UNIT
11
0
ETHERS
NEB Syllabus:
11.1 Aliphatic Ethers:
Introduction, nomenclature classification, isomerism in ether
Preparation of ethers from : (i) Alcohol, (ii) Williamson’s etherification process
Laboratory preparation of ethoxy ethane from ethanol
Physical properties of ether
Chemical properties of ethoxyethane
Action with HI, PCl5, con. HCl, conc. H2SO4, air and Cl2
Uses of ethoxyethane
11.2 Aromatic Ether:
Preparation of methoxy benzene ( anisole )
Halogenation, nitration and sulphonation reactions
Aliphatic Ethers:
Organic compounds which contain a divalent oxygen atom bonded to two alkyl groups are generally known as ethers.
Such ethers are known as aliphatic ether. They are represented by R – O – R.
Ethers are generally classified into two types
1) Symmetrical ethers [Simple Ethers]:
The ethers in which two identical alkyl groups are bonded with oxygen are called symmetrical ethers.
CH3 - O - CH3 CH3 - CH2 - O - CH2 - CH3
Methoxymethane Ethoxyethane
2) Unsymmetrical ethers [Mixed Ethers]:
The ethers in which two different alkyl group are bonded with oxygen atom are called unsymmetrical ethers.
CH3 - O - CH2 - CH3 CH3 - O - CH2 - CH2 - CH3 CH3 - O - CH - CH3
Methoxyethane Methoxypropane CH3
2-methoxypropane
Nomenclature:
IUPAC Name: Alk + oxy + alk + ane Common Name: Alk+yl + ether
S.N. Ethers IUPAC Name Common name
1. CH3 – O – CH3 Methoxy methane Dimethyl mether
2. CH3 – O – CH2 – CH3 Methoxy ethane Ethyl methyl ether
3. CH3 – CH2 – O – CH2 – CH3 Ethoxy ethane Diethyl ether
4. CH3 – O – CH2 – CH2 – CH3 Methoxy propane Methyl propyl ether
5. CH3 – O – CH – CH3 2-methyl propane Methyl isopropyl ether
|
CH3
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Isomerism in ether:
Ethers exhibit following types of structural isomerism
1. Chain isomerism:
Differ in carbon chain length.
2. Position isomerism:
Differ in position of functional group.
3. Functional isomerism:
Differ in functional group. Ether shows functional isomerism with alcohol.
4. Metamerism:
Differ in alkyl group present on either side of the functional group.
General Methods of Preparation:
1. From haloalkane:
a) Williamson’s Etherification Process or Williamson’s synthesis:
When primary haloalkane is treated with sodium alkoxide, ether is formed. Both
symmetrical as well as unsymmetical ether can be prepared by this method.
R - O Na + X - R R - O - R + NaX
Sodium Haloalkane Ether
alkoxide
CH3 - CH2 - O Na + CH3 - CH2 - Cl CH3 - CH2 - O - CH2 - CH3 + NaCl
Sodiumethoxide Chloroethane Ethoxy ethane
CH3 - CH2 - O Na + CH3 - Cl CH3 - CH2 - O - CH3 + NaCl
Sodiumethoxide Chloromethane Methoxyethane
CH3 - Br + CH3 - CH - O Na CH3 - CH - O - CH3 + NaBr
Bromomethane
CH3 CH3
Sodium isopropoxide 2- methoxy propane
b) Reaction with dry Ag2O:
When haloalkane is heated with dry silver oxide, symmetrical ether is formed.
2 R - X + Ag2O ( dry) R - O - R + 2 AgX
Haloalkane Ether
2 CH3 - CH2 - Cl + Ag2O ( dry) CH3 - CH2 - O - CH2 - CH3 + AgCl
Chloroethane Ethoxyethane
2. From alcohol:
When excess alcohol is heated with conc. H2SO4 at 140 oC, symmetrical ether is formed.
Conc. H2SO4
2 R - OH R - O - R + H2O
Alcohol 140oC Ether
Conc. H2SO4
2 CH3 - CH2 - OH CH3 - CH2 - O - CH2 - CH3 + H2O
140oC Ethoxyethane
Ethanol
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Laboratory Preparation of Diethyl ether (Ethoxyethane)
Principle:
In lab, diethyl ether is prepared by heating excess ethyl alcohol with conc. H2SO4 at 140 oC.
Conc. H2SO4
2 CH3 - CH2 - OH CH3 - CH2 - O - CH2 - CH3 + H2O
140 oC
Ethyl alcohol Diethyl ether
This reaction actually involves the following two steps.
Step 1: Ethyl alcohol reacts with conc. H2SO4 at 100 to 110 oC to form ethyl hydrogen sulphate.
110 oC CH3 CH2 HSO4 + H2O
CH3 CH2 OH + Conc. H2SO4
(Ethanol) (Ethyl hydrogen sulphate)
Step 2: Ethyl hydrogen sulphate reacts with another molecule of ethyl alcohol at 140 oC to form diethyl ether.
