STUDIES OF DYNAMIC ISOMERISM, PART VIII. 807

XCI.-Sttudies o f Dynuw& Isomerism. Part VLU. The Relationship Between A b s o q h o n Spectra and Isomeric Change. A b s o q h o n Spectra o f Halogen, Nitro-, and Jrlethyl Derivatives oj* Canaphoi-.

By THOMAS MARTIN LOWRY and CECIL HENRY DESCH.

T.-Introductorp.

IN the earlier papers of the present series a considerable number of cases of reversible isomeric change have been investigated by ‘( dynamic ” methods depending on observations of changes occurring in freshly-prepared solutions of one or other of the isomerides. The use of such methods is in practice confined to crystalline cQmpounds, since only these can, as a rule, be separated in a pure form from mixtures of isomerides such as are normally produced in solution or in the liquid state. This limitation has the disadvantage of excluding some of the most important cases of dynamic isomerism, as, for instance, cyanic and hydrocyenic acids (Butleroff, Annalen, 1877, 189, 77) and ethyl acetoacetate. Much information has been obtained by studying crystalline derivatives of these compounds, such as cyanocamphor, diethyl diacetylsuccinate (Knorr, Annalen, 1899, 30.6, 332), or menthyl acetoacetate (Lapworth, Trans., 1902, 81, 1499), but such observations can never entirely take the place of experiments on the original parent substauces. Great importance attaches, therefore, to the discovery of ‘‘ static ” methods by which the presence of oscillatory isomeric changes may be detected after the final balance has been attained, and especially of methods tha t will also reveal the proportions in which the isomerides are present and the velocities with which they change from one form into the other.

Work on these lines has been undertaken by Perkin (Trans., 1892, 61, Sol), using magnetic rotatory powers as a test, and by Bruhl (compare Ber., 1902, 35, 3510, 3619, 4030, 4113, etc.), who has made use of refractive indices; but compounds containing the group -CO*CH,*CO* usually possess abnormal optical properties (Kay and

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808 LOWRY AND DESCH:

Perkin, Trans., 1906, 89, 839 ; Briihl, Trans., 1907, 91, 115) which seriously restrict the possibility of determining the proportions of ketone and enol, and lead in some instances t o values, not intermediate, but entirely outside the values calculated for the two modifications (see, for instance, Briihl, Ber., 1902, 35, 4034). As these methods cannot in any case throw light on the velocities of the opposing isomeric changes, special interest attaches to the suggestion of Baly and Desch (Trang., 1904, 85, 1029; 1905, 87, 766) tha t a band in the ultra-violet absorption spectrum of a compound may not only indicate the occurrence of reversible isomeric change, but by its persistence may show the extent of the agitation hidden beneath the apparently placid behaviour of the substance.

This new view of the significance of absorption bands was based on a series of experiments with compounds containing the group

and was justified by the following observations. In contrast with ethyl acetoacetate, CH;CO*CH,*CO*OEt, which is generally regarded as almost wholly ketonic and gave no absorption band, acetylacetone, CH,*CO.CH,*CO*CH,, a compound which Perkin had shown to be very largely enolised, gave a well-marked band. By adding an alkali, a n equally strong band was developed in ethyl acetoacetate, but this could not be attributed to mere enolisation, since the enolic ethyl derivative, CH,*C(OEt):CH.CO*OEt, in which the compound is com- pletely locked up in this forin, gave no absorption band whatever. As neither type of structure was capable of producing a band, its appearance in the spectra of the diketones and their metallic derivatives was attributed to an oscillation between the isomeric forms of these substances. Unfortunately, very little is known of the extent or velocity of isomeric cbange amongst the diketones, and it was therefore decided to make a series of observations of the absorption spectra of the simpler derivatives of camphor in the hope of establishing a quantitative relationship between their known velocities of isomeric change and the persistence of their absorption bands.

