198 Scientific Proceedings, Royal Dublin Society. 



We see that the reactivity is uniformly about twice as great in 

 methyl as in ethyl alcohol. This higher velocity in methyl as 

 compared with ethyl alcohol has been previously observed by 

 Menschutkin, Carrara, and others, and may be connected with the 

 superior dissociating power of the former. 



On examining the results obtained in ethyl alcohol, they appear 

 to present a very extraordinary irregularity. If we plot velocity- 

 coefficient against mass of alkyl radical, the nature of this 

 apparent irregularity is at once brought out. Methyl and isobutyl 

 iodides are seen to lie off the "curve," i.e., their reactivities 

 appear to be abnormally low. This result is particularly remark- 

 able in the case of methyl iodide, for in nearly all previous 

 kinetic investigations of the reactivity of the alkyl iodides, methyl 

 iodide has been characterised by a pre-eminently great reactivity ; 

 and this behaviour may indeed be said to characterise the lowest 

 member of a homologous series, just as the lowest homologue of 

 a natural family of elements stands out from the others. 



The low reactivity in the case of isobutyl iodide need not 



surprise us, for most of the iodides compared above possess a 



normal chain. The intimate relation between the structure of the 



alkyl radical and the reactivity of the iodide is strikingly shown in 



the case of isopropyl iodide. This substance is not included in the 



above list, because, owing to its enormous reactivity, it was not 



N 

 possible to determine the velocity-coefficient in — solution at 



N . 



24"5°. By employing —^ - equivalent solutions of isopropyl 



iodide and silver nitrate, the velocity of the reaction was sufficiently 

 slowed down to permit of accurate measurement. Thus at 17°, 

 the value of k was found to be about 2000 x 10~ 5 . Considering 

 that the velocity of a chemical reaction is increased two or three 

 times by a rise of temperature of 10°, we may say that isopropyl 

 iodide is at least seventeen times more reactive than ethyl iodide, 

 as measured by silver nitrate. 



Unlike the cases of acids and bases (and, to a less degree, 

 salts), it is not in general possible, however, to construct tables of 

 the relative reactivities of neutral organic substances — even of 

 substances of such a simple type as the alkyl iodides — which shall 

 be independent of the reagent and of the particular reaction 



