfer a proton to water to a very small
  extent
 Produces H3O+ and an alkoxide ion, RO, or
  a phenoxide ion, ArO




                                                   9
Acidity Measurements
 The acidity constant, Ka, measures the extent to
  which a Brønsted acid transfers a proton to water
              [A] [H3O+]
       Ka = —————           and pKa = log Ka
              [HA]
 Relative acidities are more conveniently presented on
  a logarithmic scale, pKa, which is directly proportional
  to the free energy of the equilibrium
 Differences in pKa correspond to differences in free
  energy
 Table 17.1 presents a range of acids and their pKa
  values

                                                             10
pKa Values for Typical OH Compounds




                                  11
Relative Acidities of Alcohols
 Simple alcohols are about as acidic as water
 Alkyl groups make an alcohol a weaker acid
 The more easily the alkoxide ion is solvated by water
  the more its formation is energetically favored
 Steric effects are important




                                                          12
Inductive Effects
 Electron-withdrawing groups make an alcohol a
  stronger acid by stabilizing the conjugate base
  (alkoxide)




                                                    13
Generating Alkoxides from Alcohols
  Alcohols are weak acids – requires a strong base to
   form an alkoxide such as NaH, sodium amide
   NaNH2, and Grignard reagents (RMgX)
  Alkoxides are bases used as reagents in organic
   chemistry




                                                         14
Phenol Acidity
 Phenols (pKa ~10) are much more acidic than
  alcohols (pKa ~ 16) due to resonance stabilization of
  the phenoxide ion
 Phenols react with NaOH solutions (but alcohols do
  not), forming salts that are soluble in dilute aqueous
  solution
 A phenolic component can be separated from an
  organic solution by extraction into basic aqueous
  solution and is isolated after acid is added to the
  solution




                                                           15
Nitro-Phenols
 Phenols with nitro groups at the ortho and para
  positions are much stronger acids




                                                    16
17.3 Preparation of Alcohols: A
Review
 Alcohols are derived from many types of compounds
 The alcohol hydroxyl can be converted to many other
  functional groups
 This makes alcohols useful in synthesis




                                                        17
Review: Preparation of Alcohols by
Regiospecific Hydration of Alkenes
 Hydroboration/oxidation: syn, non-Markovnikov
  hydration
 Oxymercuration/reduction: Markovnikov hydration




                                                    18
1,2-Diols
 Review: Cis-1,2-diols from hydroxylation of an alkene
  with OsO4 followed by reduction with NaHSO3
 Trans-1,2-diols from acid-catalyzed hydrolysis of
  epoxides




                                                          19
17.4 Alcohols from Reduction of
Carbonyl Compounds
 Reduction of a carbonyl compound in general gives
  an alcohol
 Note that organic reduction reactions add the
  equivalent of H2 to a molecule




                                                      20
Reduction of Aldehydes and Ketones
  Aldehydes gives primary alcohols
  Ketones gives secondary alcohols




                                      21
Reduction Reagent: Sodium
Borohydride
 NaBH4 is not sensitive to moisture and it does not
  reduce other common functional groups
 Lithium aluminum hydride (LiAlH4) is more powerful,
  less specific, and very reactive with water
 Both add the equivalent of ―H-‖




                                                        22
Mechanism of Reduction
 The reagent adds the equivalent of hydride to the
  carbon of C=O and polarizes the group as well




                                                      23
Reduction of Carboxylic Acids and
Esters
 Carboxylic acids and esters are reduced to give
  primary alcohols
 LiAlH4 is used because NaBH4 is not effective




                                                    24
17.5 Alcohols from Reaction of Carbonyl
Compounds with Grignard Reagents
 Alkyl, aryl, and vinylic halides react with magnesium
  in ether or tetrahydrofuran to generate Grignard
  reagents, RMgX
 Grignard reagents react with carbonyl compounds to
  yield alcohols




                                                          25
Reactions of Grignard Reagents with
Carbonyl Compounds




                                      26
Reactions of Esters and Grignard
Reagents
 Yields tertiary alcohols in which two of the
  substituents carbon come from the Grignard reagent
 Grignard reagents do not add to carboxylic acids –
  they undergo an acid-base reaction, generating the
  hydrocarbon of the Grignard reagent




                                                       27
Grignard Reagents and Other Functional
Groups in the Same Molecule

 Can't be prepared if there are reactive functional
  groups in the same molecule, including proton donors




                                                         28
Mechanism of the Addition of a
Grignard Reagent
 Grignard reagents act as nucleophilic carbon anions
  (carbanions, : R) in adding to a carbonyl group
 The intermediate alkoxide is then protonated to
  produce the alcohol




                                                        29
17.6 Reactions of Alcohols
 Conversion of alcohols into alkyl halides:
- 3˚ alcohols react with HCl or HBr by SN1 through
  carbocation intermediate
- 1˚ and 2˚ alcohols converted into halides by treatment with
  SOCl2 or PBr3 via SN2 mechanism