*CO*CH,*CO*,

II. ---Nitro -compounds.

CH*NO, - C:NO,H Nitrocamphor, CsH14< co I +-- C8H14<&) .--Nitrocamphor,

which had supplied the material for the earliest series of experiments on dynamic isomerism, was selected as the first substance for spectro- scopic examination, since in this case information was already available, not only in reference to the extent of the change (Trans., 1904, 85, 1541), but also as to its velocity in different solvents (Trans., 1899, 75,

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STUDIES OF DYNAMIC ISOMERISM. PART VIII. 809

219 ; 1908, 93, 110) and in presence of different agents by which the normal rate of change can be accelerated (Trans., 1908, 93, 107) or retarded (ibid., 119). As the period occupied by the change could be varied a t will from a few seconds to a year or more, the conditions appeared to be particularly favourable for establishing a definite relationship between the velocity of the change and the persistence of the absorption band.

The solvents used were alcohol, other, ethylene dichloride, and finally, ethylene dichloride to which a trace of acetyl chloride had been added in order completely to arrest the isomeric change. In spite of the wide range of velocities (roughly 10,000 to l ) , no great alteration was seen in the form of the absorption curve. Each spectrum showed a ‘Cstep-out” or rapid extension of the limit of transmission when the thickness was reduced to about 10 mm. of N/100 solution, but this did not disappear when isomeric chahge was practically stopped by the addition of acetyl chloride, and did not mature into a band when the period of change was reduced to a few hours by the use of alcohol as solvent.

Experiments on the use OF alcohol and water as affording a more active solvent than pure alcohol had to be abandoned, as it was not practicable under the ordinary conditions of experiment to maintain the high standard of purity that was necessary. A n alternative method of accelerating the change without converting the nitrocamphor into a salt was found, however, in the use of an acid catalyst (Trans., 1908, 93, 116). For this purpose an alcoholic solution was made up to contain nitrocamphor N/100 and trichloroacetic acid N/100 ; its absorption curve showed a horizontal step-out ” as in the case of the neutral solutions, but no band was developed, and the catalytic action of the acid appeared to be entirely without influence on the form of the absorption curve.

It is elear, then, that the isomeric change of nitrocamphor, even although it involves the actual transference of a mobile hydrogen atom and takes place with a high velocity in both directions,* can proceed incessantly without resulting in the production OF a band.

Alkali Salts of Nitrocamphor.

On the addition of alkali to the nitrocamphor solutions a very deep band was developed, extending down to a thickness of only a few mm. of N/10,000 solution, but only under such conditions that muta- rotation experiments were quite out of the question, The band appeared, indeed, to be characteristic OF the salts of nitrocamphor and

* The diketones change much less readily.

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810 LOWRY AND DESCH:

not in any way of the nitro-compound itself, since on comparing solu- tions containing :

( a ) Nitrocamphor, i V / l O O O Sodium ethoxide, iV/lO,OOO ( b ) (4

3 9 iV/10,000 Y ,) N/10,000 * Y N/10,000 9 ) 9 ) W O Q

no intensification of the band could be detected as a result of adding nine additional equivalents of nitrocamphor in (u), or ninety-nine equi- valents of sodium ethoxide in ( c ) . The latter result is in contrast with the behaviour of the diketones, which develop a maximum absorptive power only in presence of a large excess of alkali, but is in

PIG. 1.

Nitro-darivatizeu of camphor.

l/h=2800 3200 3600 4000 2800 3200 3600 40CO

250 h

$ 100 3 'p.)

0

0 0 0 7-i

25

10

2 5

1

Absorption curves for (a) natrocainphor ; ( b ) p - and r-bromonitrocamphor i n ethylene chloride, ether, and alcohol N/100; and for N/10,000 solutions of their sodium salts in alcohol.

agreement with the known hehaviour of nitrocamphor, as a strong acid which would be neutralised by a simple equivalent of alkali, forming salts which would not be hydrolysed in solution.

p- and rr-Bromonitrocamphors,

A preliminary test had shown tha t the mutarotation of nitrocamphor was not accompanied byany marked change i n the form of the absorption

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STUDIES OF DYNAMIC ISOMERISM. PART VIII. 81 1

curve, in spite of the fact that the composition of the solution changed

. As the formation of an pseudo N/600

(normal N/100 \pseudo - from

alkali salt could probably be detected a t a concentration of N/iOO,OOO, it was evident that the absorptive power of the $-nitro-compound must have been something like one thousand times smaller than that of the salts derived from it.

