By ZELDA KAIIAN, B.Sc. SOME Years Ago, Ur. Travers, Wishing to Prepare Butane, Employed for This Purpose the Well Known Method O

Home , Isopropyl iodide

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 dissociation. As regards the alkyl halides, Nef assumes that the first stage in their reaction in solution, whatever the other reacting substance 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

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

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 decompose 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. These results, as well as nearly all those given in this paper, are but typical instances of a large number of similar experiments. B. A large quantity of gas was next prepared by heating several tubes to 270' for three to five hours, and fractionated by means of liquid air and also liquid air and alcohol, the gas being passed previously through potassium hydroxide ; the fractions were then analysed separately. The only product of the reaction was ethane, together with about 0.3 per cent. of hydrogen and about 0.5 per cent. of butane. C. In these experiments, the tube was heated to about 255' for two hours in the first heating, and to about 270' in each of the subsequent heatings, theexcess of gas being removed after each heating : 1st Heating. 2nd Heating. 3rd Heating. Ethylene ...... 35.7 per cent. 3 per cent. 0 per cent. Ethane ...... 64'3 ,, 90-3 ,, 57 7, Butane ...... - 7'0 97 trace Hydrogen ...... - - 12 >) Methane ...... - - trace Air ...... - trace trace B. In this series, an attempt was made to collect the gases while still hot. This was done in order to prevent the prolonged reaction of the constituents while the tube was cooling; it was also hoped to separate, at least partly, the hydrogen iodide, if any were formed, or else the iodine from the hydrogen if it had decomposed, and to prevent the re-association of the first products of the reaction. Moreover, the Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. experiments could be conducted at lower temperatures, and without allowing air to enter the reaction tube at any stage of the reaction, The following apparatus was constructed specially for this purpose. The tube, A, was of the usual kind, and was drawn out to a capillary at c, and bent as shown, so that, when the desired quantity of iodide had been introduced through 0, it could be sealed off at 0. The capillary tap and tube, T,was fused on to A, B and F being only two pieces of ordinary glass tubing joined by a capillary, P. The other side of T was joined to a capillary tube leading to the arrangement shown, S being filled previously with asbestos, and E with an easily fusible metal. The air was driven out of A through T by boiling the ethyl iodide in it. Twas then shut, and the metal in E gently heated and allowed to run into P, Fserved to retain any metal that might escape solidification in 1'. This constituted an effective closing of the View Article Online

tube A. This device had to be adopted because repeated experiments showed that the taps would not withstand the pressure of the hot vapours and gases produced in A. Mercury or water was next introduced through the open end of tube N to F1 by suction at T2, the latter was then joined to the pump, and the space Fto T1evacuated. When A had been heated to the required temperature, the tap, T,was opened and P heated gently, T1 being opened simultaneously; the gases from A at once forced their may through, and were collected at N. As soon as the excess of gas had been evolved, E was heated to seG off the tube, which was then re-heated. As grease is dissolved Iny ethyl iodide, phosphoric oxide was used as LZ lubricant for the taps, but although the apparatus worked successfully enough, the appearance of the taps showed that they were not air-tight ; probably a better result would be obtained by employing taps working in mercury cups, this was not attempted in this series of experiments. As, however,

the air could not have entered the tube, A, it may be of interest to give the results of one complete experiment. The thermometer in the furnace indicated 230°, but from tlie long time taken to reach this point, and from the appearance of the tube, the temperature must have been Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. considerably higher : 1st Heating, 2nd Heating, 3rd Heating, 230" (?). 230". 260". Etliyleiic ...... 12 per ccnt. 6.9 per cent. 0 per cent. Explosion results after removal of ethylene. Gas ...... 7-49 per cent. 4.71 per cent. 2.35 per cent. Coiitractioii ...... 9 '82 ,, 7-71 ,, 5-06 ,, CO, ...... 7.41 ,, 5-87 ,, 3'51 ,, Ijatio contractionlC0, 1-33 ,, 1.31 ,, 1'44 ,, It was evident that tlie gas contained air, and this was further confirmed by burning the residual oxygen after the removal of carbon dioxide with pliosphorns, when some nitrogen always remained behind, hit the results show clearly that inore ethylene is obtained at first than on subsequent heatings, and that only small qusn tities of hydrogen are producccl. lu the next period, the ethylene has been reduced and View Article Online 138 KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES.

