_ + Chapter 11 Reactions of Alcohols Organic Chemistry, 6 th Edition L. G. Wade, Jr.
Chapter 11 Reactions of Alcohols
Feb 21, 2016
Organic Chemistry , 6 th Edition L. G. Wade, Jr. Chapter 11 Reactions of Alcohols. Types of Alcohol Reactions. Dehydration to alkene Oxidation to aldehyde, ketone Substitution to form alkyl halide Reduction to alkane Esterification Tosylation Williamson synthesis of ether =>. - PowerPoint PPT Presentation
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Chapter 11Reactions of Alcohols
Organic Chemistry, 6th EditionL. G. Wade, Jr.
Chapter 11 2
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Types of Alcohol Reactions
• Dehydration to alkene• Oxidation to aldehyde, ketone• Substitution to form alkyl halide• Reduction to alkane• Esterification• Tosylation• Williamson synthesis of ether =>
Chapter 11 3
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Summary Table
=>
Chapter 11 4
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Oxidation States• Easy for inorganic salts
CrO42- reduced to Cr2O3
KMnO4 reduced to MnO2
• Oxidation: loss of H2, gain of O, O2, or X2
• Reduction: gain of H2 or H-, loss of O, O2, or X2
• Neither: gain or loss of H+, H2O, HX =>
Chapter 11 5
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1º, 2º, 3º Carbons
=>
Chapter 11 6
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Oxidation of 2° Alcohols• 2° alcohol becomes a ketone• Reagent is Na2Cr2O7/H2SO4
• Active reagent probably H2CrO4
• Color change: orange to greenish-blue
CH3CHCH2CH3
OHNa2Cr2O7 / H2SO4
CH3CCH2CH3
O
=>
Chapter 11 7
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Oxidation of 1° Alcohols• 1° alcohol to aldehyde to carboxylic acid• Difficult to stop at aldehyde• Use pyridinium chlorochromate (PCC)
to limit the oxidation.• PCC can also be used to oxidize 2°
alcohols to ketones.
CH3CH2CH2CH2
OH N H CrO3Cl
CH3CH2CH2CH
O
=>
Chapter 11 8
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3° Alcohols Don’t Oxidize• Cannot lose 2 H’s• Basis for chromic acid test
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Chapter 11 9
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Other Oxidation Reagents• Collins reagent: Cr2O3 in pyridine• Jones reagent: chromic acid in acetone• KMnO4 (strong oxidizer)• Nitric acid (strong oxidizer)• CuO, 300°C (industrial dehydrogenation)• Swern oxidation: dimethylsulfoxide, with
oxalyl chloride and hindered base, oxidizes 2 alcohols to ketones and 1 alcohols to aldehydes. =>
Chapter 11 10
_
+Biological Oxidation
• Catalyzed by ADH, alcohol dehydrogenase.• Oxidizing agent is NAD+, nicotinamide
adenine dinucleotide.• Ethanol oxidizes to acetaldehyde, then acetic
acid, a normal metabolite.• Methanol oxidizes to formaldehyde, then
formic acid, more toxic than methanol.• Ethylene glycol oxidizes to oxalic acid, toxic.• Treatment for poisoning is excess ethanol.
=>
Chapter 11 11
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Alcohol as a Nucleophile
• ROH is weak nucleophile• RO- is strong nucleophile• New O-C bond forms, O-H bond breaks.
=>
CO
H
R X
Chapter 11 12
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Alcohol as an Electrophile• OH- is not a good leaving
group unless it is protonated, but most nucleophiles are strong bases which would remove H+.
• Convert to tosylate (good leaving group) to react with strong nucleophile (base). =>
CO
H
+
C-Nuc bond forms, C-O bond breaks
Chapter 11 13
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Formation of Tosylate Ester
p-toluenesulfonyl chlorideTsCl, “tosyl chloride”
CO
H
CH3
S
Cl
OO N
CH3
S OO
OH
C
CH3
S
O
OO
C
ROTs,a tosylate ester
=>
Chapter 11 14
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SN2 Reactions of Tosylates
• With hydroxide produces alcohol• With cyanide produces nitrile• With halide ion produces alkyl halide• With alkoxide ion produces ether• With ammonia produces amine salt• With LiAlH4 produces alkane
=>
Chapter 11 15
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Summary of Tosylate Reactions
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Chapter 11 16
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Reaction with HBr• -OH of alcohol is protonated• -OH2
+ is good leaving group
• 3° and 2° alcohols react with Br- via SN1
• 1° alcohols react via SN2
H3O+ Br-
R O H R O H
H
R Br =>
Chapter 11 17
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Reaction with HCl• Chloride is a weaker nucleophile than
bromide.• Add ZnCl2, which bonds strongly with
-OH, to promote the reaction.• The chloride product is insoluble.• Lucas test: ZnCl2 in conc. HCl
1° alcohols react slowly or not at all.2 alcohols react in 1-5 minutes.3 alcohols react in less than 1 minute.
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Chapter 11 18
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Limitations of HX Reactions• HI does not react• Poor yields of 1° and 2° chlorides• May get alkene instead of alkyl halide• Carbocation intermediate may rearrange.
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Chapter 11 19
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Reactions with Phosphorus Halides
• Good yields with 1° and 2° alcohols• PCl3 for alkyl chloride (but SOCl2 better)
• PBr3 for alkyl bromide• P and I2 for alkyl iodide (PI3 not stable)
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Chapter 11 20
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Mechanism with PBr3
• P bonds to -OH as Br- leaves• Br- attacks backside (SN2)
• HOPBr2 leaves =>
Chapter 11 21
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+Reaction with
Thionyl Chloride
• Produces alkyl chloride, SO2, HCl
• S bonds to -OH, Cl- leaves• Cl- abstracts H+ from OH• C-O bond breaks as Cl- transferred to C
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Chapter 11 22
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Dehydration Reactions
• Conc. H2SO4 produces alkene• Carbocation intermediate• Zaitsev product• Bimolecular dehydration produces ether• Low temp, 140°C and below, favors ether• High temp, 180°C and above, favors
alkene =>
Chapter 11 23
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Dehydration Mechanisms
CH3CHCH3
OHH2SO4
alcoholCH3CHCH3
OHH
CH3CHCH3
CH2 CHCH3H2O
CH3OH
H3O+
CH3OH CH3 OH2 CH3 O
H
CH3
H2OCH3OCH3
=>
Chapter 11 24
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Energy Diagram, E1
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Chapter 11 25
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Alkoxide Ions• ROH + Na (or NaH) yields sodium alkoxide• RO- + 1° alkyl halide yields ether (Williamson
ether synthesis)
CH3CH2CHCH3
OCH3CH2 Br+ CH2CH2CH
CH3
O CH2CH3
=>
Chapter 11 26
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End of Chapter 11
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