In order to test this conclusion more closely, experiments were made with p- and r-bromonitrocamphors, the former of which crystallises out in the norrnul form, and when dissolved shows a change of rotatory power from left towards right, whilst the latter crystallises in the pseudo-form aud shows a large change from right to left, owing to the conversion of five-sixths of the material into the normal form. Freshly prepared alcoholic solutions showed a somewhat greater transmission in the thicker layers by the normal /3-compound than by the pseudo-r- compound, but the curves became identical lower down and were throughout of precisely similar type ; freshly prepared solutions in ethylene dichloride gave identical curves of the same type as in the case of nitrocamphor. There is, therefore, evidently no special feature in the spectra of the +nitro-compounds to mark them out from the noPmal modifications, and the band which is produced, not only by the saltsof nitrocamphor, but in practically identical form by the salts of its p- and r-bromo-derivatives, must be attributed to some other cause than the presence of a +-nitro-structure.

Derivatives of Nitrocamphor not Exhibiting Mutarotation.

Identical conclusions were arrived at from the study of derivatives in which the structure is fixed in the normal or in the pseudo-form by eliminating the mobile hydrogen atom. The compounds examined

C:NO*O*NO:q were the anhydride, C,H,,<),l, OC>C,H,4, of y?-ni tro-

camphor, and several halogen derivatives of the normal type,

CX*NO, and C,H,,X<I (X = C1 or Br). F]X*NO,

CsH14<(7O co The curves for the anhydride and for the two stereoisomeric aa'-chloro- nitrocamphors showed a somewhat rapid extension between 25 and 8 mm. of N/lOO-solution ; the aa'-bromonitro-compound gave an almost linear curve. I n no case did the curves show a band* or other special

* A band was produced by the chlorobromonitrocainphor formed as a by-product in attempting t o purify crude ?ra-chlorobrornocamphor with nitric acid (Trans., 1906, 89, 1041), but no importance can be attached to this until the constitution of the compound has been established by reduction and other methods.

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812 LOWRY AND DESCH:

feature, and it is therefore obvious that neither the normal nor the pseudo-type of structure is capable per 8e of generating an absorption band.

By the kindness of Dr. Forster, who supplied us with a considerable quantity of material, we were able to compare the behaviour of nitrocamphane with that of nitrocamphor. The chief differences tha t result from the elimination of the ketonic group are :

(a) A displacement of tho equilibrium, so tha t the normal compound

PIG. 2.

(1) ‘ ‘3ixed ” derivatives of nitrocamphor, -anhydride, aa’-chZorunitroca~phors (2), and aa‘-bromonitroca~phor; (2) Nitrocamphmte, alone and with 1 and 9 eqtcivalents of alkali ; in centinormal alcoholic soZutions.

exhibits a conatant rotatory power in solution, whilst the labile pseudo- compound passes over completely into the normal form.

( 6 ) A. weakening of the acid properties of the substance, so that neutral salts no longer exert an accelerating action on the isomeric change; as a consequence, the pseudo-compound can be isolated by merely acidifying a n aqueous solution of its salts.

Spectroscopic observations showed the existence of a slight extension in the absorption curves of nitrocamphane and of freshly prepared solutions of the labile $-isomeride, but this was even less pronounced than in the case of nitrocamphor, and only of about the aame order of magnitude as in the “fixed” +-anhydride and normal chloro- derivatives. llIoreover, in marked contrast with the behaviour of nitrocamphor, the addition of an equivalent of sodium ethoxide does not give rise t o a band, and even nine equivalents do no more than

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STUDIES OF DYNAMlC 180MEBISM. PAKT VlII . 813

bring the (‘ step-out ” to a horizontal position, as in neutral solutions of nitrocamphor. It appears, therefore, that in this simpler nitro-corn- pound isomeric change gives rise to no absorption band either in the parent substance or in its alkali salts.

Resnlts which have been obtained elsewhere since these experiments were commenced support this view. Thus Hedley (Ber., 1908, 41, 1195) found that simple nitro-compounds, even when in the +-nitro- form, as in solutions of their salts, did not give an absorption band, but that the presence of a second nitro-group was necessary in order that a band might be developed. Baly and Desch (Trans., 1908, 93, 1749) found, contrary to Hedley’s statement, that a small but distinct band was given by nitromethane and nitroethane. The behaviour of this band in alkaline solutions was, however, somewhat anomalous, and apart from this instance, it was found that conjugation of the nitro-group with an ethylenic linking, as in w-nitrostyrene,

C,K,*CH: C H * NO,, or with a group possessing residual affinity, as in methylnitroamide, CH,*NH*NO,, was necessary to produce a band. I n nitrocamphor, -G-Y H- the neighbourhood of the carbonyl group, , provides the 0 NO, required conjugation.

