Preparation and Properties of Ethers, Chemistry tutorial

General Methods of Preparation:

Ethers can be prepared from the given methods:

a) Dehydration of alcohols:

Simple or symmetrical ethers are obtained whenever excess alcohol is heated by the concentrated tetraoxosulphate (VI) acid or tetraoxophosphoric (V) acid. In the method, two molecules of alcohol lose a water molecule amongst themselves to make ethers.

ROH + HOR → (H2SO4/heat) → ROR + H2O

The process is used industrially in the preparation of lower ethers. Dehydration of primary alcohols might as well be effected via passing alcohol vapor over heated catalysts such as alumina at high temperature and pressure.

ROH + HOR → (Al2O3 250oC/pressure) → ROR + H2O

If secondary and tertiary alcohols are utilized under these conditions, alkene is the major product.

b) Williamson Synthesis:

This process is the most significant method for laboratory preparation of ethers. It comprises heating an alkyl halide by sodium or potassium alkoxides. 

R O Na + R'X + (heat) → R O R' + Nax

Sodium    alkyl                  ether

alkoxide   halide

For illustration:

C2H5ONa + C2H5Cl + (heat) → C2H5OC2H5 + NaCl

Sodium        ethyl                    Ethoxyethane

ethoxide      chloride

The reaction comprises nucleophilic substitution of alkoxide ion for the halide ion.

1432_Nucleophilic substitution of alkoxide ion.jpg

Fig: Nucleophilic substitution of alkoxide ion

This is the most appropriate for preparing mixed or unsymmetrical ethers.

c) Heating Alkyl halides with dry silver oxide:

Ethers might be made up by heating alkyl halides with dry silver oxide (Ag2O).

2RX + Ag2O + (heat) → R O R + 2AgX

For illustration:

2C2H5I + Ag2O → heat → C2H5OC2H5 + 2Ag2I

Mixed ethers can be made up by taking various alkyl halides in the equimolar ratio.

2RX + Ag2O + R'X + (heat) → R O R' + 2AgX

CH3I + Ag2O + C2H5I + (heat) → CH3OC2H5 + 2Ag2I

d) Reaction of lower halogenated ethers by Grignard reagents:

Higher homologues of ethers can be made up by the action of Grignard reagent on the lower halogenated ethers.

ROCH2X + R'MgX → ROCH2R' + MgX2

Halogenated Grignard      ether

Ether            reagent 

For illustration:

CH3OCH2Cl      +     C2H5MgBr     →      CH3OCH2C2H5 + MgBrCl

Chloromethoxy    ethyl magnesium         Methoxy propane

Methane                     bromide

e) Oxymecuration-demecuration:

Alkene reacts by mercuric trifluoroacetate in the presence of alcohol to provide alkoxymecurial compound that on reduction provides ether.

244_Oxymecuration-demecuration.jpg

Fig: Oxymecuration-demecuration

f) Action of diazomethane on alcohols:

Methyl ethers might be made up by the action of diazomethane on alcohols in the presence of catalyst such as boron trifluoride or fluoroboric acid.

ROH + CH2N2 + (BF3) → ROCH3 + N2

       Diazomethane

For illustration:

C2H5OH + CH2N2 + (BF3) → C2H5OCH3 + N2

                                         Methoxy ethane

General Physical Properties:

1) The lower members are gases whereas higher members are colourless, pleasant smelling, volatile liquids and low boiling point liquids.

2) They are lighter as compared to water.

3) They are sparingly soluble in water however readily soluble in the organic solvents such as benzene and chloroform. Their solubility rises in the presence of alcohol most likely, due to the hydrogen bonding.

4) Ethers are as well very significant solvents in the laboratory.

5) Their melting points boiling points and specific gravity rises with the increase in molecular weight.

6) The boiling point of ethers is much lower as compare to the isomeric alcohols. The higher boiling points of alcohols are because of the intermolecular hydrogen bonding that is not possible in ethers.

7) Lower ethers acts as anesthetic and their vapor are very inflammable.

8) Ethers are polar molecules and encompass definite dipole moments. The value of dipole moment of Ethoxy ethane is 1.12D. The C-O-C valency angle in the methoxy methane is 111o whereas that of Ethoxy ethane is 118o

Chemical Properties of Ethers:

Ethers are fairly unreactive compounds. The ether linkage is rather stable towards bases, oxidizing agents and reducing agents- they don't react with alkalis, by dilute acids, phosphoric halides and so on in the cold. This is due to the reason that they lack the active hydrogen attached to oxygen as present in alcohol. Though, they exhibit reactions due to the presence of:

1) Alkyl radicals: They go through substitution reactions as in the case of alkanes.

