Download Chapter 10:Alcohols, Phenols and Ethers

Chapter 10:Alcohols, Phenols and Ethers
§10.1.Alcohols
Alcohols are organic compounds containing hydroxyl (-OH) groups. They are some
of the most common and useful compounds in nature, in industry, and around the
house.
1. Classifications, Nomenclature and Structure of Alcohols
1) Classification of Alcohols
primary alcohol
secondary alcohol
tertiary alcohol
unitary alcohol
duality alcohol
ternary alcohol
2)
Nomenclature of Alcohols
Common Names：
OH
H3C
OH
methyl alcohol
H3CH2CH2C
OH
n-propyl alcohol
H3C
CH
isopropyl alcohol
OH
H3CH2CH2CH2C
OH H C
3
n-butyl alcohol
allyl alcohol
and
CH
H
C
H2C
CH3
sec-butyl alcohol
C
CH3
t-butyl alcohol
OH
allyl alcohol
CH3
CH3
CH2CH3 H3C
H2
C
OH
H3C
CH
H2
C
isobutyl alcohol
isopropyl alcohol
IUPAC Names：
The formal rules are summarized according three steps:
1. Name the longest carbon chain bearing the—OH group.
2. Number the longest carbon chain starting at the end nearest the hydroxyl
group.
3. Name all the substituents and give numbers, as you would for and an alkane
or an alkene.
OH
CH3 OH
H3C
4
3
C
CH
2
H2
C
Br
1
CH3
1- bromo-3,3-dimethylbutan-2-ol
3,3-二甲基- 1-溴-2-丁醇
.
OH
OH
OH
H3C
CH
OH
CH2OH
OH
propane-1,2-diol
1-cyclohexylbutane-1,3-diol
H2C
CH2
H2C
OH OH
IUPAC name: ethane-1,2-diol
common name: ethylene glycol
H
C
trans-cyclopentane-1,2-diol
OH
CH3
OH
OH OH
propane-1,2-diol
propylene glycol
cis-cyclohexane-1,2-diol
cis-cyclohexene glycol
3）Structure of Alcohols
H
109°
C
143ppm
O
u=5.70x10-30
H
H
H
2．Physical Properties of Alcohols
Boiling Points
Solubility Properties
3．Spectroscopy
Infrared Spectroscopy
O-H 3300cm-1
C-O 1000-1200cm-1
Proton NMR Spectroscopy
O-H
δ:0.5-4.5 ppm (solvent, concentration, temperature)
RCH2OH δ:3.4-4.0 ppm
4. Reactions of Alcohols
Alcohols are important organic compounds because the hydroxyl group is easily
converted to almost any other functional group. In Chapter 10, we studied reactions
that form alcohols. In this chapter, we will study reactions of hydroxyl group.
1) Reaction with lively metals
form alkoxides:
R
Na
+
OH
CH3CH2OH
R
_1
2 H2
+
CH3CH2 O + Na
Na
+
O + Na
_1
2 H2
+
The reactivity of alcohols: methyl>1°>2°>3°
2) Reactions of Alcohols with Hydrohalic Acids
R
OH
+
HX
R
Example:
CH3(CH2)2
CH2OH
NaBr,H2SO4
+
X
H2O
CH3(CH2)2
CH2Br
1-bromobutane(99%)
1-butanol
Mechanism: SN1 or SN2
H
R
O
H
+
H
R
poor leaving group
X-
+
O
H
good leaving group
The reactivity:
tertiary >secondary >primary
HI > HBr > HCl >> HF
Lucas reagents:
SN1 or SN2
R
X
Concentrated HCl and ZnCl2 distinguish the structure（1°，2°，3°）of the
alcohol.
primary alcohol very fast
secondary alcohol slow
tertiary alcohol
no reaction (r.t.)
