Transcript ch11 by dr. Dina

Chapter 11
Alcohols and Ethers
Nomenclature
Nomenclature of Alcohols (Sec. 4.3F)
Nomenclature of Ethers
Chapter 11
2
 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
3
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 eth
Chapter 11
4
 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
5
Chapter 11
6
 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
7
 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
8
 Organic Synthesis: Functional Group
Transformations Using SN2 Reactions
Stereochemistry can be controlled in SN2 reactions
Chapter 6
9
 Hydroboration-Oxidation
This addition reaction occurs with anti-Markovnikov
regiochemistry and syn stereochemistry
Chapter 11
10
Chapter 11
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
12
Chapter 11
13
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
14
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
15

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
16
 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
17
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
18
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
19
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
20
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
21
 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
22
In unsymmetrical epoxides, the nucleophile attacks primarily at
the most substituted carbon of the epoxide
Chapter 11
23
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
24
Chapter 11
25