2) Ethereal oxygen: This coordinates by electron deficient molecules or the Lewis acids.

3) Carbon-oxygen bond: It exhibits some cleavage reactions identical to the carbon-carbon cleavage. Though, the carbon-oxygen linkage is not as stable as the carbon-carbon linkage.

Reactions of the alkyl groups: Substitution Reaction

i) Halogenation:

Ethers go through substitution at the alkyl radical whenever reacted with chlorine or bromine in the absence of sunlight. They make halogen substituted ethers.

ROCH2CH3 + Cl2 + (dark) → ROCHClCH3 + HCl

Generally the hydrogen of the α-carbon is replaced most readily.

For illustration:

C2H5OC2H5 + Cl2 + (dark/-HCl) → C2H5OCHClCH3 + (Cl2/-HCl) → CH3CHCl-O-CHClCH3

During sunlight, all the hydrogen is replaced to form perhalo ethers. For illustration:

C2H5OC2H5 + 10Cl2 + (light) → C2Cl5OC2Cl5 + 10 HCl

                                               Perchloroethoxy

                                                      Ethane

ii) Combustion:

Ethers are basically volatile and highly inflammable. They burn in air to form carbon (IV) oxide and water.

C2H5OC2H5 + 4O2 → 4CO2 + 5H2O

Reactions of ethereal oxygen:

The oxygen atom in ether consists of two unshared lone pairs of electrons therefore ether acts like Lewis bases and coordinates with Lewis acids or substances deficient in the electrons.

For illustration:

i) Formation of peroxides:

Peroxides are made if ethers are taken via a prolonged action of atmospheric oxygen or ozonized oxygen due to the co-ordination of one lone pair of electrons of the ethereal oxygen by other oxygen atom.

2347_Formation of peroxides.jpg

Fig: Formation of peroxides

For illustration:  

C2H5OC2H5 + O → (C2H5)2O → O

These peroxides are unstable compounds and decompose aggressively on heating.

ii) Formation of oxonium salts:

The Ethers react by strong mineral acids to form oxonium salts that are stable at low temperature in high concentration. The oxonium salts whenever formed dissolved in the solution.

1349_Formation of oxonium salts.jpg

Fig: Formation of oxonium salts

This describes why they are employed as solvent for the Grignard reagents.

Reactions involving cleavage of carbon-oxygen bond:

i) Hydrolysis:

If ethers are boiled with water or treated with steam, they are hydrolyzed to form alcohols. The rate of hydrolysis rises in the presence of dilute acid.

ii) Action of tetraoxosulphate (VI) acid:

Whenever ethers react by hot tetraxoxsulphate (VI) acid, cleavage of the carbon-oxygen bond takes place.

ROR' + H2SO4 (conc.) + (heat) → ROH + R'HSO4

For illustration:

C2H5OC2H5 + H2SO4 (conc.) + (heat) → C2H5OH + C2H5HSO4

                                                                 Ethanol    Ethyl hydrogentetraoxosulphate (VI)

iii) Action of hydroboric or hydriodic acid:

Ethers react by hydroboric or hydriodic acids in the cold to form an alcohol and alkyl halide. Whenever unsymmetrical ether is used, the halogen links itself to the smaller of the two alkyl groups of the ether.  

ROR + 2HI + (Hot) → 2RI + H2O C2H5OC2H5

    + 2HI + (Heat) + (2C2H5I) → H2O CH3OC2H5

    + 2HI + (Heat) + CH3I + C2H5I + H2O

The order of reactivity of the acids is: HI > HBr > HCl

iv) Action of Phosphorus pentachloride (PCl5):

Ethers react by hot phosphorus pentachloride, the reaction comprise cleavage of the carbon-oxygen bond to outcome alkyl chlorides.

ROR' + PCl5 → RCl + R'Cl + POCl3

C2H5OC2H5 + PCl5 → 2C2H5Cl + POCl3

v) Action of acid derivatives:

Ethers react by acid chlorides and acid anhydrides in the presence of the catalyst such as aluminium chloride or zinc chloride to form esters and alkyl halides.

ROR + R'COCl + (AlCl3) → RCl + R'COOR

                                                        Ester

For illustration:

C2H5OC2H5 + CH3COCl + (AlCl3) → C2H5Cl + CH3COOC2H5

                    Acyl chloride                              Ethyl ethanoate

C2H5OC2H5 + (CH3CO)2O + (ZnCl2) → CH3COOC2H5

                   Ethanoic acid

                     Anhydride

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