Rearrangement
Meerwein rearrangement
The pinacol Rearrangement
H3C
CH3
CH3
C
C
OH
OH
CH3
CH3
pinacol
(2,3-dimethyl-2,3-butanediol )
Mechanism:
H2SO4
100℃
H3C
C
C
O
CH3
CH3
pinacolone
( 3,3-dimethyl-2-butanone)
+
H2O
Step 1:Protonation of a hydroxyl group
Step 2: Loss of water gives a carbocation
CH3CH3
CH3CH3
H3C
C
C
CH3
+
+ H
H3C
C
C
CH3
CH3
H3C
C
HO OH2
HO OH
CH3
+ H2O
C
CH3
HO
Step 3: Meth yl migration forms a resonance-stabilized carbocation
CH3
H3C
H
C
O
~CH3
(methyl migration)
CH3
C
CH3
CH3
H3C
H
C
O
C
CH3
CH3
H3C C C CH3
H O OH
CH3
2
resonance-stabilized carbocation
Step 4: Deprotonation gives the product.
CH3
H3C
C
O
C
CH3
CH3
H3C
C
O
CH3
H2O H
resonance-stabilized carbocation
H
C
CH3
CH3
H3C
OH2
C
O
C
CH3
+
OH2
pinacolone
3) Reations of alcohols with phosphorus halides
3ROH + PX3
3RX + P(OH)3
ROH + PCl5
RCl + POCl3 + HCl
RCl + SO2 + HCl
ROH + SOCl2
4) Esterification of Alcohols
O
O
+
R
O
H
alcohol
+
H
O
C
acid
R'
H
R
O
C
ester
R'
+
H
O
H
H3O+
H2SO4
(CH3)2CHOH + CH3COOH
isopropyl alcohol acetic acid
(CH3)2CHOOCCH3 + H2O
isopropyl acetate
5)Esters of inorganic acid(nitric acid, sulfuric acid, phosphoric acid)
Tosylate esters: para-toluenesulfonic acid and alcohols.
O
ROH + HO
O
S
CH3
O
para-toluenesulfonic acid
(TsOH)
alcohol
O
R
S
CH3
+ H2 O
O
para-toluenesulfonate ester
(ROTs)
A sulfate ester :
O
OH
O
O
S
CH3OH
OH
H3C
O
sulfuric acid
O
S
O
methyl sulfate
CH3OH
OH
H3C
O
+ H2O
S
CH3
O
+ H2O
O
dimethyl sulfate
Nitrate esters are formed from alcohols and nitric acid.
O
R
O
+
H
H
alcohol
O
O
N
nitric acid
R
O
O
+ H2O
N
O
alkyl nitrite eater
The best known nitrate ester is“nitroglycerine”: the reaction of glycerol（1，
2，3-propanetriol） with three molecules of nitric acid.
H2C
O
H
HC
O
H
H2C
O
H
+
3 HO
glycerol
(glycerine)
NO2
nitric acid
H2C
O
NO2
HC
O
NO2
H2C
O
NO2
+
3H2O
glyceryl trinitrate
(nitroglycerine)
Alkyl phosphates are composed of 1 mole of phosphoric acid combined with 3
moles of an alcohol. For example, methanol forms three phosphate esters.
HO
H CO
3
3CH3OH
+ HO
P
O
HO
6) Dehydration Reactions of Alcohols
H3CO
H3CO
P
O
+
3H2O
(1) Formation of Alkenes
Mechanism:acid-catalyzed Dehydration of an alcohol
H
H
OH
C
C
H+
H
O
C
C
H
H
C
H2O
C
C
C
+ H3O+
We can summarize the dehydration and give guidelines for predicting the
products:
1). Dehydration usually goes by the E1 mechanism. Rearrangements may occur
to form more stable carbonations.
2). Dehydration works best with tertiary alcohols and almost as well with
secondary alcohols. Rearrangements and poor yields are common with primary
alcohols.
3). (Saytzeff rule) if two or more alkenes might be formed by deprotonation of
the carbonations, the most highly substituted alkene usually predominates.
