chap. 6 alcohols and ethers solomons: chapter 11
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Chap. 6 Alcohols and Ethers Solomons: Chapter 11. Physical Properties of Alcohols and Ethers Ether boiling points are roughly comparable to hydrocarbons of the same molecular weight Molecules of ethers cannot hydrogen bond to each other Alcohols have considerably higher boiling points - PowerPoint PPT PresentationTRANSCRIPT
Chap. 6
Alcohols and Ethers
Solomons: Chapter 11
Chapter 11 2
Physical Properties of Alcohols and Ethers Ether boiling points are roughly comparable to hydrocarbons of
the same molecular weight Molecules of ethers cannot hydrogen bond to each other
Alcohols have considerably higher boiling points Molecules of alcohols hydrogen bond to each other
Both alcohols and ethers can hydrogen bond to water and have similar solubilities in water
Diethyl ether and 1-butanol have solubilites of about 8 g per 100 mL in water
Chapter 11 3
アルコール類の合成法
1. Alcohols from Alkenes (酸触媒存在下での水和)
2. Hydroboration-Oxidation of Akenes (水素ホウ素化)
Chapter 11 4
Synthesis of Alcohols from Alkenes
Acid-Catalyzed Hydration of AlkenesThis is a reversible reaction with Markovnikov regioselectivity
Chapter 11 5
Hydroboration-Oxidation
This addition reaction occurs with anti-Markovnikov regiochemistry and syn stereochemistry
Chapter 11 6
アルコール類の反応
1. Conversion of Alcohols into Alkyl Halides
2. Sulfonate誘導体を経る変換反応 (SN2反応)
Chapter 11 7
Conversion of Alcohols into Alkyl HalidesHydroxyl groups are poor leaving groups, and as such, are
often converted to alkyl halides when a good leaving group is needed
Three general methods
Chapter 11 8
Alkyl Halides from the Reaction of Alcohols with Hydrogen Halides
The order of reactivity is as followsHydrogen halide HI > HBr > HCl > HF Type of alcohol 3o > 2o > 1o < methyl
Mechanism of the Reaction of Alcohols with HX SN1 mechanism for 3o, 2o, allylic and benzylic alcohols
Chapter 11 9
Primary and methyl alcohols undergo substitution by an SN2 mechanism
Primary and secondary chlorides can only be made with the assistance of a Lewis acid such as zinc chloride
Chapter 11 10
Alkyl Halides from the Reaction of Alcohols with PBr3 and SOCl2
These reagents only react with 1o and 2o alcohols in SN2 reactions
Reaction of phosphorous tribromide to give alkyl bromides
Chapter 11 11
Reaction of thionyl chloride to give alkyl chloridesOften an amine is added to react with HCl formed in the
reaction
Chapter 11 12
Tosylates, Mesylates, and Triflates:
Leaving Group Derivatives of Alcohols The hydroxyl group of an alcohol can be converted to a good leaving group
by conversion to a sulfonate ester
Sulfonyl chlorides are used to convert alcohols to sulfonate esters Base is added to react with the HCl generated
Chapter 11 13
A sulfonate ion (a weak base) is an excellent leaving group
If the alcohol hydroxyl group is at a stereogenic center then the overall reaction with the nucleophile proceeds with inversion of configuration The reaction to form a sulfonate ester proceeds with retention
of configuration
Chapter 11 14
Ethers
Chapter 11 15
Synthesis of Ethers Ethers by Intermolecular Dehydration of Alcohol
Primary alcohols can dehydrate to ethers This reaction occurs at lower temperature than the
competing dehydration to an alkene This method generally does not work with secondary or
tertiary alcohols because elimination competes strongly
Chapter 11 16
The mechanism is an SN2 reaction
Chapter 11 17
Williamson Ether Synthesis This is a good route for synthesis of unsymmetrical ethers
The alkyl halide (or alkyl sulfonate) should be primary
to avoid E2 reaction
Substitution is favored over elimination at lower temperatures
Chapter 11 18
tert-Butyl Ethers by Alkylation of Alcohols: Protecting Groups
This method is used to protect primary alcohols The protecting group is removed using dilute acid
Chapter 11 19
Silyl Ether Protecting GroupsSilyl ethers are widely used protecting groups for alcohols
The tert-butyl dimethysilyl (TBDMS) ether is common The protecting group is introduced by reaction of the alcohol with
the chlorosilane in the presence of an aromatic amine base such as imidazole or pyridine
The silyl ether protecting group is removed by treatment with fluoride ion (e.g. from tetrabutyl ammonium fluoride)
Chapter 11 20
Reactions of EthersAcyclic ethers are generally unreactive, except for cleavage by very
strong acids to form the corresponding alkyl halidesDialkyl ethers undergo SN2 reaction to form 2 equivalents of the
alkyl bromide
Chapter 11 21
EpoxidesEpoxides are three-membered ring cyclic ethers
These groups are also called oxiranes
Epoxides are usually formed by reaction of alkenes
with peroxy acids This process is called epoxidation and involves syn addition of
oxygen
Chapter 11 22
Magnesium monoperoxyphthalate (MMPP) is a common and safe peroxy acid for epoxidation
Chapter 11 23
Reaction of EpoxidesEpoxides are considerably more reactive than regular ethers
The three-membered ring is highly strained and therefore very reactive
Acid-catalyzed opening of an epoxide occurs by initial protonation of the epoxide oxygen, making the epoxide even more reactiveAcid-catalyzed hydrolysis of an epoxide leads to a 1,2-diol
Chapter 11 24
In unsymmetrical epoxides, the nucleophile attacks primarily at the most substituted carbon of the epoxide
Chapter 11 25
Base-catalyzed reaction with strong nucleophiles (e.g. an alkoxide or hydroxide) occurs by an SN2 mechanism
The nucleophile attacks at the least sterically hindered carbon of the epoxide
Chapter 11 26
Anti 1,2-Dihydroxylation of Alkenes via Epoxides Opening of the following epoxide with water under acid catalyzed
conditions gives the trans diol