Chapter 11 Alcohols and Ethers

Nomenclature Nomenclature of Alcohols (Sec. 4 Nomenclature Nomenclature of Alcohols (Sec. 4.3F) Nomenclature of Ethers Chapter 11

Nomenclature Nomenclature of Ethers Common Names The groups attached to the oxygen are listed in alphabetical order IUPAC Ethers are named as having an alkoxyl substituent on the main chain Chapter 11

Ethers are described as symmetrical or unsymmetrical depending on whether the two groups bonded to oxygen are the same or different. Unsymmetrical ethers are also called mixed ethers. Diethyl ether is a symmetrical ether; ethyl methyl ether is an unsymmetrical ether. Cyclic ethers have their oxygen as part of a ring–they are heterocyclic compounds. Several have specific IUPAC Chapter 11

Cyclic ethers can be named using the prefix oxa- Three-membered ring ethers can be called oxiranes; Four-membered ring ethers can be called oxetanes Chapter 11

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 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

Synthesis of Alcohols from Alkenes Acid-Catalyzed Hydration of Alkenes This is a reversible reaction with Markovnikov regioselectivity HA = acid ex H2SO4 ( H+ HSO4-) Oxymercuration-demercuration This is a Markovnikov addition which occurs without rearrangement Chapter 11

Organic Synthesis: Functional Group Transformations Using SN2 Reactions Stereochemistry can be controlled in SN2 reactions Chapter 6

Hydroboration-Oxidation This addition reaction occurs with anti-Markovnikov regiochemistry and syn stereochemistry Chapter 11

Chapter 11

Alcohols as Acids Alcohols have acidities similar to water Sterically hindered alcohols such as tert-butyl alcohol are less acidic (have higher pKa values) Why? 1. The conjugate base is not well solvated and so is not stable 2. the alkyl group is electron donated group, so the electrons density is increased on the -C-O- Alcohols are stronger acids than terminal alkynes and primary or secondary amines An alkoxide can be prepared by the reaction of an alcohol with sodium or potassium metal Chapter 11

Chapter 11

Conversion of Alcohols into Alkyl Halides Hydroxyl groups are poor leaving groups, and as such, are often converted to alkyl halides when a good leaving group is needed Three general methods exist for conversion of alcohols to alkyl halides, depending on the classification of the alcohol and the halogen desired Reaction can occur with phosphorus tribromide, thionyl chloride or hydrogen halides Chapter 11

the Reaction of Alcohols + Hydrogen Halides Alkyl Halides from the Reaction of Alcohols + Hydrogen Halides The order of reactivity is as follows Hydrogen 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 These reactions are prone to carbocation rearrangements In step 1 the hydroxyl is converted to a good leaving group In step 2 the leaving group departs as a water molecule, leaving behind a carbocation Chapter 11

Primary and methyl alcohols undergo substitution by an SN2 mechanism In step 3 the halide, a good nucleophile, reacts with the carbocation 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

Alkyl Halides from the Reaction of Alcohols with PBr3 and SOCl2 These reagents only react with 1o and 2o alcohols in SN2 reactions In each case the reagent converts the hydroxyl to an excellent leaving group No rearrangements are seen Reaction of phosphorous tribromide to give alkyl bromides Chapter 11

Synthesis of Ethers Ethers (symetrical) 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 The mechanism is an SN2 reaction Chapter 11

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

Reactions of Ethers Acyclic ethers are generally unreactive, except for cleavage by very strong acids to form the corresponding alkyl halides Dialkyl ethers undergo SN2 reaction to form 2 equivalents of the alkyl bromide Chapter 11

Epoxides Epoxides 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

Reaction of Epoxides Epoxides 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 reactive Acid-catalyzed hydrolysis of an epoxide leads to a 1,2-diol Chapter 11

In unsymmetrical epoxides, the nucleophile attacks primarily at the most substituted carbon of the epoxide Chapter 11

Base-catalyzed reaction with strong nucleophiles (e. g 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

Chapter 11