notes of ether

1. ETHERS
CHEMICAL PROPERTIES OF ETHERS
Ethers are quite stable compounds. These are not easily attacked by alkalies, dilute mineral
acids, active metals, reducing agents or oxidising agents under ordinary conditions.
Being Lewis bases, ethers form complexes with Lewis acids such as BF3, AlCl3, FeCl3, etc.
These complexes are called etherates
Similarly, diethyl ether reacts with Grignard reagent forming Grignard reagent etherate.
Due to the formation of the etherate, Grignard reagents dissolve in ether. That is why Grignard
reagents are usually prepared in ethers. However, they cannot be prepared in benzene, because
benzene has no lone pair of electrons and therefore, cannot form complexes with them.
(ii) Action of hydroiodic acid
(a) With cold HI
Cold
C2H5OC2H5 + HI —————→ C2H5I + C2H5OH
Diethyl ether Ethyl iodide Ethyl alcohol
(b) With hot HI
R – O – R' + 2HI ——————→ RI + R'I + H2O
(iii) Zeisel method : RI + AgNO3 (alc.) → AgI ↓ + RNO3
• The silver iodide thus form can be detected and estimated. This is the basis of Zeisel method
for the detection and estimation of alkoxy group in a compound.
(iv) Action of PCl5
R – O – R + PCl5 ————→ 2RCl + POCl3. There is no reaction in cold.
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2. ETHERS
(v) Reaction with acetyl chloride
ZnCl2
CH3COCl + C2H5 . O . C2H5 ——————→ CH3COOC2H5
heat
Acetylchloride Diethyl ether Ethyl acetate
(vi) Reaction with acid anhydride
ZnCl2
CH3CO . O . OCCH3 + C2H5 . O . C2H5 —————→ 2CH3COOC2H5
Acetic anhydride Diethyl ether
heat
Ethyl acetate
Al2O3
C2H5OC2H5 ———————→ 2CH = CH2 + H2O
300o
C
x-small;"> BF3/150o
C
C2H5OC2H5 + CO ———————→ C2H5COOC2H5
Diethyl ehter
500 atm. Ethyl propionate
(ix) Action of bases
Friedel Craft reaction
The aromatic ethers formthe acetophenone with the aluminium trichloride.
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3. ETHERS
HALOGENATION OF ETHER
Halogenation of ether refers to the treating the ether with the halogens such as chlorine,
bromine, iodine etc.
Cl2
CH3CH2OCH2CH3 —————→ CH3CHClOCH2CH3
Diethyl ether
dark (α-Monochloro diethyl ether)
Cl2
CH3CH2OCH2CH3 —————→ CH3CHClOCHCICH3
Diethyl ether
dark (α, α'-Dichlorodiethyl ether)
Cl2
C2H5OC2H5 —————→ C2Cl5OC2Cl5 + 10HCl
Diethyl ether
dark (α, α'-Dichlorodiethyl ether)
Hydrolysis of Ether
(a) With dil. H2SO4: ROR + H2O ————→ 2ROH
H2SO4
C2H5OC2H5 + H2O ——————→ 2C2H5OH
Diethyl ether Ethanol
(b) With conc. H2SO4:
C2H5OC2H5 + H2SO4 → C2H5OH + C2H5HSO4
C2H5OH + H2SO4 → C2H5HSO4 + H2O
—————————————————————————————
C2H5OC2H5 + 2H2SO4 → 2C2H5HSO4 + H2O
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4. ETHERS
Diethyl ether Ethyl hydrogen sulphate
Peroxide formation
• • • •
C2H5 O C2H5 + O : → (C2H5)2 O → O.
• • • •
(a) The boiling point of peroxide is higher than that of ether. It is left as residue in the
distillation of ether and may cause explosion. Therefore ether may never be evaporated to
dryness.
(b) Absolute ether can be prepared by distillation of ordinary ether from conc. H2SO4 and
subsequent storing over metallic sodium.
• Formation of peroxide can be prevented by adding small amount of Cu2O to ether.
• With strong oxidising agent like acid, dichromate ethers are oxidised to aldehydes.
2[O]
CH3CH2OCH2CH3 ———→ 2CH3CHO + H2O
Acetaldehyde
• The presence of peroxide can be indicated by the formation of blood red colour complex in
the following reaction.
SCN–
Peroxide + Fe+2
→ Fe+3
——————→ [Fe(SCN)n]3–n
Blood red colour
(n=1 to 6)
Ring substitution in aromatic ethers
Alkoxy group is ortho and para directing and it directs the incoming groups to ortho and para
position. It activates the aromatic ring towards electrophilic substitution reaction.
III, IV and V show high electron density at ortho and para position.
Williamson synthesis
It is a nucleophilic substitution reaction and proceed through SN
2
mechanism.