less of it is obtained, whilst after the third heating only ethane and hydrogen are produced, The tubes, when heated to 250° for about six hours, or to 270' for a, short time, contained a considerable amount of carbon. No carbon was produced in tubes heated to legs than 250' or to that temperature for about one to four hours, Although, from the above experiments, it is to be concluded that hydrogen iodide is one of the products of the reaction, it was scarcely ever directly shown to be present ; the water through which the gases were led never giving a decidedly acid reaction. Taking advantage of the fact mentioned by Mendelbeff (" Principles of Chemistry '.'), that silver chloride is very slowly decomposed by hydrogen bade at lSOO, it was determined to try the action of it on ethyl iodide. If hydrogen iodide is indeed produced, it should act on the silver chloride, forming hydrogen chloride, which, since it does not readily act on ethylene and should not decompose at the temperature, would appear as an end product. This expectation was realised. A tube containing dry silver chloride and ethyl iodide mas heated to 250" for three hours. There was considerable pressure on opening the tube, and the water through which some of the gas was passed had a distinctly acid reaction, gave an almost white precipitate with silver nitrate, and with potassium dichromate and sulphuric acid gave the characteristic chromyl chloride reaction. The gas itself before being passed through water fumed slightly in air, and burned with a pale green flame; after removal of hydrogen chloride, the gas consisted of 27 per cent. of ethylene, 70 per cent. of ethane, and 2.6 per cent. of hydrogen. The experiment was repeated several times with exactly similar results.

Action of Zinc on, Ethyl Iodide ; Wt. of Wt. of Ethylene, Butane, Ethane, Hydrogen, Kind of Zn in C2H, in Time and per Per per per Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. zinc used. grams. grams. temp. cent. cent. cent. cent. Stick ...... 5 3 100-160" 20 10'4 69.6 0 4+ hours Sbick ...... 2 6 100-160" 26.4 3'6 70 0 39 hours Powdered ... 2 6 100-160" 24.8 20.8 54.4 0 34 hours Stick ...... 2 6 270" 0 0 96.9 3 39 hours Stick ...... 6 8 270" 3.2 32.3 64'3 0 34 hours

It is seen that, although the zinc was always in excess of that theoretically necessary for complete double decomposition into butane and zinc iodide, yet the greater the proportion of zinc employed the greater the amount of butane obtained. It is difficult to explain such a result on the hypothesis of a primary dissociation into R and HI j View Article Online KAHAN: THE EE'FEC'I' OF ILEAT ON THE ALKYL 10DIDES. 139

two explanations may, however, be given : (1) that, in addition to alkylidene or alkylene dissociation, the iodide is also dissociated to a very slight extent into C,H,+I. A large excess of zinc removes the iodine as quickly as formed, leaving u larger number of ethyl groups, which polymerise and form butane. (2) The dissociation is wholly into R + HI, but zinc in the active state also exists to a small extent as ::::>Zn<, tho dotted lines indicating the residual affinity of the zinc atom. The greater the amount of zinc uscd the larger tho number of activt' n,toms of ;zXh< present. Then ::J:*Zn< 4- 2CH,*CI'I< -3

