THE ETHERIPICATION PROCESS S. T. BOWDEN University College, Cardiff
S T ~ Eare Nusually T S taught that ethyl ether is produced through the interaction of ethyl dcohol and sulfuric acid in with the mechanism known as of organic ~ i l l i theory. ~ ~ ~ ~ most ~ vtextbook ~ chemistry the etherification process is represented as involving the intermediate formation of an ester as shown by the two consecutive reactions
+
(2) I n general, the ester of the acid is not involved in the formation of ether. Hydrochloric acid is even a better catalyst than sulfuric aoid a t 155-160°, and if the reaction be represented according to the Williamson we have
+
+
EtOH HC1 = EtCl &O E ~ C I E ~ O H= ~ t o - E t HC~
+
+
EtOH HISO, = E~HSOI H ~ O EtHSO. f EtOH = EM--Et HzSO,
+
This formulation, however, is incorrect since ethyl chloride does not react with alcohol under these conditions. ~ of ~ ~h t h ~ ~r i f i ~~ ~ t i ~ ~ The , v assumption that an ester is formed as an his first paper on the n intermediate product which reacts with a fresh molecule Williamson (1) wrote as follows: of alcohol is untrue in the case of hydrochloric acid and MYtask is now to explain the process of a e t h e i f i ~ t i o nby the is superfluous in the case of sulfuric acid. ~h~ only action of sulphuric acid upon alcohol; and in order to accomphsh difference lies in the fact that the ester of sulfuric acid I must the connexion between those suhstsncas and I must f i s t show how a substance anallo- is itself an acid and does not escape from the reaction the reagents used. gous to my iodide of aethylo is formed, and then how by douhle mixture on account of its high boiling point. It may decomposition withdrahol it produces aether. This isveryeasy; be noted that ether is formed when alcohol is heated for sulphovinic acid is strictly anqlagous to iodide of aethyle with sulfuric acid as dilute as half a per cent. plus iodide of hydrogen, which we should obtain by replacing SO, (3) Salts of strong acids and weak bases ~roduceether in its fomuls. by an equivalent of iodine; and in order to sent the formation of this sulphovinic acid, which is well known to when heated with aqueous alcohol. Thus, ether is precede that of aether, the simplest mode is at the same time the formed in 75 per cent yield by heating anhydrous ferric one most free from hypothesis; it consists in stating the fact that sulwith alcohol (96 per cent) at 1550, copper sulphuric aoid and alcohol are transformed into sulphovinic aoid fate may be employed at a higher temperature for and water. . . . Now from this point it is clear that the process is the same as in the decompositions above described; for by this the same Purpose. The salts of trivalent metals are the sulphovinic acid coming into contact with an atom of alcohol, it best catalysts, and the sulfates are to be preferred to reacts exactly in the same manner as the iodide did, forming of the chlorides since the latter also give rise to ethyl course sulphuric seid and aether. The sulphurio acid thus repro- chloride. The conversion of glycol into dioxane is duced comes again in contact with alcohol, forming sulphovinic as with with ferric acid, which reacts as before; and so the process goes on oontinu- effected as acid, and a better yield of the cyclic ether is obtained ously, as found in practice . . . and the alternate formation and decomposition of sulphovinic acid is to me, as to the partisans with the former since it does not catalyse the side reof the chemical theory, the key to explaining the process of aetheri- actionsto the same extentas sulfuric acid, fication. Other theories have been proposed to explain the Accordmg to Van Alphen (8) a study of the original formation of ether from alcohol (5) and Boesekeu (4), in publications reveals that Williamson merely tried to particular, has rejected the Williamson theory in so far prove--and this he did successfully-that one molecule as it postulates the intermediate formation of ethyl of ether is produced from two molecules of alcohol and hydrogen sulfate as an essential feature of the process. that ether has the double and alcohol the simple The reaction has been studied in detail by Van Alphen formula. This investigation also points out that the (5) who has examined the action of several acids and a oft-quoted experiment in which three ethers are pro- number of salts on ethyl alcohol under various thermal duced by heating two alcohols with sulfuric acid fur- conditions. According to the theory of Van Alphen, the nishes no proof of the above mechanism since it simply formation of ether in this way involves the equilibrium shows that an ether can be derived from water by re- reaction placing the two hydrogen atoms by alkyl groups. 2EtOH F? E t U E t H20 The objections to the so-called Williamson theory which is catalysed by hydrogen ions (H30+)a t temperamay be briefly summarised as follows: That the hydrogen ion is the (1) All strong and moderately strong acids can con- tures above 100'. vert alcohol into ether. The reaction occun in the catalyst is proved by the fact that the salts of strong presence of hydrochloric, hydrobromic, hydriodic, sul- acids and weak bases (which in the presence of water furous, phosphoric, arsenic, chloroacetic, tartaric, give rise to hydrogen ions) can be used instead of the rnaleic, and benzenesulfonic acids, but not in the acids themselves. This formulation brings the reaction presence of weak acids, such as acetic acid. into l i e with the well-known group of reactions, com-
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..
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432
SEPTEMBER, 1947
prising the esterification of acids, the hvdrolvsis of estersythe formation of acetals, the fissiod of &aldehyde, the formation of lactones, the inversion of sugars, etc., which may be represented schematically COH
inmuch as drogen ions.
+ HOC Ft C-0-C + HzO
of these Feactions am catalysed Gy hy-
433
LITERATURE CITED London, Edinburgh, Dublin Phil. Mag,, [31 350 (1850). (2) Rec. hau. 49,754 (1930). (3) LIEBIG, Ann., 9, 1 (1834); 23, 12 (1837); 30, 129 (1839); GRAHAM, ibid., 75, 108 (1850); MITSCHERLICA, Ann. Physik., 31, 350 (1834); ROSE, ibid., 48, 1463 (1839). (4) "Koolwatmtotfen1," Delft, 191% P. 78. (5) Rec.trav. chim., 49,492,754,1040 (1930).
