Chemistry PREPARATION OF ETHERS : WILLIAMSON SYNTHESIS ETC.

From Alcohols :

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

(a) Acid -catalysed dehydration :

The yield of alcohol however depends on the nature of alcohols (`1^o` ,`2^o` or `3^o`) `2^o` alcohol mainly gives alkenes with low yields of ethers whereas `3^o` exclusively gives alkenes.

Order of dehydration of alcohol or formation of ethers : `1^o > 2^o > 3^o`

`CH_3CH_2OH underset(140^oC)overset(H_2SO_4)-> undersettext(Ether)(CH_3CH_2 -O - CH_2CH_3)`

`text(Mechanism of)` `S_N2` : See fig.

Catalytic dehydration : Dehydration of alcohols to ethers can also be achieved by passing the vapours of an alcohol over heated alumnia at `523` `K`.

e.g., `CH_3CH_2-OH + H-OCH_2CH_3 underset(523 K)overset(Al_2O_3)-> CH_3CH_2-O-CH_2CH_3 + H_2O`

(b) By the action of diazomethane on alcohols - Methyl ethers can also be prepared by action of `CH_2N_2` on alcohols in presence of fluoroboric acid (`HBF_4`) as catalyst.

`CH_3CH_2OH + CH_2N_2 overset(HBF_4)-> CH_3CH_2-O-CH_3 + N_2`

Williamson's Synthesis :

It is the most important method for formation of ethers. It is a nucleophilic substitution reaction. Nucleophile (`S_N2`) attack by alkoxide ion on an alkyl halide/alkyl sulphate/alkyl sulphonato which are known as substrates.

a) Substrates should have good leaving group like `X^(-)`, `- OSO_2`, `-OSO_2R`

b) Substrates must have a primary alkyl group for good yield.

c) In case of tertiary substrate elimination occurs giving alkenes.

d) With a secondary alkyl halide, both elimination and substitution products are obtained.

`R - X + Na^(+) - text()^(-)O - R' -> R - O- R' + Na^(+)X^(-)`

`text(For example :)` See fig.1.

Reaction involves nucleophilic substitution of alkoxide ion for halide ion; it is strictly analogous to the formation of alcohols by treatment of alkyl halides with aqueous hydroxide. See fig.2.

Since alkoxides and alkyl halides are both prepared from alcohols, the Williamson method ultimately involves the synthesis of an ether from two alcohols.

If we wish to make an unsymmetrical dialkyl ether, we have a choice of two combinations of reagents; one of these is nearly always better than the other. In the preparation of tert-butyl ethyl ether, for example, the following combinations are conceivable : See fig.3.

Alkoxides are not only nucleophiles, but also strong bases which tend to react with alkyl halides by elimination to yield alkenes. Whenever we are trying to carry out nucleophilic substitution, one must be aware of the danger of a competing elimination reaction. The tendency of alkyl halides to undergo elimination is `3^o > 2^o > 1^o`

In the above example, the use of the tertiary halide is rejected as it would be expected to yield mostly or all elimination product; hence the other combination is used. Aromatic ethers are formed when phenoxides react with alkyl sulphates following `S_N2` mechanism.

`C_6H_5- Na^(+) + CH_3-OSO_2O- CH_3 -> C_6H_5OCH_3 + NaSO_2OCH_3`

From Alkenes :

Ethers can also be prepared by the addition of alcohols to alkenes in presence of acids as catalyst.

From Grignard Reagent :

Higher ethers can be prepared by treating a - halo ethers with suitable Grignard reagents.

Stability of Ethers :

On standing in contact with air, most aliphatic ethers are converted slowly into unstable peroxides. Although present in only low, these peroxides are very dangerous, since they can cause violent explosions during the distillation that normally follow extractions with ether.

The presence of peroxides is indicated by formation of a red colour when the ether is shaken with an aqueous solution of ferrous ammonium sulfate and potassium thiocyanate; the peroxide oxidizes ferrous ion to ferric ion, which reacts with thiocyanate ion to give the characteristic blood-red colour of the complex.

`text(peroxide) + Fe^(2+) -> Fe^(3+) overset(SCN^(-))-> undersettext(Red)(Fe(SCN)_n^((3-n)-)) (n-1 text(to) 6)`

Peroxides can be removed from ethers in a number of ways, including washing with solutions of ferrous ion (which reduces peroxides), or distillation from concentrated `H_2SO_4` (which oxidizes peroxides).

For use in the preparation of Grignard reagents, the ether (usually diethyl) must be free of traces of water and alcohol. This so-called absolute ether can be prepared by distillation of ordinary ether from concentrated `H_2SO_4` (which removes not only water and alcohol but also peroxides), and subsequent storing over metallic sodium.