Chapter 17 Alcohols and Phenols - s3.· 1 Chapter 17 Alcohols and Phenols Alcohols and Phenols -...

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Transcript of Chapter 17 Alcohols and Phenols - s3.· 1 Chapter 17 Alcohols and Phenols Alcohols and Phenols -...

  • 1

    Chapter 17

    Alcohols and Phenols

    Alcohols and Phenols

    - alcohols

    - compounds that have hydroxyl groups bonded to saturated, sp3-hybridized carbon atoms

    - phenols

    - compounds that have hydroxyl groups bonded to aromatic rings

    - enols

    - compounds that have a hydroxyl group bonded to a vinylic carbon

    OH

    OH

    C COH

    Naming Alcohols and Phenols

    - classified as primary (1o), secondary (2o), or tertiary (3o)

    - named as derivatives of the parent alkane

    C

    H

    OHR

    H

    C

    H

    OHR

    R

    C

    R

    OHR

    R

    primary (1o) secondary (2o) tertiary (2o)

  • 2

    Rules for Alcohols1) Select longest carbon chain containing the hydroxyl group; derive the parent name -e with -ol

    2) Number the alkane chain beginning at the end nearer the hydroxyl group

    3) Number substituents according to position on the chain and list substituents in alphabetical order

    Examples:

    C

    OH

    CH3

    CH2CH2CH3H3C

    HO H

    HO H

    CH

    CH3CHCH3OH

    2-methyl-2-pentanol cis-1,4-cyclohexanediol 3-phenyl-2-butanol

    Common Alcohols

    CH2OHH2C CHCH2OH C

    CH3

    CH3

    OHH3C

    HOCH2 CH2OH HOCH2 CHCH2OH

    OH

    benzyl alcoholallyl alcohol

    tert-butyl alcohol

    ethylene glycolglycerol

    Naming Phenols

    - phenol is both the name of the hydroxy compound and family name for hydroxy-substituted aromatic compounds

    OHH3C OH

    O2N NO2

    m-methylphenol 2,4-dinitrophenol

  • 3

    Properties of Alcohols and Phenols

    - alcohols and phenols have geometry nearly the same as water

    - R-O-H angle ~ 109o

    - oxygen atom is sp3 hybridized

    Physical Properties of Alcohols

    - alcohols have much higher boiling points than hydrocarbons and alkyl halides

    Compound Molecular Mass Boiling Point

    1-propanol

    butane

    chloroethane

    60 g/mol

    58 g/mol

    65 g/mol

    97 oC

    -0.5 oC

    12.5 oC

    Comparison of Boiling Points

  • 4

    - phenols have elevated boiling points relative to hydrocarbons

    phenol: bp = 181.7oC toluene: bp = 110.6oC

    OH CH3

    Physical Properties of Phenols

    Hydrogen Bonding by Alcohols and Phenols

    Basicity and Acidity

    - alcohols and phenols are both weakly basic and weakly acidic

    - reversibly protonated by strong acids to yield oxonium ions, ROH2+

    - dissociate slightly in dilute aqueous solution by donating a proton to water, generating H3O+ and alkoxide ion RO-, or a phenoxide ion, ArO-:

    OHH

    H X+ OH

    H HX

    ORH O

    HH+ R O O

    H

    H H+

  • 5

    Factors Affecting Basicity and AcidityAlcohols1) solvation and steric effects

    - smaller substituents promote solvation of the alkoxide ion that results from dissociation

    - the more easily the alkoxide ion is solvated, the more stable it is

    - the more stable the alkoxide ion, the more acidic the parent alcohol

    methoxide ion, CH3O-(pKa = 15.5) t-butoxide ion, (CH3)3CO

    -

    (pKa = 18.0)

    OH3C OC

    CH3H3C

    CH3

    2) inductive effects

    - electron-withdrawing substitutents stabilize an alkoxide ion by spreading the charge over a large volume, making the alcohol more acidic

    (pKa = 5.4) (pKa = 18.0)

    OC

    CH3H3C

    CH3

    OC

    CF3F3C

    CF3

    Generation of Alkoxides

    - alcohols react with alkali metals and with strong bases to form alkoxides

    C

    CH3

    CH3

    OHH3C + 2 K CCH3

    CH3

    OH3C K+ + H2

    tert-butyl alcohol potassium tert-butoxide

  • 6

    CH3OH + NaH CH3O- Na+ + H2methanol sodium methoxide

    CH3CH2OH + NaNH2 CH3CH2O- Na+ + H2sodium ethoxideethanol

    OH + CH3MgBr O +MgBr + H2O

    cyclohexanol bromomagnesiumcyclohexoxide

    Examples

    Phenols

    - phenols are a million times more acidic than alcohols

    - greater acidity is because the phenoxide ion is resonance-stabilized

    - delocalization of the negative charge over the ortho and para positions of the aromatic ring results in increased stability of the phenoxide anion

    - phenols with an electron-withdrawing substituent are generally more acidic since the substitutents delocalize the negative charge

    - phenols with an electron-donating substituent are generally less acidic since the substitutents destabilize the phenoxide ion

    O O

    EWG EDG

    Resonance Stabilization of thePhenoxide Ion

  • 7

    Preparation of AlcoholsReview

    1) hydration of alkenes by way of hydroboration/oxidation and oxymercuration/reduction

