By ZELDA KAIIAN, B.Sc. SOME Years Ago, Ur. Travers, Wishing to Prepare Butane, Employed for This Purpose the Well Known Method O
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View Article Online / Journal Homepage / Table of Contents for this issue 132 KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES. By ZELDAKAIIAN, B.Sc. SOMEyears ago, Ur. Travers, wishing to prepare butane, employed for this purpose the well known method of heating together ethyl iodide and zinc. He found, however, that, instead of obtaining pure butane, a mixture of gases was formed, the chief constituent of which was ethane, To investigate the cause of this, a few preliminary experi- Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. ments were performed, but the work was abandoned owing to lack of time. The investigation of the influence of heat on the alkyl iodides is so much the more important in view of the interesting experiments performed by Nef, and the theory of the bivalent carbon atom based upon them. If it could be proved that in the state of vapour the alkyl iodides are dissociated, it would at least be strong evidence of their being in a similar state in solution. As a result of a large number of experiments (ilnnalen, 1892, 270, 267 ; 1897, 297, 202 ; 1899, 309, 126; 1901, 318, I), Nef has arrived at the conclusion that all chemical actions, whether of addition or substitution, are to be explained by an application, in each case, of the principle of dis- sociation. As regards the alkyl halides, Nef assumes that the first stage in their reaction in solution, whatever the other reacting sub- stance may be, is a dissociation into an alkylidene or methylene deriv- View Article Online KAIIAN: THE EFFECT OF HEAT ON THE ALKYI, IODIDES. 133 X ative and the halogen acid; thus a compound of the type R<Y would yield R< + (X + Y), these two active residues then react with the active residues of the other substance (produced in a similar manner), yielding the observed products of the reaction. Without discussing the large number of interesting experiments performed by Nef in proof of his theory, it may be mentioned that much of the earlier work on the subject is either directly in favour of tlhis theory, or at least of tho dissociat,ion of tlie dkyl halide into olefine and haiwgan acid. Thus all the facts ohserved by Frankland in his exhaustive study of the dkyl iodides can be explained very simply on the hypothesis of their dissociation, whereas any other explanation is wholly inaclequate to elucidate all the observed facts. Among the most instructive of his experiments are those of the reaction of zinc with ethyl iodide in presence of water, alcohol, and ether. With water, the only gas evolved was ethane, the large number of hydrogen H- atomsnntiir.allyredncing t,he alkylidene, thus : MeCH< + i-= MeCH,, also MeCII< + H-OH-+ EtOH( + AleC H<) -+ Et*O*Et,ether, also, being a product of the reaction. As is to be expected, the products of the reaction are the same with alcohol, but with ether, ethylene and butane are also formed. That this explanation is probably the t,rue one is further confirmed by the work of Niederist (Jozcm. Chem. SOC.,1577, ii, 422); Butleroff (Compt. rend., 1861, 53, 247); Buckeisen and Wanklyn (Jount. Chem. Soc., 1861, 13, 140); Erlen- meyer (Ann. I’hp. Chem., 1866, 139, 228) ; Aronstein (Bey., 1881, 14, 607 ; 1880, 13, 4S9), and many others. The most interesting case of all is perhaps the production of ethers and olefines when the alkyl halides react with potassium or sodium othoxides, or with silver nitrate in alcoholic solution, and in this con- Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. nexion it might be mentioned that the criticism passed on this theory by Burke and Donnan (Trans., 1904, 85,561) on account of Brussof’s results (Zeitsch. physikal. Chem., 1900, 34, 129) is not really valid. Brussof measured the total amount of olefine obtained from the various alkyl iodides under similar conditions, and found that isopropyl iodide gave very nearly the sitme quantity of olefine as isobutyl iodide. From this, it would appear that the amount of alkylene dissociation was practically the same for isopropyl as for isobutyl iodide, whereas, according to Burke and Uonnan’s experiments, the velocity of reaction for isopropyl iodide ~iclsilver nitrate in alcoholic solution is at least one hundred and forty times as great as that for isobutyl iodide. Now, although it is ti-ite that hydrogen iodide obtained from either kind of dissocitition is ocjI1itlly ofticient for reacting with silver nitrate, the :mount of oletino