(2)Bimolecular Dehydration to form Ethers
I
H
H3C
CH3
H
SN2
CH3CH2 O
C O
CH3CH2 O C
O
H
H
H
H
H
H
H
H
nucleophilic
electrophilic
protonated ether
water
Substitution to give the ether, a bimolecular dehydration
H2SO4 , 140℃
2CH3CH2OH
CH3CH2 O CH2CH3
ethanol
diethyl ether
Elimination to give the alkene, a unimolecular dehydration
H2SO4 , 180℃
CH3CH2OH
+ H2O
H2C
CH2
ethanol
ethylene
CH3
CH3CH2 O
C
H
H
+ H3O+
diethyl ether
+
H2O
7) Oxidation-reduction reaction
Primary and secondary alcohols are easily oxidized by a variety of reagents, including
chromium reagents, permanganate, nitric acid, and even household bleach（NaOCl, sodium
hypochlorite）.
H
O
OH
Na2Cr2O7
H2SO4
cyclohexanol
cyclohexanone
(90%)
OH
R
C
O
[O]
H
primary alcohol
R
O
C
H
[O]
R
C
acid
aldehyde
OH
Oppenauer Oxidaxion
A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium
chlorochromate（PCC）, a complex of chromium trioxide with pyridine and HCl.
OH
R
C
O
CrO3.pyridine.HCl(PCC)
H
R
CH2Cl2
H
primary alcohol
Example:
H
aldehyde
+
H3C(H2C)3
C
CH2OH
1-heptanol
7) Reaction of polylol
O
-
pyH CrO3Cl (PCC)
CH2Cl2
H3C(H2C)3
C
heptanal 78%
H
5.
Synthesis of Alcohols
One of the reasons alcohols are important synthetic intermediates is that they can
be synthesized directly from a wide variety of other functional groups. We have
examined the conversion of alky halides to alcohols by substitution and the
conversion of alkenes to alcohols by hydration, hydroboration, and hydroxylation.
These reactions are summarized below:
1) Nucleophilic substitution on an alkyl halide
R
R
C
HO
X
HO
H H
R
X
HO
H
H
transition state
H
X
H
Example:
H
Br
HO
KOH
H
C
C
H3C
H3C
CH2CH3
(s)-2- bromobutane
CH2CH3
(R)-2-butanol,100% inverted confi guration
2) synthesis of alcohols from alkenes
1). Acid-catalyzed hydration
C
+
C
H+
H2O
C
C
H
OH
2).Oxymercuration-demercuration
C
+ Hg(OAc)2
C
H2O
C
(AcO)Hg
C
NaBH4
OH
C
C
H
OH
Example:
H3C
CH3
C
H
C
Hg(OAc)2
H2O
CH3
3).Hydrobortion-oxidation
H
CH3
OH
C
C
(AcO)Hg
CH3
CH3
NaBH4
H
CH3
OH
C
C
H
CH3
CH3
C
(1)BH3.THF
(2)H2O2,NaOH
C
C
C
H
OH
Example:
CH3
CH3
CH3
H
H
H2O2,NaOH
BH3.THF
OH
BH2
H
H
H
1-methylcyclopentene
trans-2-methylcyclopentanol
(85%)
4)Hydroxylation: synthesis of 1,2-diols from alkenes
C
OsO4,H2O2
C
or KMnO4,OH(cold,dilute)
C
C
OH
OH
syn hydroxylation
Example:
H
H3CH2C
CH2CH3
O
H
O
KMnO4
H
H
CH2CH3
O-
O
CH2CH3
H3CH2C
H
OH
OHH2O
=
H
H3CH2C
H
OH
H
OH
OH
H3CH2C
cis-3-hexaene
CH2CH3
meso-3,4-hexanediol
(60%)
OH
C
C
RCOOOH, H3O+
C
C
OH
anti hydroxylation
Example:
H
H3CH2C
CH2CH3
HCO3H
H
O
H
H
CH2CH3
cis-3-hexaene
H3CH2C
CH2CH3
H3CH2C
H
OH
H3O+
=
HO
CH2CH3
H
HO
OH
H
H
( )-3,4-hexanediol
(70%)
CH2CH3
3) Synthesis of alcohols from carbonyl compounds
(1) Addition of organometallic reagents to carbonyl compounds
Grignard and organolithium reagents are strong nucleophiles and strong bases.
Key mechanism: Grignard reactions
First reaction: the Grignard reagent attacks a carbonyl compound to form an alkoxide
salt.