I stablo compouucl than zinc ethyl, I>Za<:*'cEI~'CH3 **CH,*CH, -3 ZnT, + C,H,,. The assumption that zinc can also exist as a qusdrivalent element, certainly seems to be rather an improbalde one, since the metals of the same nntl neighbouring groups of the periodic system, if they are ever anything but bivalent, are usually univalent ; the suggestion is only matlc here, bocause it gives perhaps the most probable explanation of ths behaviour of the propyl iotlides. On the other hand, there is nothing inherently impossible in the assumption. We know that in the case of many elements, the valencies ara not fixed and unalterable, Such a stablo bivalent element as oxygen, for example, has been shown to bo qundrivalent under certain conditions. Again, the phenomena of double salt formation and of water, alcohol, or ammonia of crystallisation ~vould find an easy explanation by assuming the presence of ordinarily latent residual affinities in the elements. It is thus conceivable that zinc, just like tin, oxygen, or other elements, can act sometimes as though it mere quadrivalent (compare also Abegg, Zeitsch. anorg. Cheni., 1904, 39, 330; Marozoff, J. Russ. Phys. Chem. Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. Xoc., 1906, 38, 480). A mixture of the gases obtained as above was also fractionated by iiieans of liquid air, and, in addition to the gases already mentioned, 1-1.5 per cent. of hydrogen and, possibly, 0.5-1 per cent. of methane were also detected,

Action oJ Sodium on Etlql Iodide. One gram of sodium in small pieces mas heated with about 3 grams of ethyl iodido at 130-150" for three to four hours. The gas was fractionated and analysecl, and the percentage composition mas approximately 40 of ethylene, 45 of ethnne, 15 of butane, 05-1 of hydrogen, nncl 0.5-1 of inetliaue. View Article Online 40 KAZTAN : 'I'HE EFFECT OF HEAT ON THE ALKYL IODIDES.

Action qf Heat on Methyl Iodide. Tubes charged with methyl iodide, when heated to 265' for three hours, were found to contain no gas but, when heated to 270" for six hours about 10 C.C. were obtained, which consisted of ethylene and methane, the former varying between 3 and 12 per cent. in the different tubes. Hydrogen iodide was not detected. Methyl rodide and Silver Chloride.-The tubes were heated to 275' for five hours; when collected over water, four-fifths of the resulting gas was dissolved, the rest consisting of methane with 2 per cent- of ethylene. A large quantity of gas was prepared, and collected over mercury. Some of it was passed through a little water, which afterwards gave a distinctly acid reaction and an almost white precipitate with silver nitrate. Fourteen per cent. of the gas was absorbed by potassium hydroxide, the rest on fractionation was found to consist of nearly 25 per cent of inetlxinc, about 74 per cent. of methyl chloride, mixed with a little methyl iodide vapour, and possibly about 2 per cent. of ethylene and 0.5 per cent. of hydrogen. When the methyl chloride was exploded vith osygen, a thick incrustation of mercuric chloride and a little iodide w;w formed on the sides of the burette. When the mercury was raised SO that it reached this incrustation, a pungent gas with an acid reaction was given off. After this operation had been repeated two or three times, no more gas was evolved. Methyl chloride could have been formed either according to : (1) CH,T + AgCl= AgI + CH,C,l or (2) CH,I ---+ CH,< +HI ; HI + AgC1= AgI +?71 H CH,< + 7 71= CH,

Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. Action (1) is evidently inadequate to explain the production of methane or ethylene; the temperature was too low for a total des- truction of methyl iodide, and, even if this were decomposed according to CH,T -+ CH,+I, no methane could be produced unless the reaction CH, -+ CH, + H took place, also some ethane should have been formed. If (2) is the actual course of the reaction, then it shows : (1) that metliylene combines with hydrogen chloride more easily than ethylene. (2) That rnethylene unites with other substances more readily than it polymerises, for very little ethylene is produced. (3) That methylene combines with hydrogen chloride as readily as, if not more so than, with hydrogen iodide. In view of the behaviour of ethylene, this soerns more tiulilcsly ; however, it is quite possible that methylene, as the first iiwrnber of t,he scrics, presents many striking differences from the other members. View Article Online KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES. 141