111.-Halogen, Derivatives.

I n view of the negative character of the results described above, it was decided to enlarge the scope of the research and to include all the simpler derivatives of camphor which had been investigated in previous papers of the series, together with a number of other compounds, the examination of which might throw light on the conditions under which an ultra-violet absorption band might be produced. The general result was to show that, not only may isomeric changes occur in the case of the nitro-compounds without any development of a band, but conversely amongst the halogen derivatives bands may be developed by compounds or under conditions such that all possibility of isomeric change was excluded. I n order to make this clear, it will be con- venient, before describing the spectroscopic observations, to summarise the information that is available in reference to isomeric changes in this series of compounds.

So far as the parent substance is concerned, it is generally recognised tha t the properties of camphor are those of the cyclic ketone, C,H,,<bo CH2 , To this view no exception can be taken on the

ground that i t yields an enolic benzoyl derivative when boiled with benzoyl chloride (Lees, Trans., 1903, 83, 152), since this may result

VOL. xcv. 3 G

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814 LOWBY AND DESCH:

from the formation and decomposition of an intermediate compound without involving the formation of any trace of enolic camphor :

Camphor.

Camphor itself

Additive compoiind.

Enolic benzoyl derivative.

is, however, not an ideal substance on which to investigate isomeric change, since its crystallisation from solution is unsatisfactory and its solubility excessive. The P-bromo-derivative,

C,H1313r<X5, has the advantage of excellent crystalline properties

and a smaller solubility, whilst retaining most of the character- istic properties of the parent substance. Careful observations, including a number of tests carried out specially for the purpose of the present investigation, have failed to reveal any changes of rotatory power in the freshly prepared alcoholic solutions, and it has already been shown (Trans., 1906, 89, 1037) that no increase of solubility results from the addition of a trace of alkali to the saturated solution. Enolisation, if it takes place at all, must there- fore be limited to a very small proportion of the material in solution.

I n the case of a-bromocamphor the evidence is much more decisive, owing to the fact that the keto-enolic change is associated with possibilities of stereoisomerism, the hydrogen atom of the enol reverting either to the a- or to the a'-position in the ketone :

a-Bromocamphor. Enolic form. a'-Brumocamphor.

This change has been shown by Kipping (Proc., 1905, 21, 125) t o take place in alkaline, but not in neutral or acid, solutions; in spite of the fact that only about 6 per cent. of the a'-compound is formed, it is accompanied by a decrease of rotatory power from [a], 135' to 122O, which renders it very easy to detect by polarimotric observations. It is important to notice that the proportion of enol affects the velocity, but not the extent, of the alteration of rotatory power ; i t is therefore possible, by extending the observations over a considerable period, to detect the formation of the most minute quantities of the enolic form. The advantage gained by this method of procedure is such that, whilst observations of rotatory power might detect the formation of 1 per cent. of enol from P-bromocamphor, in the case of the a-compound the corresponding proportion would be more like one part in a million,

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STUDI’ES OF DYNAMIC ISOMERISM. PART VIII. 815

and this minute proportion may be taken as an upper limit of the amount of enolisation in neutral solutions of a-bromocamphor.

Similar statements may be made in reference to all the halogen derivatives of camphor which contain the group *CHX*CO*, since these are all completely stable in neutral solutions, but undergo stereoisomeric change when enolisation is induced by the addition of a trace of alkali. If, therefore, the appearance of an absorption band were directly related to the occurrence of reversible isomeric change, such a band sbould appear (as in the case of nitrocamphor) only on the addition of alkali, and not, as is actually the case, indifferently in neutral and alkaline solutions.

Derivatives containing the group *CH,*CO* or *CXY*CO* show no change of rotatory power or of solubilityon the addition of alkali. I n the former case it may reasonably be assumed that compounds, such as camphor or P-bromocamphor, behave in essentially the same way as a-bromocamphor, that is, that they are enolised by the addition of alkali, although to so small an extent that, in the absence of a further stereoisomeric change no alteration can bs detected in the properties of the solution. The disubstituted compounds, on the other hand, from which both a-hy I-ogen atoms have been displaced are presumably exempt from all possibility of enolisation.

