1880.] 



The Aluminium-Iodine Reaction. 



549 



heated it splits up into alumina, propylene, and bodies of the cresylic 

 group. One of these is a solid body, which when resnblimed or 

 crystallised from alcohol presents itself in most beautiful pearly plates. 

 Its properties resemble rafcher those of an ether, and concordant analyses 

 of different specimens and determinations of vapour density give the 

 anomalous molecular formula C 14 H 13 0, but its real constitution is still 

 the subject of inquiry. 



Theory of Reaction. 



The reaction by which the compounds described above are formed 

 is by no means clear at first sight. It is difficult to imagine that the 

 presence of a haloid salt should determine the direct replacement of 

 hydrogen by aluminium in water or in alcohol. We believe however 

 that the presence of hydriodic acid in the reaction with the aromatic 

 alcohols furnishes the true key to the chemical change. The first 

 action is in all probability that of a reciprocal decomposition between 

 two binary compounds — the alcohol and aluminium iodide — forming a 

 certain proportion of aluminium alcohol and hydriodic acid. But the 

 metallic aluminium present at the same time decomposes the hydracid 

 with evolution of hydrogen and the production of more aluminium 

 iodide, which brings about a further redistribution of the elements 

 and the formation of more of the aluminium alcohol and the hydriodic 

 acid, and so on till the chemical change is complete. The following 

 equations express these changes, which take place alternately or rather 

 continuously : — 



(1) 6(C w H 2ra _ 7 O.H)+AU 6 =Al 2 (C K H 2M . 7 0) 6 + 6HI. 



(2) 6HI+A1 2 =A1 2 I 6 + 6H. 



It is evident that as the iodine does not enter into the final products 

 a very small quantity of it is sufficient to carry on the continuous 

 chemical change. 



There can be little doubt that the same reactions occur in the case of 

 the alcohols of the C w H 2W+1 O.H series, but the amount of hydriodic acid 

 formed in the reciprocal decomposition is perhaps very small, and the 

 tenacity with which it is held by the alcohol has made it impossible 

 for us to prove its separate existence. The same remark applies also 

 to water, which we believe to act in the same way. Indeed we know 

 from the experiments of Hoscoe that there would be no chance of 

 separating minute quantities of acid from an aqueous solution under 

 the circumstances. 



The part which the free elements take in the decomposition of the 

 ethers is doubtless analogous to that which the combined elements 

 play in the action with water and alcohol. The reason why the free 

 elements do, and aluminic iodide does not attack the ethers, is probably 



2 q 2 



