Ether – Isomers, Preparation, Properties and Uses.

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What are ethers ?

Ethers are organic compounds having general formula R – O – R’.

If both R and R’ are same then it is called simple or symmetrical ether but if R and R’ are different then it is called mixed or unsymmetrical ether.

nomenclature of ether

Examples :ethers

Isomerism in ethers

1. Functional isomerism : Ethers are always functional isomers of alcohols. Eg.

functional isomers of ether

2. Metamerism : Ethers also show the phenomenon of metamerism in which ethers have same molecular formula differ in the size of alkyl groups attached on either sides. Eg.

metamerism in ether

3. Chain isomers : Ethers having same alkyl group on either side of the oxygen atom but differ in the arrangement of the carbon chain within the alkyl group are called chain isomers.

Eg. In the above examples A, C, D and F are also chain isomers of one another while B and E are also chain isomers of one another.

General methods of preparation of ethers

1. Preparation of ether from alkyl halides (Williamson’s synthesis) :

The reaction in which alkyl halide and sodium or potassium alkoxide are reacted to form ether is known as Williamson’s etherification reaction. Eg.

Both symmetrical and unsymmetrical ether can be prepared from this reaction. Eg.

preparation of ether by williamsons method

2. Preparation of ether from alcohols : By dehydration

Ethers may be prepared by dehydration of alcohols either in the presence of acids or heated with alumina. Eg.

preparation of ether from alcohol

Laboratory preparation of diethyl ether (ethoxy ethane)

When an excess of ethyl alcohol is heated with conc. H2SO4 at 1400C, diethyl ether is obtained.

lab preparation of diethyl ether

lab preparation of diethyl ether(ethoxy ethane)

Procedure : A mixture of ethyl alcohol and conc. H2SO4 in the ratio of 1:1 by volume is taken in the distillation flask. The flask is then fitted with a dropping funnel containing alcohol. The flask is heated on a sand bath at 1400C. Ethanol is added at nearly the same rate that of the distillation so that the ether formed is continuously received in the receiver kept cold in the ice cold water.

Purification : The distillate thus obtained contains ether, ethyl alcohol, water and sulphurous acid. The acid is removed by washing with KOH or NaOH solution. The solution is then stirred with anhydrous CaCl2 to remove alcohol. Finally it is redistilled to obtain almost pure ether.

Physical properties of ether

  1. Solubility : Ethers are slightly soluble in water because some H-bonding can exist between ether and water molecules. However, they are freely soluble in non-polar i.e. organic solvents like alcohol and chloroform.
  2. Boiling point : The boiling point of ethers show a gradual increase with rise in molecular weight. Ethers are isomeric with monohydric alcohols but their boiling points are much lower than the isomeric alcohols. This is because unlike alcohols, ethers can not form intermolecular H-bonds.

Chemical properties of ether

1. Reactions involving the ethereal oxygen (Addition reaction):

a. Formation of peroxide :

Q) It is dangerous to boil sample of ether stored for a long time, why?

→ Due to presence of lone pair of electrons on the ethereal oxygen, ether when comes in contact with atmospheric oxygen in the presence of sunlight, it reacts with oxygen to form ether peroxide. Ether peroxide is highly unstable and explodes violently on heating causing serious accidents. Therefore, it is dangerous to boil the sample of ether stored for a long time.

reaction of ether with oxygen

b. Formation of oxonium salts :

Since ethers can behave as weak Lewis bases, they dissolve in cold and conc. mineral acids like HCl or H2SO4 to form oxonium salts. (i.e. Protonation from the acid).

reaction of ether with HCl and H2SO4

2. Reactions involving the cleavage of C – O bond ( Fission reaction) :

a. Reaction with halogen acids (HX) :

When ethers are heated with conc. halogen acids (specially HI and HBr), the C – O bond is cleaved and gives alcohol and alkyl halide.

reaction of ether with HI

In case of unsymmetrical ethers, smaller alkyl group forms alkyl iodide and larger alkyl group forms alcohol.

b. Hydrolysis : Ether on acidic hydrolysis forms alcohol. Eg.

hydrolysis of ether

c. Reaction with PCl5 :

When ether is heated with PCl5, the C – O bonds in the ether are cleaved and alkyl chloride is formed. Eg.

reaction of ether with PCl5

3. Reaction involving alkyl groups :

Halogenation ( Reaction with chlorine) :

The products we get when ethers react with halogens largely depend on the condition.

For example, we get α,α’-dichlorodiethyl ether when diethyl ether reacts with Cl2 in dark.

reaction of ether with chlorine

In presence of light and excess of chlorine, all the hydrogen atoms are replaced by chlorine.

Aromatic Ethers

aromatic ethers nomenclature

Preparation of anisole by Williamson’s method

When methyl halide is treated with sodium phenoxide, anisole is obtained.

Preparation of anisole

Note :

In aryl halides the C-X bond has partial double bond character due to resonance. This makes the C-X bond stronger and hence it becomes difficult to break the bond by nucleophile.