140oC CH3 CH2 O CH2 CH3 + H2SO4
CH3 CH2 HSO4 + CH3 CH2 OH
(Diethyl ether)
Fig. Laboratory preparation of diethyl ether
Procedure:
100 ml ethyl alcohol and 50 ml Conc. H2SO4 are dropping in distillation flask from dropping funnel fitted with
thermometer and water condenser. When the reaction mixture is heated over sand bath at 140oC , ether is distilled
and collected in receiver immersed in ice cold water. To continue the process, ethyl alcohol is added through the
dropping funnel at the same rate at which ether formed. This process is called continuous etherification process.
Purification:
Ether obtained by this process may contain water, ethyl alcohol and acidic impurities. To remove these impurities
first of all Ether is treated with aq. NaOH to remove acidic impurities, then treated with 50 % CaCl2 solution to
remove excess ethyl alcohol. It is then washed with water and dried over anhydrous CaCl2 to remove moisture and
finally redistilled around 34.5oC to get pure and dry diethyl ether.
Absolute ether: 100% pure diethyl ether is known as absolute ether
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Physical Properties:
1. State, Color and Odour:
Methoxy methane and methoxy ethane are gases. Other members are colorless volatile liquids with
pleasant smell
2. Solubility:
Lower members of ether are slightly soluble in water because some hydrogen bonding can exist between
ether and water molecule whereas higher members of ethers are insoluble.
3. Boiling point:
Ethers have lower boiling point than its isomeric alcohols because ethers cannot form intermolecular H-bond
with each other whereas alcohol can form intermolecular H- bond with each other.
Intermolecular H-bonds
No Intermolecular H-bonds
H O H O
H3C O H3C O
CH2CH3 CH2CH3
CH3 CH3
H-bond between ethanol molecules No H-bond between methoxy methane molecules
Chemical Properties:
A. Reaction due to ethereal oxygen:
1. Formation of oxonium salt:
+
H
Cold
CH3 - CH2 - O - CH2 - CH3 + HCl(Conc.) CH3 - CH2 - O - CH2 - CH3 Cl-
diethylether
Diethyloxoniumchloride
+
H
Cold
CH3 - CH2 - O - CH2 - CH3 + H2SO4(Conc.) CH3 - CH2 - O - CH2 - CH3 HSO4-
diethylether
Diethyloxoniumhydrogensulphate
Oxonium salts are unstable at high temperature and in dilution solution.
2. Formation of peroxide:
O
Light
CH3 - CH2 - O - CH2 - CH3 + O2 (air) CH3 - CH2 - O - CH2 - CH3
diethylether diethylperoxide
Peroxide of ethers are unstable compounds and decomposed violently on heating causes serious
accident. So it is dangerous to distill or boil the old sample of ether stored for long time.
B. Reaction due to cleavage of C – O bond:
1. Reaction of HX:
Cold
CH3 - CH2 - O - CH2 - CH3 + HI CH3 - CH2 - OH + CH3 - CH2 - I
Diethylether Ethanol Iodoethane
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If excess hot HI is used, only iodoalkane is formed.
Hot
CH3 - CH2 - O - CH2 - CH3 + HI 2 CH3 - CH2 - I + H2O
Diethylether Excess Iodoethane
If ether is unsymmetrical then halogen goes with smaller alkyl group.
Cold
CH3 - CH2 - O - CH3 + HI CH3 - CH2 - OH + CH3 - I
Ethylmethylether Ethanol Iodomethane
2. Reaction with H2SO4:
Hot
CH3 - CH2 - O - CH2 - CH3 + H2SO4 CH3 - CH2 - OH + CH3 - CH2 - HSO4
Diethylether Conc. Ethanol Ethylhydrogensulphate
If ether is heated with dil. H2SO4 under pressure, alcohols are formed.
dil. H2SO4
CH3 - CH2 - O - CH2 - CH3 + H2O 2 CH3 - CH2 - OH
Diethylether Ethanol
3. Reaction with PCl5
CH3 - CH2 - O - CH2 - CH3 + PCl5 2 CH3 - CH2 - Cl + POCl3
diethylether Chloroethane
CH3 - CH2 - O - CH3 + PCl5 CH3 - CH2 - Cl + CH3 - Cl + POCl3
ethylmethylether Chloroethane Chloromethane
C. Reaction due to alkyl group:
Reaction with Chlorine
Cl2
CH3 - CH2 - O - CH2 - CH3 CH3 - CH - O - CH - CH3
Dark
Diethylether
Cl Cl
1,1-dichlorodiethylether
Cl2
CH3 - CH2 - O - CH2 - CH3 CCl3 - CCl2 - O - CCl2 - CCl3
Light
Diethylether Perchlorodiethylether
Uses:
1) Diethyl ether is used as a general anesthetic agent.