δ
δ
R
MgX
+
C
O
R
ether
C
O
- +
MgX
magnesium alkoxide salt
Second reaction: after the first reaction is complete, water or dilute acid is added to
protonate the alkoxide.
H
R
C
O
- +
O
H
MgX
R
magnesium alkoxide salt
O
H
+
XMgOH
alcohol
the primary alcohol:
H
H
CH3CH2CH2CH2
MgBr
+
C
(1) ether solvent
(2)H3O+
O
H
butylmagnesium bromode
formaldehyde
CH3CH2CH2CH2
C
1-pentanol (92%)
H
the secondary alcohol:
CH3
H3C
CH3CH2
MgBr
+
C
O
(1)ether solvent
(2)H3O+
CH3CH2
C
H
OH
H
acetaldehyde
2-butanol (85%)
the tertiary alcohol:.
CH2CH2CH3
H3CH2CH2C
CH3CH2
MgBr
+
C
H3C
2-pentanone
reaction with epoxides:
Example:
O
(1)ether solvent
(2)H3O+
CH3CH2
C
OH
CH3
3-methyl-3-hexanol (90%)
OH
O
O
H3C(H2C)3
MgX
H2C
CH2
H2C
ether
- +
CH2
C4H9
butylmagnesium bromide
OH
MgX
H3O+
H2C
CH2
C4H9
1-hexanol (61%)
ethylene oxide
(2)Catalytic Hydrogenation of Ketones and Aldehydes
O
OH
+ H2
C
Raney Ni
CH3
H2C
H
C
H2
C
C
CH3
CH
CH3
O
+ 2H2
C
Raney Ni
H3C
H2
C
H
C
CH2OH
CH3
2,2-dimethyl-4-pentenal
2,2-dimethyl-4-pentenol (94%)
CH3
NaBH4
(for comparison)
H2
C
H2C
H
C
H2
C
C
CH3
2,2-dimethylpent-4-en-1-ol
6. Some important Alcohols (Self-study)
CH2OH
§2. Phenols
1. Classifification, Nomenclature and Structure of Phenols
O
OH
N
OH
OH
O
Br
H3CH2C
IUPAC:
2-bromophenol
common name: ortho-bromophenol
IUPAC:
2-methylphenol
common name: ortho-cresol
3-nitrophenol
meta-nitrophenol
OH
OH
CH3
OH
benzene-1,2-diol
catechol
4-ethylphenol
para-ethylphenol
OH
OH
HO
OH
benzene-1,3-diol
resorcinol
benzene-1,4-diol
hydr oquinone
2. Physical Properties of Phenols
IR: O-H ν=3600cm-1
1
HNMR chemical shifts :δO-H = 4.5 - 8.0 in the 1HNMR spectrum.
3.Reactions of Phenols
Much of the chemistry of phenols is like that of aliphatic alcohols. For example,
phenols can be acylated to give esters, and phenoxide ions can serve as nucleophiles
in the Williamson ether synthesis. Formation of phenoxide ions is particularly easy
because phenols are more acidic than water, aqueous sodium hydroxide deprotonates
phenols to give phenoxide ions.
All the alcohol-like reactions involve breaking of the phenolic O-H bond. This is
a common way for phenols to react. It is far more difficult to break the C-O bond of a
phenol, however. Most alcohol reactions in which the C-O bond breaks are not
possible with phenols. For example, phenols do not undergo acidcatalyzed elimination
or SN2 back-side attack.
Phenols also undergo reactions that are not possible with aliphatic alcohols. Let’s
consider some reactions that are peculiar to phenols.
1) Acidity of Phenols
carbolic acid: sodium or potassium metal.
ONa
OH
+
NaOH
+
H2O
2) Oxidation of phenols to quinones
OH
O
N a 2 C r2 O 7
H 2S O 4
CH
m -cres o l
CH
3
3
O
2 -m eth yl-1 ,4 -b e n zo q u in o n e
2.3.3.
Eletrophilic Aromatic Substitution of phenols
Phenols are highly reactive substrates for elevtrophilic aromtic substitution
because the nonbonding electrons of the hydroxyl group stabilize the sigma complex
formed by attack at the ortho or para position (Section 17-6B). Therefore, the
hydroxyl group is strongly activating and ortho, para-directing. Phenols are excellent
substrates for halogention, nitration, sulfonation, and Friedel-Crafts reactions.
