Download CHAPTER 9 Further Reactions of Alcohols and the Chemistry of

CHAPTER 11
Alcohol Reactivity
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Summary Table
Oxidation: Adding O, Losing H
Chromic Acid (Cr(VI)) is a powerful
oxidizing agent
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Oxidation Mechanism
When Aldehydes are formed by Cr(VI) in water, they are are in
equilibrium with their hydrates (geminal alcohols)
These geminal alcohols undergo a second oxidation with Cr(VI) to form
carboxylic acids quantitatively
H
H 2O
O
OH
H
OH
OH
H
OH
H 2CrO 4
OH
H
OH 2
O CrO H
3
OH
O
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PCC is a specialized Cr(VI) reagent that dissolves in dry
solvents and thereby prevents hydrate formation.
Primary alcohols stop at the aldehyde stage
Sodium Hypochlorite (NaOCl)
Sodium hypochlorite (household bleach) can
oxidize alcohols under relatively mild conditions
without generating hazardous waste.
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Swern Oxidation
Dimethylsulfoxide (DMSO), with oxalyl chloride
and hindered base, oxidizes 2 alcohols to
ketones and 1 alcohols to aldehydes.
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Swern
Oxidation
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Dess-Martin Periodinane
(DMP)
• Can oxidize primary alcohols to aldehydes
and secondary alcohols to ketones.
• The reaction with DMP takes place under
mild conditions (room temperature, neutral
pH) and gives excellent yields.
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Reaction of Alcohols with HX
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The stereochemistry of the product provides evidence of the mechanism
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While 2° and 3° alcohols react with HCl via an SN1 mechanism, 1°
alcohols react via SN2.
Addition of ZnCl2 facilitates the SN2 reaction with the relatively weak
nucleophile Cl–
Carbocation Rearrangements:
Unexpected products via rearrangement of the
carbocation intermediate
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The stereochemistry of the product provides evidence of the mechanism
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Even Primary alcohols may undergo rearrangement.
Alkyl and hydride shifts occur without the formation of primary
carbocations when they are concerted with the loss of the leaving
group:
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Reaction with PBr3
PBr3 is an alternative to HBr. Advantages:
•Safer
•No rearrangement
•Fewer side reactions
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Mechanism with PBr3
Reaction of Alcohols with Thionyl Chloride
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Dehydration of Alcohols Using POCl3 and Pyridine
Reactions of Alcohols:
Conversion of Alcohols to Alkyl sulfonates creates a good leaving group for
subsequent displacement by an anionic nucleophile
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Examples of SN2 Nucleophilic Attack on Alkyl Sulfonates:
Inversion of the Chiral a-carbon
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Dehydration of Alcohols: Review
• Alcohol dehydration generally takes place through
the E1 mechanism.
• Rearrangements are possible.
• The rate of the reaction follows the same rate as the
ease of formation of carbocations: 3o > 2o > 1o.
• Primary alcohols rearrange, so this is not a good
reaction for converting 1° alcohols into alkenes.
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Unique Reactions of Diols
Vicinal diols can undergo the following two
reactions:
– Pinacol rearrangement
– Periodic acid cleavage
Pinacol Rearrangement
Propose a Mechanism of this
Rearrangement.
Why does a rearrangement take place?
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Mechanism of the Pinacol Rearrangement
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Periodic Acid Cleavage of Glycols
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Esterification of Alcohols
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•
•
•
•
Fischer: Alcohol + carboxylic acid
Tosylate esters
Sulfate esters
Nitrate esters
Phosphate esters
Fischer Esterification
The Fischer Esterification is a reversible
reaction but yeilds are high when water is
distilled as it forms, or alcohol is
introduced in large excess
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Reaction of Alcohols with Acyl
Chlorides
• The esterification reaction achieves better results by
reacting the alcohol with an acyl chloride.
• The reaction is exothermic and produces the
corresponding ester in high yields with only HCl as a
by-product.
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