dfetlql Iodide cml 13ai.iuwt i'eyoxide.-In order to see whether the hydrogen iodide might be oxidised, thus leaving the methylene to remain free or to polymerise, the action of barium peroxide was tried. The result, however, was quite contrary to what had been expected at first. A tube, containing methyl iodide and dry barium peroxide, was heated to about 250°, the resulting gas fractionated with liquid air, and analysed. The percentage composition was 50 of methane, 18 of carbon dioxide, 14 of carbon monoxide, 6.5 of hydrogen, 5.5 of an unsaturated hydrocarbon, probably ethylene, and 6 (by difference) of a higher saturated hydrocarbon, possibly ethane. Although not fulfilling the purpose for which the experiment was started, the results are still interesting, for the question arises as to what gave rise to carbon mono- and di-oxides. Methane is excluded, for it only commences to unite with oxygen at 300°, and then only slowly if kept at that temperature for a week or so (Bone and Wheeler, Trans., 2902, 81, 535). It is also very unlikely that either ethylene or ethane should be the cause (Trans., 1904, 85, 693, 1637). If, however, we assume the primary production of methylene and hydrogen iodide, all difficulty disappears, thus we have : IH HI II HI I1 Ba:O:O --f Ea:O*O -+ l3a:O.O --+ Ba*O*O -+ Bzt*O*O -+ II iH iil iH Ih12+ HgO, -+ H,O + 0- H CH,< + 01 =O:C<,, --+ GO t H, I

Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. 0 I >C<" -+ CO, + H,O, 0 OH 112 :ko>C If, + IT, = GI€, and >C H2+ >CH, = C,H4 -+ C,H, ; the last two prcjcesses taking place very slowly. This agrees witLhthe property a1re;idy attributed to methylene of combining with other substances more readily than of polyinerising. We should in any case expect metliylene to be oxidised and reducctl very rapidly. Metlql iodide and Zinc.-The results are shown in the following table : Zinc, Cl131, Tiiiie and tcin- Ethylene, Ethane, Methane, in grams. in granis. lwature of heating. per cent. per cent. per cent. 2 5 3 linnrs to 1SO" 5.5 14'3 80 5.5 3 >> 1' ti -5 38% 55.6

2 5 11 275" 1-2 0 95--100 View Article Online 142 KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES.

On fractionation of a mixture of tlliese gases, about 0.5 per cent. of hydrogen wns nlso detected. Thi~the results are very .analogous to those furnished by ethyl iodide, and the reason for obtaining a larger proportion of etliano when a large excess of zinc is used would be tho same as that for obtaining butane from cthyl iodide in similar circum- stances. On tlie other liand, the production of methane at such low temperatures can only be explained by assuming the formation of >GEL,, and its reduction by the hydrogen iodide. The comparative stability of methyl iodide, and the fact that it is easier to estimate nietlioxy- than ethoxy-groups by Zeisel’s method (Decker, Ber., 1 903, 36, 2895 ; Perkin, ‘l‘i*:~ns.,1903, 83, 1367), find an cx&mation in the readiness with wliicli niethyleno unites with other elements. This view is further supported by the experiments of Moureu and Valeur on the reaction of sparteiue with methyl and ethyl iodides with and without tlie corresponding alcohols as solvents (Compt. rend., 1905, 141, 49), and by those of Chabltty with methyl chloride and sodamide (Compt. q.ewi?., 1905, 140, 1262).