Becaues they are highly reactive, phenols are usually alkylated or acylated using
relatively weak Friedel-Crafts catalysts (such as HF) to avoid overalkylation or
overacylation.
OH
OH
OH
OH
+
H3C
CH
CH( CH3)2
CH3
HF
+
CH(CH3)2
2.4. Preparation of Phenols
2.4.1. Incorporation
Phenol groups can be incorporated into an aromatic ring by sulfonation of the aromatic rings
followed by melting the product with sodium hydroxide to convert the sulfonic acid group to a
phenol. The reaction conditions are harsh and only alkyl-substituted phenols can be prepared by
this method.
SO3H
OH
SO3
H2SO4
1) NaOH
2) H3O+
CH3
CH3
CH3
A more general method of synthesizing phenols is to hydrolyze a diazonium salt, prepared from
an aniline group (NH2)
NH2
NO2
H2NO3
Sn
HCl
H2SO4
OH
N2HSO4
HNO2
H2SO4
H3O+
2.4.2. Functional group transformation
Various functional groups can be converted to phenols. Sulfonic acids and amino groups have
already been mentioned. Phenol esters can be hydrolyzed (a). Aryl ethers can be cleaved (b). The
bond between the alkyl group and oxygen is specifically cleaved since the Ar-OH bond is too
strong to be cleaved.
O
R
C
a)
H3O+ or
NaOH
OH
b)
Conc. HX
heat
O
HO2C-R
2.5.Some important Phenols (Self-study)
X-R
O
R
§3. Ethers
3.1. Nomenclature ,Structure and Spectroscopy of Ethers
Ethers are compounds of formula R-O-Rˊ,where R and Rˊmay be alkyl groups
or aryl(benzene ring)groups. Like alcohols, ethers are related to water, with alkyl
groups replacing the hydrogen atoms. In an alcohol, one hydrogen atom of water is
replaced by an alkyl group. In an ether, both hydrogens are replaced by alkyl groups.
The two alkyl groups are the same in a symmetrical ether and different in an
unsymmetrical ether.
H
Examples of
H3CH2C
O
H
R
O
water
ethers:
O
H
R
alcohol
CH2CH3
O
O
R'
ether
CH3
O
diethyl ether
(a symmetrical ether)
methyl phenyl ether
(an unsymmetrical ether)
tetrahydrofuran
(a symmetrical,cyclic ether)
Common names of ethers are formed by naming the two alkyl groups on oxygen
and adding the word ether. Under the current system, the old system, which named
the groups in order of increasing complexity. For example, if one of the alkyl groups
is methyl and the other is t-butyl, the current common name should be “t-butyl methyl
ether”, but most chemists use the older common name, “methyl t-butyl ether”(or
MTBE). If both groups are methyl, the name is “dimethyl ether”. If just one alkyl
group is described in the name, it implies the ether is symmetrical, as in “ethyl ether”.
IUPAC names use the more complex group as the root name, and the rest of the
ether as an alkoxy group. For example, cyclohexyl methyl ether is named
merhoxycyclohexane. This systematic nomenclature is often the only clear way to
name complex ethers.
OCH3
H3C
O
IUPAC name: methoxyethane
common name: ethyl methyl ether
H3C
H2
C
Cl
CH2CH3
methyoxylbenzene
ethyl phenyl ether, anisole
O
CH3
chloromethoxymethane
chloromethyl methyl ether
CH3
Cl
H
H
OCH2CH3
IUPAC name: 3-ethoxy-1,1'-dimethylcyclohexane
OCH3
H
H2C
OH
H2C
O
CH2CH3
2-ethoxyethanol
trans-1-chloro-2-methoxycyclobutane
Cyclic ethers are our first examples of heterocyclic compounds, containing a ring
in which a ring atom is an element other than carbon. This atom, called the
heteroatom, is numbered l in numbering the ring atoms. Heterocyalic ethers are an
especially important and useful class of ethers.