RONa + R'X → ROR' + NaX
C2H5ONa + CH3 – I → CH3OC2H5 + NaI
Sodium ethoxide Ethylmethylether
C2H5ONa + C2H5Br → C2H5OC2H5 + NaBr
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5. ETHERS
Sodium ethoxide Ethyl bromide Ethoxyethane
(a) Order of reactivity of primary halide is CH3X > CH3CH2X > CH3CH2CH2X.
(b) Tendency of alkyl halide to undergo elimination is 3o
> 2o
> 1o
.
(c) For better yield alkyl halide should be primary and alkoxide should be secondary or tertiary.
CH3 CH3
| |
C2H5Br + NaO – C – CH3 → C2H5 – O – C – CH3
Ethyl bromide
| |
CH3 CH3
Sodium salt of Ethyl tert. butyl ether
tert. butyl alcohol
(d) Secondary and tertiary alkyl halides readily undergo E2 elimination in the presence of a
strong base to form alkenes.
CH3 CH3
| C2H5ONa |
CH3 – C – Cl ——————→ CH3 – C +
Cl–
,
| |
CH3 CH3
CH3 CH3
| |
CH3 – C+
+ C2H5O–
→ CH3 – C + C2H5OH
| ||
CH2–H CH2
• Aryl halide and sodium alkoxide cannot be used for preparing phenolic ethers because aryl
halide are less reactive toward nucleophilic substitution reaction than alkyl halides.
PREPARATION OF ETHERS
Ethers are anhydride of alcohols, they may be obtained by elimination of a water molecule from
two alcohol molecules.
R – OH + HO – R → R – O – R + H2O
Ether
General formula is CnH2n+2O
General methods of preparation of ethers
(1) From alkyl halides
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6. ETHERS
(ii) By heating alkyl halide with dry silver oxide
heat
2RX + Ag2O ————→ R – O – R + 2AgX,
heat
2C2H5Br + Ag2O ————→ C2H5OC2H5 + 2AgBr
Ethyl bromide Diethyl ether
(2) From alcohols
(i) By dehydration of alcohols
(a) With conc. H2SO4 at 140° C
ROH + HOR ——————→ ROR + H2O.
2 molecules of alcohol
140o
C Ether
• In this reaction alcohol must be present in excess.
• This reaction is mainly applicable for the dehydration of primary alcohols. Secondary and
tertiary alcohols form alkenes mainly.
• When this reaction is carried out between different alcohols then there is a mixture of different
ethers is obtained.
(b) With Al2O3 at 250° C:
Al2O3
2ROH ——————→ R – O – R + H2O
250o
C
(ii) By the action of diazomethane on alcohols : This reaction is in presence of catalyst, boron
trifluoride or HBF4.
BF3
ROH + CH2N2 ——————→ R – O – CH3 + N2
(a) This method is very useful for preparing mixed ethers.
(b) In higher cases, there can be 1, 2-hydride or 1, 2-methyl shift to form more stable carbonium
ion.
(3) Reaction of lower halogenated ether with Grignard reagent
ROCH2X + XMgR' ——→ ROCH2R' + MgX2
Halogeneted Grignard Higher
ether reagent ether
(i) Higher members can be prepared by the action of grignard reagent on lower halogenated
ethers.
(ii) Ether form soluble coordinated complexes with grignard reagent.
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7. ETHERS
(ii) By heating alkyl halide with dry silver oxide
heat
2RX + Ag2O ————→ R – O – R + 2AgX,
heat
2C2H5Br + Ag2O ————→ C2H5OC2H5 + 2AgBr
Ethyl bromide Diethyl ether
(2) From alcohols
(i) By dehydration of alcohols
(a) With conc. H2SO4 at 140° C
ROH + HOR ——————→ ROR + H2O.
2 molecules of alcohol
140o
C Ether
• In this reaction alcohol must be present in excess.
• This reaction is mainly applicable for the dehydration of primary alcohols. Secondary and
tertiary alcohols form alkenes mainly.
• When this reaction is carried out between different alcohols then there is a mixture of different
ethers is obtained.
(b) With Al2O3 at 250° C:
Al2O3
2ROH ——————→ R – O – R + H2O
250o
C
(ii) By the action of diazomethane on alcohols : This reaction is in presence of catalyst, boron
trifluoride or HBF4.
BF3
ROH + CH2N2 ——————→ R – O – CH3 + N2
(a) This method is very useful for preparing mixed ethers.
(b) In higher cases, there can be 1, 2-hydride or 1, 2-methyl shift to form more stable carbonium
ion.
(3) Reaction of lower halogenated ether with Grignard reagent
ROCH2X + XMgR' ——→ ROCH2R' + MgX2
Halogeneted Grignard Higher
ether reagent ether
(i) Higher members can be prepared by the action of grignard reagent on lower halogenated
ethers.
(ii) Ether form soluble coordinated complexes with grignard reagent.
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