Organic Chemistry Fifth Edition

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Organic Chemistry II (Chem 234)Professor Duncan
J. Wardrop
Spring 2004
University of Illinois at Chicago
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Chapter 15Alcohols, Diols, and Thiols
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Sources of Alcohols
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Methanol

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

CO 2H2 ? 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 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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Natural Product Alcohols
Natural product Any organic substance isolated
from living organisms or from material derived
from living material.
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Review - Sources of Alcohol (table 15.1)
Hydroboration-oxidation of alkenes
Hydration of alkenes
Nucleophilic 1,2-Addition of Organometallic Reagen
ts to Carbonyl Compounds
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New Ways to Prepare Alcohols

• Reduction of aldehydes and ketones
• Reduction of carboxylic acids
• Reduction of esters
• Reaction of Grignard reagents with epoxides
• 1,2-Diols by dihydroxylation of alkenes

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15.2Preparation of AlcoholsbyReduction of
Aldehydes and Ketones
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Reduction of Aldehydes Gives Primary Alcohols
Primary Alcohol
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Example Catalytic Hydrogenation
q. Why isnt the carbonyl group reduced? a. CC
Bonds are weaker than CO bonds.
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Reduction of Ketones Gives Secondary Alcohols
Secondary Alcohol
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Example Catalytic Hydrogenation
H
OH
Pt

H2
ethanol
(93-95)

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Retrosynthetic Analysis
H
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Metal Hydride Reducing Agents
Sodiumborohydride

• both reagent act as donors of hydride (H-) donors

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Examples Sodium Borohydride
Aldehydes are reduced to 1 alcohols
Ketones are reduced to 2 alcohols
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Lithium aluminum hydride (LiAlH4 or LAH)

• more reactive than sodium borohydride
• cannot use water, ethanol, methanol etc. as
  solvents - they react violently!
• LiAlH4 4 H2O ? LiOH Al(OH)3 4 H2
• LiAlH4 4 EtOH ? LiOEt Al(OEt)3 4 H2
• diethyl ether is most commonly used solvent

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Examples Lithium Aluminum Hydride
Aldehyde
Ketone
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Chemoselectivity
neither NaBH4 or LiAlH4reduces isolated CC bonds
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15.3Preparation of Alcohols By Reductionof
Carboxylic Acids and Esters
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Reduction of Carboxylic AcidsGives Primary
Alcohols
Lithium aluminum hydride is one of only a few
effective reducing agents, which mediate this
transformation. In other words, you need a
powerful reducing agent to reduce acids. Why?
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Example Reduction of a Carboxylic Acid
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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 an Ester
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15.4Preparation of Alcohols From Epoxides
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Reaction of Grignard Reagentswith Epoxides
Epoxide rings are strained (29 kcalmol-1) and
therefore prone to nucleophilic attack at the
carbon centers.
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Examples
Cuprates are particularly adept at opening
epoxide rings (low basicity-high
nucleophilicity)
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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
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Example 2 reduction of a diester
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Hydroxylation of AlkenesGives Vicinal Diols
Vicinal diols have hydroxy groups on adjacent
carbons Ethylene glycol (HOCH2CH2OH in
anti-freeze) is a familiar example
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Osmium Tetraoxide is Key Reagent
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Example
(73)
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Example
Os(VIII)-mediated dihydroxylation is a
stereospecific reaction. Only the cis
1,2-diol can be obtained because of the mechanism
of the intial cycloaddition step both oxygen
atoms on OsO4 must attack the same face of the
olefin.
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Information
Suggested Problems 15.17-25, 15.29-15.32 --------
--------------------------------------------- Offi
ce Hour Thursday, 3.15 p.m., SES
4446 ---------------------------------------------
-------- Next Week Recitation Quiz 1 Covers
Chapters 14 15