Chapter 15 Alcohols, Diols, and Thiols

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Chapter 15Alcohols, Diols, and Thiols
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15.1Sources of Alcohols
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Methanol

• Methanol is an industrial chemical
• end uses solvent, antifreeze, fuel
• principal use preparation of formaldehyde

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Methanol

• Methanol is an industrial chemical
• end uses solvent, antifreeze, fuel
• principal use preparation of formaldehyde
• prepared by hydrogenation of carbon monoxide

CO 2H2 Æ CH3OH
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Ethanol

• Ethanol is an industrial chemical
• Most ethanol comes from fermentation
• Synthetic ethanol is produced by hydrationof
  ethylene
• Synthetic ethanol is denatured (madeunfit for
  drinking) by adding methanol, benzene,pyridine,
  castor oil, gasoline, etc.

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Other alcohols

• Isopropyl alcohol is prepared by hydration of
  propene.
• All alcohols with four carbons or fewer are
  readily available.
• Most alcohols with five or six carbons are
  readily available.

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Sources of alcohols
Reactions discussed in earlier chapters (Table
15.1)

• Hydration of alkenes
• Hydroboration-oxidation of alkenes
• Hydrolysis of alkyl halides
• Syntheses using Grignard reagents organolithium
  reagents

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Sources of alcohols
New methods in Chapter 15

• Reduction of aldehydes and ketones
• Reduction of carboxylic acids
• Reduction of esters
• Reaction of Grignard reagents with epoxides
• Diols by hydroxylation of alkenes

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15.2Preparation of AlcoholsbyReduction of
Aldehydes and Ketones
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Reduction of Aldehydes Gives Primary Alcohols
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Example Catalytic Hydrogenation
Pt, ethanol

(92)
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Reduction of Ketones Gives Secondary Alcohols
R
C
O
R'
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Example Catalytic Hydrogenation
H
OH
Pt

H2
ethanol
(93-95)

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Retrosynthetic Analysis
H
H
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Metal Hydride Reducing Agents
Sodiumborohydride
Lithiumaluminum hydride

• act as hydride donors

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Examples Sodium Borohydride
Aldehyde
NaBH4
CH2OH
methanol
(82)
Ketone

NaBH4
ethanol
(84)
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Lithium aluminum hydride

• more reactive than sodium borohydride
• cannot use water, ethanol, methanol etc.as
  solvents
• diethyl ether is most commonly used solvent

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Examples Lithium Aluminum Hydride
Aldehyde
1. LiAlH4diethyl ether
CH3(CH2)5CH2OH
2. H2O
(86)
Ketone

1. LiAlH4diethyl ether
(C6H5)2CHCCH3
2. H2O
(84)
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Selectivity

• neither NaBH4 or LiAlH4reduces isolateddouble
  bonds

1. LiAlH4diethyl ether
2. H2O
(90)
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15.3Preparation of Alcohols By Reductionof
Carboxylic Acids and Esters
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Reduction of Carboxylic AcidsGives Primary
Alcohols
R
C
O
HO

• lithium aluminum hydride is only effective
  reducing agent

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Example Reduction of a Carboxylic Acid
1. LiAlH4diethyl ether
2. H2O
(78)
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Reduction of EstersGives Primary Alcohols

• Lithium aluminum hydride preferred forlaboratory
  reductions
• Sodium borohydride reduction is too slowto be
  useful
• Catalytic hydrogenolysis used in industrybut
  conditions difficult or dangerous to duplicate
  in the laboratory (special catalyst,
  hightemperature, high pressure

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Example Reduction of a Carboxylic Acid
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15.4Preparation of Alcohols From Epoxides
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Reaction of Grignard Reagentswith Epoxides
R
CH2
CH2
OMgX

H3O
RCH2CH2OH
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Example
CH2
H2C
CH3(CH2)4CH2MgBr


O
1. diethyl ether 2. H3O
CH3(CH2)4CH2CH2CH2OH
(71)
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15.5Preparation of Diols
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Diols are prepared by...

• reactions used to prepare alcohols
• hydroxylation of alkenes

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Example reduction of a dialdehyde
H2 (100 atm)
Ni, 125C
3-Methyl-1,5-pentanediol
(81-83)
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Hydroxylation of AlkenesGives Vicinal Diols

• vicinal diols have hydroxyl groups on adjacent
  carbons
• ethylene glycol (HOCH2CH2OH) is most familiar
  example

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Osmium Tetraoxide is Key Reagent

• syn addition of OH groups to each carbonof
  double bond

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Example
(73)
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Example
(CH3)3COOHOsO4 (cat)
tert-Butyl alcoholHO
(62)