Infrared spectroscopy of ethers: Infrared spectra do not show obvious or reliable
absorptions for ethers. Most ethers give a moderate to strong C-O stretch around 1000
to 2000cm-1(in the fingerprint region), but many compounds other than ethers give
similar absorptions. Nevertheless, the IR spetrum can be useful because it shows the
absence of carbonyl(C=O) groups. If the molecular formula contains an oxygen atom,
the lack of carbonyl or hydroxyl absorptions in the IR suggests ether.
NMR spectroscopy of ethers In the 13CNMR spectrum, a carbon atom bonded
to oxygen generally absorbs between δ 65 and δ 90.Protons on carbon atoms
bonded to oxygen usually absorb at chemical shifts betweenδ3.5and δ4 in the
1HNMR. spectrum Both alcohols and ethers have resonances in this range.
3.2.
Physical properties of Ether
Like water, ethers have a bent structure, with an sp3 hybrid oxygen atom giving a
nearly tetrahedral bond angle. In water, the bulk of the alkyl groups enlarges the
H-O-H bond angle to 104.5゜, but in a typical ether, the bulk of the alkyl groups
enlarges the bond angle..
Although ethers lack the polar hydroxyl group of alcohols, they are still strongly
polar compounds. The dipole moment of ether is vector sum of two polar C-O bonds,
with a substantial contribution the two lone pairs of electrons. Table
Table 14-1 Comparison of the Boiling points of Ethers, Alkanes, and Alcohols
of similar Molecular Weights
Compound
Formula
MW
Dipole
bp(℃)
Moment(D)
18
100
1.9
water
H2O
ethanol
CH3CH2-OH
46
78
1.7
dimethyl ether
CH3-O-CH3
46
-25
1.3
propane
CH3CH2CH3
44
-42
0.1
n-butanol
CH3CH2CH2CH2-OH
74
118
1.7
72
66
1.6
tetrahydrofuran
O
diethyl ether
CH3CH2-O-CH2CH3
74
35
1.2
pentane
CH3CH2CH2CH2CH3
72
36
0.1
Note: The alcohols are hydrogen bonded, giving them much higher boiling points.
The ethers have boiling points that are closer to those of alkanes with similar
molecular weights.
compares the dipole moments of dimethyl ether, diethyl ether, and tetrahydrofuran
(THF) with those of alkanes and alcohols of similar molecular weights. An ether such
as THF provides a strongly polar solvent without the reactivity of a hydroxyl group.
Boiling points of Ethers; Hydrogen Bonding
Table 14-1 compares the boiling points of several ethers, alcohols, and alkanes.
Notice that the boiling points of dimethyl ether and diethyl ether are nearly 100℃
lower than those of alcohols having similar molecular weights. This large difference
results mostly from hydrogen bonding in the alcohols. Pure ethers cannot engage in
hydrogen bonding because they have no O-H groups. Ethers do have large dipole
moments, resulting in dipole-dipole attractions, but these attractions appear to have
relatively little effect on their boiling points.
Although pure ethers have on hydroxyl groups to engage in hydrogen bonding,
they can hydrogen bond with other compounds that do have O-H or N-H groups.
Figure 14-2 shows that a hydrogen bond requires both a hydrogen bond donor and a
hydrogen.
TABLE14-2 physical properties of some Representative Ethers
Name
Structure
mp(゜C) bp(゜C)
eimethyl ether
CH3-O-CH3
-140
-25
ethyl methyl ether
CH3CH2-O-CH3
8
diethyl ether
CH3CH2-O-CH2CH3
-116
35
di-n-propyl ether
CH3CH2CH2-O-CH2CH2CH2
-122
91
diisopropyl ether
(CH3)2CH-O-CH(CH3)2
-86
68
1,2-dimethoxyethane
CH3-O-CH2CH2-O-CH3
-58
83
(DME)
-37
154
Methyl phenyl ether
H3C O
(anisole)
27
259
diphenyl ether
O
furan
Density(g/ml)
0.66
0.72
0.71
0.74
0.86
0.99
1.07
0.94
-86
32
0.89
-108
65
1.03
11
101
O
tetrahydrofuran
(THF)
1,4-dioxane
O
O
O
Bond acceptor. The donor is the molecule with an O-H or N-H group. The acceptor is
the molecule whose lone pair of electrons forms a weak partial bond to the hydrogen
atom provided by the donor. An ether molecule has the lone pair to form a hydrogen
bond with an alcohol (or other hydrogen bond donor), but it cannot form a hydrogen
bond with another ether more volatile than alcohols having similar molecular weights.