oltaiiied is not necessarily a measure of the View Article Online 134 IChllhN : 'L'EIE EFFECT OF HEA'l' ON THE ALKYTJ lOI)IDES, velocity of dissociation; at most, it only shows how readily the butylidene or propylidene passes into the corresponding olefine, thus : 1 1 li. C,II,T. -+ cir,aI< -+ CH,-CH, -+- CH,:CH,, II 1 , 3 B. (CH3),CHI + (CH3)2C< -+ CH,*CH*CH, -+ CH,*CH:CH,, II 1 > c'. (CR3),CII*CH2T---+ (C€€3)2C€I*CI!C< -+ and what Brussof's experiments show at most is, that in d the changes (2) and (3) are less mpid than in B ancl C, whilst the two latter may be approximately equal. This result is in complete agreement with Nef's own results (Annalen, 1901, 318, 1). The initial dissociation or its velocity may be far more in one case than another, with con- sequent production of much acid, but if less oletine is formed, then there is more alkylene available for the production of polymericles or addition products. AS n matter of fact, Brussof himself found that the amount of ether formed is the greater the less the olefine evolved. It seems more difficult, however, to account for the fact that, whereas in the renction of isobutyl iodide with alcoholic potash Nef obtained only isobutylene but no isobntyl ethyl ether, in the reaction with silver nitrate the ether is produced besides butylene. May this fact, however, not depend on the relative stability of the butyl ethyl ether in alkaline ant1 acid solution 1 The function of the sodium hydroxide, as of the silver nitrate, is, indeed, to remove the hydrogen iodide, but in the case of sodium hydroxide the reacting mixture is throughout an alkaline one, and if the ether is unstable in alkaline solution H it would at once break up, thus : Me,CH*CH<O.C -+ 25 Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. Me,CH*CH< + C,H,*OH ; there woulcl thus be a large number of alkylidene residues which would be favourable to the formation of isobutylene. Silver nitrate, however, produces the free acid, and this may be favourable to the formation of the ether. EXPERIMENTAL. Action OJ neat on EthJ Iodide. From the foregoing and other work that might be cited, it will be seen that a good deal of the experimental evidence is eithcr in favour of or not against the theory that the first stage in the react'ion of the Rlkyl iodides is their dissociation into linlogen acid and alkylidene or alkylene, aucl the objcct of the experiments described in this paper was to throw further light on the subject by a more detailed study of View Article Online KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES. 135 the behaviour of the alkyl iodides under the influence of heat. The iodides were generally allowed to stand over silver oxide before re- distillation over phosphoric oxide. isoPropyl iodide seemed to decom- pose whon distilled over the latter, and had therefore to be dried with calcium chloride ; it could, also, never be obtained colourless, unless first decolorisod completely with silver oxide. The vapour density of ethyl iodide was first determined by Victor Meyer's method, with tho following results, which must be regarded as qualitative only : Exp. Temp. Density. z. Wnlm ........................... 100" 78.5 11. Aiiiliiic ........................ 180 111. Dinictliyluiiiliii~ ............ 192 IV. Ethyl benxoata ............... 210 57 '6 Calculating the percentage dissociation by means of the formula : Temp. Extcnt of tlissocistioii =a, 100" - 180 - 192 1-1 21 0 6'4 280 25 *1(density = 62 '3) 305 35.4 (density=57'6) Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. The increase of dissociation seems to be fairly regular between 192-280°, but at 280°, when there is no longer a sharp end point to the displacement of air, there is evidently some secondary reaction, probably the dissociation of hydrogen iodide or, possibly, of C,H, into C2H,+H. Against the latter, it must be said that if C2H, were indeed formed, it should polymerise partly in to butane, when the contraction in volume should at least counterbalance the effect of the dissociation. On the other hand, on shaking out the tube with a little water, no acid reaction was obtained. At 280--305O, iodine was formed in the tube, a trace was also found at 210°, whilst below that none was produced. View Article Online 136 KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES. Ethyl Iodide Heated alone in 8ealed .Tubes ; Time of Amount removed A. heating. Temp. of KOH. Ethylene. Ethane. I. 5 hours 270" 0.06 0 100 Ir. 7 ,, 254 0'10 4.6 95.4 111. 3 ,, 270 0 '08 0 100 IV. 23 ), 254 1 '47 25.5 74.0 v. 2tt 9, 210 Amount of gas too sinall for analysis.