A’xperiw el 1 t s with the P~opgI Iodides. Both alone and with metals these decompose rnuch more readily than ethyl iotiido. Sonio of the gas evidently condensea in the reaction tubes, ils, ivlieii they mere opened, a portion of the remaining liquid very soon boiled :tmay. The iiiercury in the gas holder was attacked almost as soon as the gas entered ; on this account, the gases were usually collected over water. When a little mercury was boiled with some of the iso- or norrnal iodide, it was not attacked even after some time. The gas obtttinetl by heating either of the iodides alone fumed in air, n property wliicli it; lost after standing over mercury for some time, or directly it was passed through water, the latter in this case giving n distinctly acid reaction. The gas also had a

Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. pungent odour, ant2 burned with a pde, peach-coloured, then luminous flame. Evidently hydrogen iodide was contained in it, but the quantity absorbed by water or potassium hydroxide was very small. Most of it was soluble in alcohol. isol’yopyl .Iotlitle.-When heated alone for two and a-half hours to 250°, the pressure was so great that, in many cases, the rubber tube connecting the reaction tube to the gas holder burst with a loud report. Tho gas was fractionated after removal of hydrogen iodide, the uncondensnble portion consisted of 1-1.5 per cent. of hydrogen ; the rest contained no unsaturated gas, and consisted of pure propane, the explosion results being : 3.66 gas gave : C,H, requires : Contraction, 10.90 ...... 10.83 C;mbon ciioxitlc, 10.85 ...... 10.83 Oxjgeri used, 1S.14 ...... 18 05 View Article Online KAHAN: THE EFFECT OF HEAT ON THE ALKYL IODIDES. 143

Tubes heated to 175' from three to six hours gave similar results, the hydrogen being 0-5-1 per cent. In one experiment there seemed to be the possibility of an admixture of a minute quantity of a higher hydrocarbon. isoPropyl Iodide with Zinc at l103.-The gas did not fume, and was soluble in alcohol, but not in water or potassium hydroxide ; it had no acid reaction. It was found to consist of 35 per cent. of propylene, 5 per cent, of hydrogen, and nearly 60 per cent. of propane. With Sodium.-Schorlemmer (Proc. 12oy.$S'oc., 1867, 15,34) remarks that isopropyl iodide does not react with sodium even at its boiling point, His experimental conditions were different from those obtaining here, for in a sealed tube at 110' the reaction with sodium was found to be no less marked than with zinc, the gases formed consisting of 53 per cent. of propylene, 2 per cent. of hydrogen, and the rest was pure propane. n-Prop$l Iodide Heated alone to 1'75'.-The gas formed consisted of pure propane, mixed with 0.3 per cent. ofl hydrogen and-possibly less than 1 per cent. of unsaturated hydrocarbon. n-Pm.ygZ Iodide with Zinc at 200°.-The gas consisted of 5 per cent. of hydrogen, 45 per cent. of propane, and 50 per cent. of propylene. With Sodium.-At llOo, very little gas was formed, although, when the tube containing the sodium and ;the iodide had been sealed off after boiling out the air, the liquid disappeared in about half an hour, whereas in the case of the isoiodide the liquid remained for several days without any change. At 210°, rather more gas was produced, but there was no very great pressure; after removing the excess of gas, the tube was again heated, but no further pressure of gas was observed. The gas obtained at 210' consisted of 46 per cent. of propylene, 48 per cent. of propane, and 5.7 per cent. of hydrogen. Many attempts were made to isolate the bromide or iodide of

Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. diethylethylene, CH,*CH,*CH:CH*CH,*CH3,or of tetramethyl- ethylene, (CH,),C:C(CH,),, from the liquid remaining in the iso- and n-propyl iodide tubes when heated alone, but with no success. After removing the iodine with potassium hydroxide, a heavy, black oil remained, which was soluble in ether and alcohol. It decomposed on standing, with production of iodine and carbon, and also yielded these products when it was distilled under ordinary or reduced pressure. It probably consists of a mixture of very unstable iodides of complex hydrocarbons, and, perhaps, also of iodides of carbon. It is remarkable that scarcely any propylene should be formed when the propyl iodides are heated alone, whereas with zinc or sodium nearly half the gas obtained consists of propylene. On the assumption of a dissociation into 11 + HI, one would expect just the opposite, for the zinc or sodium should decompose the hydrogen iodide as fast as it is formed, leaving as much nascent liyclrogon to reduce propylene or View Article Online 144 KAHAN : THE EFFECT OF HEAT ON THE ALKYL IODIDES.

