Chapter 11

Alcohols and Ethers

NomenclatureNomenclature of Alcohols (Sec. 4.3F)Nomenclature of Ethers

Chapter 11 2

Chapter 11 3

Nomenclature Nomenclature of Ethers

Common NamesThe groups attached to the oxygen are listed in

alphabetical order

IUPACEthers are named as having an alkoxyl substituent on the

main chain

Chapter 11 4

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 5

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 6

Chapter 11 7

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 8

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 6 9

Organic Synthesis: Functional Group Transformations Using SN2 Reactions

Stereochemistry can be controlled in SN2 reactions

Chapter 11 10

Hydroboration-OxidationThis addition reaction occurs with anti-Markovnikov

regiochemistry and syn stereochemistry

Chapter 11 11

Chapter 11 12

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 stable2. 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 13

Chapter 11 14

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 15

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 HXSN1 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 16

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 17

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 18

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 19

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 20

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 21

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 22

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 23

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

Chapter 11 24

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 25