Donnan — On the Reactivity of the Allcyl Iodides. 201 



So far as the general order of reactivities is concerned, 

 these results are in very good agreement with those obtained 

 by Menschutkin. It may be noted that isopropyl iodide is 

 characterised by a relatively low reactivity. 



If relative reactivity be plotted against mass of alkyl radical, 

 the general resemblance between the results of Menschutkin, and 

 of Hecht, Conrad, and Bruckner, is found to be very striking. 



Considering the similarity of the results obtained in 

 these three totally different reactions {i.e. with sodio-aceto- 

 acetic ester, triethylamine, and sodium alcoholate) and the — 

 in two main respects at all events — widely divergent order of 

 reactivities as measured by silver nitrate, there is good ground for 

 the belief that, in the latter case, the reaction is intimately 

 associated with some special function or state of the alkyl iodide 

 molecule. The number and comparative complexity of the pro- 

 ducts of the reaction afford further confirmation of this. Whether 

 these results can be explained by Nef's dissociation-hypothesis, or 

 indeed by any dissociation-hypothesis, can only be decided by 

 further detailed investigation, a portion of which has been 

 already carried out without, however, yielding any perfectly 

 decisive results so far as the intimate mechanism of the reaction 

 is concerned. 



As is well known, the alkyl iodides turn yellow or brown 

 when exposed to light, the coloration of the solution being due 

 to free iodine. Possibly this reaction is connected with the dis- 

 sociation-equilibrium : — 



R-CHJ ^± R-CH + HI, 



the hydriodic acid being oxidized by the oxygen of the air usually 

 present. The latter reaction is known to be catalytically accelerated 

 by the presence of light. However that may be, an interesting 

 relation between the rate (and extent) of this production of free 

 iodine and the corresponding velocity-coefficients (as measured by 



N 

 silver nitrate) has been established. -^ - solutions of the alkyl 



iodides in dry alcohol were put into tightly-corked test-tubes 

 and exposed to uniform diffused daylight. On the following day, 

 the solution of isopropyl iodide showed a faint yellow coloration. 

 The other solutions became coloured in due course in the relative 