1~ropylitleneto pro pi^ ;LS wlicn the iodides are heated alone. Why, then, :;lioultl tliero be iuorc polymerisation in the one case than in the other? If we asmine that zinc can exist as ::>Zn< in its active state, the reaction becomes easier to understand. For, if the propyl iodides dissociate very easily into propylene and hydrogen iodide, then, when heated alone, there is always sufficient hydrogen iodide to reduce half tho propylone or propylidene to propane, leaving the rest free to form complex hydrocarbons or their derivatives ; when, however, zinc is

It might here be remarked that in the case of ethyl iodide, the proportion of ethylene produced is also greater in the presence than in the absence of zinc. Co?Lclusion.-So far as the behaviour of the alkyl iodides in a state of vapour is concerned, the evidence is wholly in favour of their dissociation into hydrogen iodide and alkylene or alkylidene, which of the two latter is actually formed still being open to doubt. There is, however, nothing to prevent the assumption that they also dissociate to a slight extent into alkyl and iodine as urged by Lobry de Bruyn and Tjimstra (Zeitsch. ph?ysiknZ. Chena., 1904, 40, 436). In support OF the view that the alkyl iodides dissociate, might also be cited the many cases in which their addition or substitution derivatives are formed only, or more readily, in the presence of a strong dissociating agent (compare Mouneyrat, Cotnpt. vend., 1898, 127, 273 ; also Bull. Soc. cl~irn.,1899, [iii], 21, 615; Kronstein, Be?.., 1S91, 24, 4245). Most of their reactions in solution are also either in agreement with, or not opposed to, this theory ; the only really serious objection being, that the velocity of tho reactions in which the alkyl halides take part have

Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37. mostly pointed to these being bimolecular (Chiminello, Gacszetta, 1895, 25, ii, 410 ; Bruyn and Steger, Rec. trav. cl~im.,1899,,18, 311 ; Burke and Donnan, Zoc. cit. ; Slator, Trans., 1904, 85, 1286), whereas, since the reaction of, for example, silver nitrate and hydrogen iodide is an ionic reaction, the reaction actually measured should be RI -+ R + HI, which mould be unimolocular, lout, as has been shown by Burke and Donnan, the experiinental results do not necessarily justify the assumption that the action is chemicully it bimolecular one. l\loreover, Slator finds (loc. cit.) that the action of ethylene chlorobromide and of ethylene cldoroiodide on sodium thiosulphate is unimolecular, and a relatively slow dissociation might explain this at least as well as the assumption he inakes of x slow tautomeric change of (‘a- ” C,H4C1Br into “t3-” C,H,UlBr. On the other hand, the rate and extent of dissociation would naturally depend both on the medium in which the View Article Online DEltIVATIVES OF S-~€lENYT~PHENB;;(OTHIOWlIJM. PART I. 145

reaction takes pltxe and on the other reacting substance. It is thus quite in accordance with the theory, or at all events not against it, that the velocity of reaction of the alkyl iodides have been found to vary greatly in the experiments of different observers, who have each studied the influenco of totally unlike substances on them. If the changes through which the other reacting substance passes during the reaction, tako place at measurable and comparative rates, the resulting constant could not tie expect,ed to correspond with a unimolecular reaction. The dynamic results (lo not therefore necessarily disprove the theory, whereas it would bo very difficult to explain the results obtained at higher temperatures in any other way. Moreover, by accepting the theory of tlissocintion in its entirety as applied to all chemical reactions, me have a more rntiomtl explanation for such processes as catalysis, many tlonble decompositions, and substitution.

In conclusion, I wish to thank Prof. Collie for the help he has given me during the progress of this work. UNIVERSITYCOLLEGE, LOXDON, W. C. Published on 01 January 1908. Downloaded by State University of New York at Stony Brook 26/10/2014 03:29:37.