                                                                30
31
Conversion of Alcohols into Tosylates
  Reaction with p-toluenesulfonyl chloride (tosyl
   chloride, p-TosCl) in pyridine yields alkyl tosylates,
   ROTos
  Formation of the tosylate does not involve the C–O
   bond so configuration at a chirality center is
   maintained
  Alkyl tosylates react like alkyl halides




                                                            32
Stereochemical Uses of Tosylates
  The SN2 reaction of an alcohol via a tosylate,
   produces inversion at the chirality center
  The SN2 reaction of an alcohol via an alkyl halide
   proceeds with two inversions, giving product with
   same arrangement as starting alcohol




                                                        33
Dehydration of Alcohols to Yield
Alkenes
 The general reaction: forming an alkene from an
  alcohol through loss of O-H and H (hence
  dehydration) of the neighboring C–H to give  bond
 Specific reagents are needed




                                                       34
Acid- Catalyzed Dehydration
 Tertiary alcohols are readily dehydrated with acid
 Secondary alcohols require severe conditions (75%
  H2SO4, 100°C) - sensitive molecules don't survive
 Primary alcohols require very harsh conditions –
  impractical
 Reactivity is the result of the nature of the
  carbocation intermediate




                                                       35
Dehydration with POCl3
 Phosphorus oxychloride in the amine solvent pyridine
  can lead to dehydration of secondary and tertiary
  alcohols at low temperatures
 An E2 via an intermediate ester of POCl2 (see Figure
  17.7)




                                                         36
Conversion of Alcohols into
Esters




                              37
17.7 Oxidation of Alcohols
 Can be accomplished by inorganic reagents, such as
  KMnO4, CrO3, and Na2Cr2O7 or by more selective,
  expensive reagents




                                                       38
Oxidation of Primary Alcohols
 To aldehyde: pyridinium chlorochromate (PCC,
  C5H6NCrO3Cl) in dichloromethane
 Other reagents produce carboxylic acids




                                                 39
Oxidation of Secondary Alcohols
 Effective with inexpensive reagents such as
  Na2Cr2O7 in acetic acid
 PCC is used for sensitive alcohols at lower
  temperatures




                                                40
Mechanism of Chromic Acid
Oxidation
 Alcohol forms a chromate ester followed by
  elimination with electron transfer to give ketone
 The mechanism was determined by observing the
  effects of isotopes on rates




                                                      41
17.8 Protection of Alcohols
 Hydroxyl groups can easily transfer their proton to a
  basic reagent
 This can prevent desired reactions
 Converting the hydroxyl to a (removable) functional
  group without an acidic proton protects the alcohol




                                                          42
Methods to Protect Alcohols
 Reaction with chlorotrimethylsilane in the presence of
  base yields an unreactive trimethylsilyl (TMS) ether
 The ether can be cleaved with acid or with fluoride
  ion to regenerate the alcohol




                                                           43
Protection-Deprotection
 An example of TMS-alcohol protection in a synthesis




                                                        44
17.9 Phenols and Their Uses
 Industrial process from readily available cumene
 Forms cumene hydroperoxide with oxygen at high
  temperature
 Converted into phenol and acetone by acid




                                                     45
17.10 Reactions of Phenols
 The hydroxyl group is a strongly activating, making
  phenols substrates for electrophilic halogenation,
  nitration, sulfonation, and Friedel–Crafts reactions
 Reaction of a phenol with strong oxidizing agents
  yields a quinone
 Fremy's salt [(KSO3)2NO] works under mild
  conditions through a radical mechanism




                                                         46
Quinones in Nature
 Ubiquinones mediate electron-transfer processes
  involved in energy production through their redox
  reactions




                                                      47
17.11 Spectroscopy of Alcohols
and Phenols
 Characteristic O–H stretching absorption at 3300 to
  3600 cm1 in the infrared
 Sharp absorption near 3600 cm -1 except if H-bonded:
  then broad absorption 3300 to 3400 cm 1 range
 Strong C–O stretching absorption near 1050 cm 1
  (See Figure 17.11)
 Phenol OH absorbs near 3500 cm -1




                                                         48
Nuclear Magnetic Resonance
Spectroscopy
   13CNMR: C bonded to OH absorbs at a lower
  field,  50 to 80
 1H NMR: electron-withdrawing effect of the nearby
  oxygen, absorbs at  3.5 to 4 (See Figure 17-13)
       Usually no spin-spin coupling between O–H proton and
        neighboring protons on C due to exchange reactions
        with moisture or acids
       Spin–spin splitting is observed between protons on the
        oxygen-bearing carbon and other neighbors
 Phenol O–H protons absorb at  3 to 8




                                                                 49
Mass Spectrometry
 Alcohols undergo alpha cleavage, a C–C bond
  nearest the hydroxyl group is broken, yielding a
  neutral radical plus a charged oxygen-containing
  fragment
 Alcohols undergo dehydration to yield an alkene
  radical anion




                                                     50

				
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Lingjuan Ma Lingjuan Ma MS
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