INDUSTRIAL A N D ENGINEERING CHEMISTRY
184
Vol. 16, No. 2
New Catalytic Effects of Zinc Chloride and Aluminium
Chloride' By J. F. Norris MASSACHUSETTS INSTITUTE O F
NE of the major problems being studied at the institute is the change in reactivity of atoms and groups in organic compounds as affectedby changes within the molecule and in its environment. Relative reactivities are measured by comparing the rates a t which the several members of a series of compounds of the same type react with a second compound which is the same in all cases. In this way the effect of changes within the molecule can be measured. The influence of the presence of substances outside the molecule, such as solvents or catalytic agents, can be measured in the same way. The solvent or catalytic agent affects the bonds in the activated compoundin the same way that changes within the molecule bring about this effect. I n many cases compounds made up of the catalyst and the activated molecule have been isolated. It is highly probable that in cases where such addition compounds are not formed, a molecular attraction exists, which results from residual affinities. Two molecules, when brought together, must exert an influence, one on the other, and this must result in a change in the attractions between the atoms within each molecule. Solvents have a marked effect on the rate a t which a given reaction proceeds. From the foregoing point of view, they act as true catalysts by altering the strength of the affinities between the atoms in the dissolved molecule. When the changes set up in the affinities lead to increased reactivity, the added substance is a positive catalyst. If, on the other hand, the reactivity is reduced, the substance functions as a negative catalyst. The normal rate of reaction between two molecules occurs when these two kinds alone are present. The work referred to above led to a knowledge of the effect of changes within the molecule on the reactivity of the following bonds: C-OH, CO-H, C-C1. In this paper will be described briefly some of the results of the study of the activation of the C-C1 and the C-OH bonds by aluminium chloride and zinc chloride, substances known to form molecular compounds with one of the reacting compounds. When benzoyl chloride reacts with aluminium chloride, the resulting compound contains a very active chlorine atom. Advantage is taken of this in preparing benzophenone by the Friedel-Crafts reaction. The molecular compound has been found to react with a wide variety of substances, and condensations have been effected with ethers, esters, ketones, unsaturated hydrocarbons, and saturated hydrocarbons. At this time, however, a second type of reaction, which has a bearing on some of the views put forward by Professor Bancroft, will be considered. When the molecular compound of triphenylchloromethane and aluminium chloride is treated with ether, triphenylmethane is formed. A study of the mechanism of the reaction led to the view that it took place in steps, the first being represented by the following equation: (CeH6)aCCI CzHbOCzHs = (CeHa)sCOCzHs f CLHSCI An analogous reaction occurs in the cold when benzoyl chloride dissolved in ether is treated with aluminium chloride. Ethyl chloride and ethyl benzoate are formed. Further action of aluminium chloride on (CBH&COC~H~ proceeds as follows :
0
+
1 Presented as a part of the Intersectional Symposium on Catalysis a t the joint meeting of the New Haven, Connecticut Valley, Rhode Island, and Northeastern Sections of the American Chemical Society, Cambridge, Mass., January 12, 1924.
TECHNOLOGY, CAMBRIDGE, MASS.
( C ~ H ~ ) B C O= C ~(CeHs)aCH H~ 4- CHBCHO In the presence of aluminium chloride the carbon-oxygen bond, the weakest in the compound, is first broken. T h e oxygen saturates itself by drawing upon the carbon, which consequently loses a hydrogen atom: H H (CeHa)aC-0-C-CHa = (CsHs)sCH 0 = C-CHs
+
H
+
It seemed of interest to determine if such a decomposition, could be produced by heat. The pure ether broke down on heating, as indicated above. By an analogous reaction butyric aldehyde was prepared from the nosma1 butyl ether of triphenylcarbinol. In these cases heat alone brought about a decomposition which aluminium chloride effected a t room temperature. The work suggests the possibility of finding other reactions in which the catalyst will produce the same results as exposure of the compound to high temperatures. The work leads to a very simple way of preparing triphenylmethane in quantity. A mixture of carbon tetrachloride, benzene, and aluminium chloride is allowed to react over night. Ether is then added, and after standing some hours the mixture is decomposed by water and triphenylmethane is obtained from the reaction product. Excellent yields and very pure materials have been obtained. The second case of activation of a chemical bond which will be considered briefly is that of the carbon-oxygen bond in primary alcohols. These alcohols will not react with aqueous hydrochloric acid in an open vessel, and as B consequence alkyl chlorides are prepared from anhydrous alcohols, zinc chloride, and hydrogen chloride. The zinc chloride has been supposed to act as a dehydrating agent. Zinc chloride forms compounds with alcohols, and it is possible that in this synthesis it acts catalytically by activating the hydroxyl group: Zinc chloride forms a compound with water. The question naturally arose whether the molecular compound of the chloride with alcohol would be stable in the presence of water. Experiments showed that primary alcohols react readily with concentrated hydrochloric acid if zinc chloride is present, indicating that even in the presence of water enough of the molecular compound of the chloride with alcohol is present to make the reaction possible. The results serve to emphasize the facts established by Whitaker in his study of the preparation of organic esters in thepresence of water, and also the observations of Kendall that t h e compounds of organic acid and hydrochloric acid exist in water solution. It is probable that other catalytic reactions involving the formation of water, which are ordinarily carried out in anhydrous solvents, can be brought about in t h e presence of water. Whitaker's work led to the preparation on an industrial scale of ethyl acetate from dilute acetic acid, and as a result a great organic chemical industry was built up. Summing up, the facts presented show, first, that a catalyst can cause, a t ordinary temperatures, the severing of the same bond that breaks when decomposition takes place as a result of high temperatures; and, second, that a substance that, has been thought to act as a dehydrating agent in bringing about a reaction involving the elimination of mater, effects. the condensation as a result of its catalytic influence, even though water is present.