Table 14-2 lists the physical properties of a representative group of common ethers.
3.Reactions of Ethers
1) Cleavage of Ethers by HBr and HI
excess HX
(X=Br or I)
Protonation and cleavage of ether
R
O
R'
Br
CH3CH2
O
CH2CH3+ H
diethyl ether
Br
R
X
+
H
H3C
C
X
Br
H
H O CH2CH3
protonated ether
Conversion of ethanol to ethyl bromide
R'
H3C
C
H +H
H
ethyl bromide
O
CH2CH3
ethanol
Br
H
O
+ H
CH2CH3
H
Br
O
ethanol
H2O + Br
CH2CH3
CH2CH3
ethyl bromide
H
Overall reaction
excess HBr/H2O
2 CH3CH2 Br
CH2CH3
CH3CH2 O
diethyl ether
ethyl bromide
We can rank the hydrohalic acids in order of their reactivity toward the cleavage
of ethers:
HI > HBr >> HCl
Phenyl Ethers Phenyl ethers (one of the groups bonded oxygen is a benzene ring)
react with HBr or HI to give alkyl halides and phenols. Phenols do not react further to
give halides because the sp2 hybridized carbon atom of the phenol cannot undergo the
SN2 (or SN1) reaction needed for conversion to the halide.
O
Br-
CH2CH3
H
CH2CH3
ethyl phenyl ether
OH
O
Br
H
protonated ether
+ Br
CH2CH3
phenol
ethyl bromide
(no further reaction)
2) Autoxidation of Ethers
When ethers are stored in the presence of atmosperis oxygen, they slowly oxidize to
produce hydroperoxides and dialkyl peroxides, both of which are explosive, Such a
spontaneous oxidation by atmospheric oxygen called an autoxidation.
OOH
R
H2
O
C
ether
excess O2
(slow)
R'
R
O CH R'
hydroperoxide
+
H2
C
dialkyl peroxide
R
O
O
R'
Example
H3C
CH
O
excess O2
(weeks or months)
CH
H3C
OOH
H3C
CH3
CH3
CH
O
C
H3C
diisopropyl ether
hydroperoxide
CH3
4. Synthesis of Ethers
1) Bimolecular Dehydration of Alcohols
Bimolecular dehydration
2R
OH
Examples
H+
R
O
R
+
H2O
H3C
CH3
+
CH3
CH
O
O
CH
H3C
CH3
diisopropyl peroxide
2CH3OH
methyl alcohol
H2SO4,140℃
H3C
2CH3CH2CH2OH
H2SO4,140℃
n-propyl alcohol
H
C
H3C
O
CH3
dimethyl ether
(100%)
CH3
H3CH2CH2C
O
n-propyl ether
(75%)
H2SO4,140℃
H
C
H2C
+
H2O
CH2CH2CH3
+
CH3
+
H2O
H2O
OH
isopropyl alcohol
unimolar dehydration
(no ether formed)
2) The Williamson Ether Synthesis
R
O
R'
X
R
O
+
R'
X
Examples
OH
OCH2CH3
(1).Na
(2) CH3CH2-OTs
cyclohexanol
ethoxycyclohexane(92%)
Synthesis of phenyl Ethers A phenol (aromatic alcohol) can be used as the alkoxide
fragment (but not the halide fragment) for the Williamson ether synthesis. Phenols are
more acidic than aliphatic alcohols (Section 10-6), and sodium hydroxide is
sufficently basic to form the phenoxide ion. As with other alkoxides, the electrophile
should have an unhindered primary alkyl group and a good leaving group.
O
OH
CH2CH2CH2CH3
NO2
NO2
(1)NaOH
(2)CH3CH2CH2CH2-I
2-nitrophenol
5. Some Important Ethers
2-butoxynitrobenzene(80%)