    CH3

    H

    BH2

    H3C

    H

    H

    OH

    H3C

    H

    OH

    H

    H3C

    HgOAc

    OH

    H3C

    H2O2

    -OH

    BH3

    THF

    Hg(OAc)2

    H2O

    NaBH4

    trans-2-methylcyclohexanol

    1-methylcyclohexanol

    1-methyl-cyclohexene

    2) hydroxylation of an alkene with OsO4 followed by reduction with NaHSO3

    CH3

    O

    O

    H3C

    H

    OsO

    O

    OH

    OH

    H3C

    H

    CH3

    H

    O

    CH3

    OH

    HO

    H

    OsO4

    pyridine

    Hg(OAc)2

    H2O

    NaHSO3

    H3O+1-methyl-

    cyclohexene

    1-methyl-trans-1,2-cyclohexanediol

    1-methyl-1,2-epoxy-cyclohexane

    1-methyl-cis-1,2-cyclohexanediol

  • 8

    Reduction of Carbonyl Compounds

    C

    OC

    OH[H]

    Reduction of Aldehydes and Ketones

    - aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols

    CR H

    O

    CR R'

    O

    CR H

    OH

    H

    CR H

    OH

    R'

    aldehyde primary alcohol

    ketone secondary alcohol

    [H]

    [H]

    Reagents for Aldehyde and Ketone Reduction

    - sodium borohydride NaBH4 is usually chosen because of its safety and ease of handling

    C H

    O

    CH3CH2CH2 C H

    OH

    H3CH2CH2C

    H

    1. NaBH4, EtOH

    2. H3O+

    butanal1-butanol (85%)

    COHH

    C

    O1. NaBH4, EtOH

    2. H3O+

    dicyclohexyl ketone dicyclohexylmethanol (88%)

    O

    COHH

    1. LiAlH4, ether

    2. H3O+

    2-cyclohexanone 2-cyclohexenol

  • 9

    Reduction of Carboxylic Acids and Esters

    - carboxylic acids and esters are reduced to give primary alcohols

    CR OH

    O

    CR OR'

    OC

    R H

    OH

    H

    primary alcohol

    [H]or

    - NaBH4 reduces esters slowly and does not reduce carboxylic acids; LiAlH4 reduces all carbonyl groups

    CH3(CH2)7CH CH(CH2)7 COH

    O1. LiAlH4, ether

    2. H3O+CH3(CH2)7CH CH(CH2)7CH2OH

    9-octadecenoic acid 9-octadecen-1-ol (87%)

    CH3CH2CH CH COCH3

    OCH3CH2CH CHCH2OH

    1. LiAlH4, ether

    2. H3O+

    methyl-2-pentenoate 2-penten-1-ol (91%)

    Examples

    Mechanism

    - can be regarded to involve attack of a hydride ion to the positively polarized, electrophilic carbon atom of the carbonyl group

    - protonation by acid gives the alcohol

    C

    O HC

    H

    O

    CH

    OH

    alcoholalkoxide

    intermediate

    carbonylcompound

  • 10

    Reactions with Grignard Reagents

    R X R MgX

    C

    OC

    R

    OH1. RMgX, ether

    2. H3O+

    Grignard reagent

    R = 1o, 2o, or 3o alkyl, aryl, vinylic

    X = Cl, Br, or I

    HOMgX+

    Formaldehyde Reaction

    Aldehyde Reaction

    MgBrC

    H H

    O CH2OH1. Mix

    2. H3O++

    cyclohexylmagnesiumbromide

    formaldehyde cyclohexylmethanol(65%)

    H3C C

    CH3CH2 CH

    O

    H

    MgBrH3C C

    CH3CH2 C

    OH

    H H

    +1. ether solvent

    2. H3O+

    3-methylbutanalphenylmagnesium

    bromide3-methyl-1-phenyl-1-

    butanol (73%)

    Ester Reaction

    Ketone Reaction

    O OH

    CH2CH31. CH3CH2MgBr, ether

    2. H3O+

    cyclohexanone 1-ethylcyclohexanol(89%)

    C

    O

    OCH2CH3CH3CH2CH2CH21. CH3CH2MgBr, ether

    2. H3O+C

    OH

    CH3CH3CH2CH2CH2CH3

    + CH3CH2OH

    ethylpentanoate2-methyl-2-hexanol (85%)

  • 11

    Carboxylic Acid Reaction

    CR' OH

    O

    CR' O

    ORMgBr + RH +

    +MgBr

    carboxylic acid carboxylic acid salt

    - carboxylic acids do not give addition products with Grignard reagents because the acidic carboxyl hydrogen reacts with the basic Grignard reagent to yield a hydrogen carbon (RH) and magnesium salt of the acid

    Limitations of Grignard Reagents

    - Grignard reagent cannot be prepared from an organohalide if there are other reactive functional groups in the same molecule

    - this limits the structures of the products

    - Grignard cannot be made where FG =

    moleculeBr FG

    = -OH, -NH, -SH, -COOH

    =CH

    OCR

    O

    CNR2

    O

    C N NO2 SO2R

    , ,

    , ,

    protonateGrignard

    adds toGrignard

    Mechanism

    C

    O RC

    R

    O

    CR

    OHH3O+

    carbonylcompound alkoxide

    intermediatealcohol

    - Grignard reagent acts as a nucleophilic carbon anion, or carbanion; the addition of the Grignard is analogous to the addition of a hydride

  • 12

    Reactions of Alcohols

    C

    OH

    O-H reactionsC-O reactions

    Dehydration of Alcohols (C-O bond)

    - a number of methods have been developed:

    1) acid-catalyzed dehydration (mild)

    2) acid-catalyzed dehydration (harsh)

    3) phosphorus oxychloride

    C COHH

    C C + H2O

    Acid-catalyzed dehydration

    - E1 mechanism that involves three steps

    OHH3CCH3

    H3O+, THF

    50oC

    1-methylcyc