Reaction of anisole with HI

The ether linkage(C – O) is highly stable but methyl group can be removed with hydroiodic acid to form phenol.

Reaction of anisole with HI

Uses of ether

  • Ether is used as a solvent for dissolving oil, resin, petrol, gum, etc.
  • It is also used as a reaction medium for carrying out certain reactions due to its almost inert nature and also because of its great dissolving power. Eg. in the preparation of Grignard reagent, in Wurtz reaction etc.
  • It is also used as an anaesthetic.
  • It is also used as a coolant.

Questions and Answers from ether

1. Convert ethoxyethane to methoxyethane.

Convert ethoxyethane to methoxyethane.

2. Convert ethoxyethane to ethanoyl chloride.

Convert ethoxyethane to ethanoyl chloride

3. How would you obtain anisole from phenol and vice-versa.

How would you obtain anisole from phenol and vice-versa.

4. Starting from CH3ONa, how would you prepare methoxybenzene ? What happens when methoxybenzene is treated with excess HI ?

5. Ether is stored in a bottle containing iron wire, why?

Ether is highly reactive towards atmospheric oxygen in the presence of sunlight and forms peroxide which explodes on heating causing serious accident.

When ether is kept in a bottle containing iron wire, the oxygen combines with iron to form iron oxide and it prevents the formation of peroxide. Therefore, ether is stored in a bottle containing iron wire.

Ether is stored in a bottle containing iron wire, why?

Exercise

1. Write down the possible unsymmetrical ethers of C4H10O and their IUPAC names.

2. Write down an isomeric ether of isopropyl alcohol. What happens when the isomeric ether is heated with excess HI?

3. Write two unsymmetrical ethers having molecular formula C4H10O and write their preparation by Williamson’s method.

4. An ether A(C5H12O) when heated with excess of hot concentrated Hl produced two alkyl halides which on hydrolysis from compounds B and C. Oxidation of B gives an acid D whereas oxidation of C gave a ketone E. Deduce the structures of A,B, C,D and E.

5. Compound (A), C7H8O, does not react with sodium metal. When (A) is treated with Hl, two compounds (B) and (C) are formed. Compound (B) reacts with sodium to produce hydrogen, and also forms a salt with NaOH. Compound (B) can be brominated, and the major product contains three bromine atoms. Compound (C) contains iodine. Compound (C) reacts with magnesium in dry ether to form a Grignard reagent. The Grignard reagent reacts with CO2, followed by acid hydrolysis to form acetic acid. Identify (A), (B) and (C). Show the reactions involved.

6. Different compounds having same molecular formula are called isomers. Different isomers are possible for molecular formula C4H10O.

a. Write all possible isomers of this molecular formula.

b. Indicate the isomers which do not decolorize acidified potassium permanganate solution.

c. Identify two isomers which give single monohaloalkane on treatment with excess conc. HI.

7. An organic compound with molecular formula C4H10O does not react with Na and gives only single haloalkane when heated with excess conc.HI. identify the organic compound.

8. An organic liquid ‘A’ has molecular formula C4H10O which is soluble in cold and conc. H2SO4 but does not decolorize pink colour of acidified potassium permanganate. ‘A’ when refluxed with phosphorus pentachlodide gives C2H5Cl Identify ‘A’ giving necessary reactions. How does ‘A’ reacts with (a) cold HI (b) hot excess HI.

9. How is ethoxyethane distinguished from ethanol?

10. An organic compound A (C2H6O) reacts with sodium to form compound B with the evolution of H2 and gives a yellow compound C when treated with iodine and NaOH. When heated with conc. H2SO4 at 1400C, it gives a compound D (C4H10O) which on treatment with conc. HI at 1000C gives E. D is also obtained when B is heated with E. identify A,B,C,D and E,and write the equations for the reactions involved.

11. Ether can be prepared by Williamson’s method.

a. Write general reaction for the preparation of symmetrical and unsymmetrical ether by this method.

b. Indicate the nucleophile and leaving group in the given reaction.

c. What is the importance of this method?

d. Give two important uses of ether.

Give reason:

  1. Small piece of iron is kept in a bottle of ether.
  2. It is dangerous to boil a sample of ether stored for a long time.
  3. Anisole is prepared by using sodium phenolate and methyl iodide but not sodium methoxide and chlorobenzene.
  4. Williamson’s method is better than dehydration of alcohol to prepare ether .
  5. Boiling point of methoxy methane is lower than isomeric alcohol.
  6. Sodium metal can be used to dehydrate ethoxyethane but not ethanol.
  7. Ditert-butyl ether canot be prepared by Williamson’s method.
  8. [Hint: This reaction proceeds via SN2 nucleophilic substitution reaction]
  9. Cleavage of phenyl alkyl ether with HI always produces phenol and alkyl iodide and not iodobenzene and alkanols.

Convert:

  1. Ethoxyethane to methoxyethane.
  2. Ethoxyethane to ethanoyl chloride.
  3. Phenol to anisole and vice-versa.

References