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Janice Gorzynski SmithUniversity of Hawai’i
Chapter 9Alcohol, Ethers, Epoxides
Organic Chemistry, Fifth Edition
Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
Modified by Dr. Juliet Hahn
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• Epoxides can be named in three different ways—epoxyalkanes, oxiranes, or alkene oxides.
• To name an epoxide as an epoxyalkane, first name thealkane chain or ring to which the O atom is attached, anduse the prefix “epoxy” to name the epoxide as asubstituent.
• Use two numbers to designate the location of the atomsto which the O is bonded.
Naming Epoxides
End class 10/25/17 Wed
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• Alcohols, ethers, and epoxides exhibit dipole-dipoleinteractions because they have a bent structure with twopolar bonds.
• Alcohols are capable of intermolecular hydrogen bonding.Thus, alcohols are more polar than ethers and epoxides.
Hydrogen Bonding in Alcohols
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Interesting Molecules with Epoxides• Eplerenone (used by heart attack patients) and tiotropium
bromide (a bronchodilator) contain epoxides.
Preparation of Alcohols, Ethers, Epoxides
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• Alcohols and ethers are both common products of nucleophilic substitution.
• The preparation of ethers by the method shown in the last equation is called the Williamson ether synthesis.
(1)Preparation of Alcohols and Ethers (SN2 with OH- & OR- )
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• In theory, unsymmetrical ethers can be synthesized in twodifferent ways.
• In practice, one path is usually preferred.• The path involving alkoxide attack on a less hindered alkyl
halide is preferred.
Williamson Ether Synthesis
SN2 path
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• An alkoxide salt is needed to make an ether.• Alkoxides can be prepared from alcohols by a Brønsted-Lowry
acid-base reaction.• For example, sodium ethoxide (NaOCH2CH3) is prepared by
treating ethanol with NaH.
• NaH is an especially good base for forming alkoxide becausethe by-product of the reaction, H2, is a gas that just bubblesout of the reaction mixture.
Preparation of Alkoxides (to do above RXN)
baseacid
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• Organic compounds that contain both a hydroxy group and ahalogen atom on adjacent carbons are called halohydrins.
• In halohydrins, an intramolecular version of the Williamsonether synthesis can occur to form epoxides.
(2) Forming Epoxides from Halohydrins
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Reactions of alcohol, ether, epoxide
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General View: Substitution and Elimination Reactions of Alcohols
• Treatment of alcohols with a strong acid protonates the O converting the bad leaving group ¯OH into H2O, a good leaving group.
• The pKa of (ROH2)+ is ~ -2, so protonation of alcohols only occurs with very strong acids.
• This makes it possible to perform substitution and elimination reactions on alcohols.
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• Like alcohols, ethers do not contain a good leaving group.• Ethers undergo fewer useful reactions than alcohols.
OR- as a Leaving Group
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• Unlike alkyl halides in which the halogen atom serves as agood leaving group, the OH group in alcohols is a very poorleaving group.
• For an alcohol to undergo nucleophilic substitution, OH- mustbe converted into a better leaving group.
OH- as a Leaving Group
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• Dehydration, like dehydrohalogenation, is a β elimination reaction in which the elements of OH and H are removed from the α and β carbon atoms respectively.
(1) Reactions of Alcohols—Dehydration
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• Typical acids used for alcohol dehydration are H2SO4 or p-toluenesulfonic acid (TsOH).
(a) Dehydration Requires Strong Acids
• Dehydration is typically carried out using H2SO4 and other strong acids, or phosphorus oxychloride (POCl3) in the presence of an amine base.
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• More substituted alcohols dehydrate more easily, givingrise to the following order of reactivity.
(b) Dehydration and Alcohol Substitution
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• When an alcohol has two or three β carbons, dehydration isregioselective and follows the Zaitsev rule.
• The more substituted alkene is the major product when amixture of constitutional isomers is possible.
(c) Zaitsev’s Rule
End class 10/27/17F