2) It is used as solvent in the preparation of Grignard reagent.
3) It is good solvent for fats, waxes oil and plastics.
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Methods of Preparation Chemical Properties
O
O2
CH3 - CH2 - O - CH2 - CH3
Sunlight
CH3CH2 ONa Diethyl peroxide
CH3 - CH2 - Cl H
Ethyl chloride HCl
Dry Ag2O [CH3 - CH2 - O - CH2 - CH3] +Cl-
Cold & Conc
CH3 - CH2 - Cl Diethyl oxonium chloride
Ethyl chloride
Conc. H2SO4 HCl (limited)
CH3 - CH2 - OH CH3 - CH2 - O - CH2 - CH3 CH3 - CH2 - Cl + CH3 - CH2 - OH
140oC Hot & Conc. Chloroethane ethanol
Ethyl alcohol
Al2O3 diethylether HCl (excess)
CH3 - CH2 - Cl
CH3 - CH2 - OH Hot & Conc.
Ethyl alcohol 250oC Chloroethane
CH2N2 HI
CH3CH2 - OH
Ethyl alcohol
Cold
CH3 - CH2 - I + CH3 - CH2 - OH
Iodoethane ethanol
H2SO4
CH3 - CH2 - OH + CH3 - CH2 - HSO4
Hot & Conc Ethanol Ethylhydrogensulphate
dil. H2SO4
CH3 - CH2 - OH
Ethanol
PCl5 CH3 - CH2 - Cl
chloroethane
Cl2
light CCl3 - CCl2 - O - CCl2 - CCl3
perchlorodiethyl ether
Cl2
CH3 - CH - O - CH - CH3
dark
| |
Cl Cl
1,1 dichlorodiethyl ether
Aromatic Ether:
Anisole [ Methoxy benzene ]
OCH3
Anisole
General Method of preparation
Williamson’s synthesis:
When iodomethane is treated with sodium phenoxide, then methoxy benzene or anisole is formed.
ONa OCH3
+ CH3 - I + NaI
Sod.phenoxide Iodomethane Anisole
But anisole cannot be prepared by reacting Iodobenzene with sodium methoxide. Because nucleophilic substitution
reaction is difficult in aryl halide due to following reason:
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Carbon - halogen bond in aryl halide has partial double bond due to resonance.
Carbon - halogen bond is less polar due to hybridization effect
partial double bond character
I
+ CH3 - ONa No reaction
Iodo benzene (sod. methoxide)
Chemical Properties
1) Reaction with HI:
OCH3 OH
+ HI + CH3 - I
Iodo methane
Anisole Phenol
But Iodobenzene and Methanol is not formed in above reaction because the oxygen- carbon bond in anisole
acquires partial double bond character by resonance and which is difficult to break.
partial double bond character
OCH3 I
+ HI
+ CH3OH
Anisole Iodobenzene Methanol
2) Electrophilic substitution reactions:
Methoxy group in anisole is o- and p- directing in electrophilic substitution reaction. Due to resonance in
anisole, electron density at o- & p- positions increases and hence incoming electrophile will prefer to attack
these positions.
OCH3 OCH 3 OCH3 OCH3 OCH3 OCH3
Anisole Resonance
Resonating structures
hybrid
7 | Page © Jay Prakash Paudel / Jaya Ram Ghimire / Department of Chemistry || Uniglobe SS/College -2075
a) Halogenation:
OCH3 OCH3 OCH3
Cl
AlCl3
+ Cl2 + + HCl
Anisole
o-chloroanisole
Cl
p-chloronisole
OCH3 OCH3 OCH3
Br
FeBr3
+ Br2 + + HBr
Anisole
o-bromoanisole
Br
p-bromoanisole
b) Nitration:
OCH3 OCH3 OCH3
NO2
Conc. H2SO4
+ Conc. HNO3 + + H2O
Anisole o-nitroanisole
NO2
p-nitroanisole
c) Sulphonation:
OCH3 OCH3 OCH3
SO3H
+ Conc. H2SO4 + + H2O
Anisole o-methoxybenzenesulphonic acid
SO3H
p-methoxybenzenesulphonic acid
8 | Page © Jay Prakash Paudel / Jaya Ram Ghimire / Department of Chemistry || Uniglobe SS/College -2075
Methods of Preparation Chemical Properties
OH
HI
+ CH3 - I
Phenol Methyl iodide
OCH3 OCH3
Br2 / FeBr3
Br
+
ONa OCH3
Br
CH3 - I o-bromo anisole p-bromo anisole
OCH3 OCH3
Anisole
Sodium phenoxide (Methoxy benzene) NO2
Conc. HNO3 / conc. H2SO4
+
NO2
o-nitro anisole p-nitro anisole
OCH3 OCH3
SO3H
conc. H2SO4
+
SO3H
o-methoxy benzene p-methoxy benzene
sulphonic acid sulphonic acid
THE-END
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