The Absorption, spectrum of Cccmphor.

The absorption spectrum of a N/lOO-solution of camphor in alcohol was examined as long ago as lSSl by Hartley (Trans., 39, 153), who reported that the pure substance was remarkably diactinic and showed no absorption band. Baly, Marsden, and Stewart (Trans., 1906, 89, 979), working with more concentrated solutions, found an absorption band persisting over a range of thickness of about 10 to 1, and this has since been confirmed by Hartley (Proc., 1908, 24, 120). As our sliding-tube permitted of observations being made with thicknesses of solution up to 250 mm., we have been able to investigate the band in N/100- as well as in N/lO-solutions. The Ar/lOO-solutions we have found, in confirmation of Hartley’s observations, to be diactinic up to thicknesses of 60 mm., but at greater thicknesses a shallow band is observed ; in neutral solutions the band persists over 2 or 3 exposures only, but the addition of an equivalent of alkali extends the range to 3 or 4 exposures, each representing a change of thickness in the ratio 1.26 to 1.

Our curve for. the N/lO-solutions is very similar to tha t recorded by Baly, Marsden, and Stewart, but the band mas found to extend over six instead of ten exposures. This difference is probably due to the fact that the upper part of Baly’s curve was plotted from observations

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816 LOWRY AND DESCH:

made with N/l-solut,ions. As camphor does not obey Beer's law, the absorption produced by a millimetre of N/1-solution would not be the same as that produced by a centimetre of "/lo- or a decimetre of N/100-solution, and the use of the stronger solution would inevitably lead to a further intensification of the band.

It was at first thought tha t the weakening of the camphor band

FIG, 3.

Absorption curves showing in$uencc of solveiit am2 conccntmtioii , The clottcd c w v e s show the cfect of an eqzdvale?tt of sod i zm ethasidc added to the N/100-solictions.

which results from diluting the solution from N/1 t o N/lOO strength might be due to the formation of an alcoholate,

C H /yH2 '*\ O H 7

C<OEt in presence of the larger excess of alcohol. This view was negatived by observations which showed a precisely similar change in ether and in ethylene dichloride. Another possibility, that the band might be due to polymerisation of the ketone, was rendered improbable, dthough not altogether disproved, by the fact that the bands were progressively weaker in alcohol, ether, and ethylene dichloride, although the latter solvents usually permit a higher degree of association than alcohol.

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STUDIES OF DYNAMIC ISOMERISM. PART VIII. 817

Mono-derivatives of Camphor.

Precisely similar changes were observed in the absorption spectrum of a-methylcamphor on diluting the alcoholic solution from N/10 t o N/100 and on adding a n equivalent of alkali to the N,lOO-solution, the curves in each case being almost identical with those for the parent substance, The three curves for P-bromocamphor differed only in that the persistence of the bands was one unit less throughout, whilst those for a-bromocarnphor (which could only be plotted for iV/lOO-solutions) were a unit stronger and showed a shifting of the head of the band from 1/X 3500 to 1 / X 3350. The three curves for a-chlorocamphor also showed an intensification of the camphor band, resulting from the introduction of the a-halogen atom, the head of the band being shifted at the same time from l / X 3500 to l / X 3400. N/lOO-solutions of the UP- and ar-dihalogen derivatives gave bands of the same persisten2e as in the case of camphor, the intensification due t o the a-halogen atom being balanced by the repressive action of the bromine in the p- or rr-position, and showed the same slight intensifi- cation on the addition of an equivalent of alkali.

The change of persistence which results from the addition of alkali, although generally observed amongst the simpler derivatives of camphor, gives no representation whatever of the effect which the alkali produces on the velocity of isomeric change. I n neutral soln- tions the change is infinitely slow ; on the addition of an equivalent of alkali, it is practically instantaneous,* but the effect on the absorption curve is so insignificant tha t it will not even bear com- parison with the change produced by merely altering the concentration of the alcoholic solution. Evidently, therefore, t,he presence of the ba ld cannot be correlated, even in a qualitative way, with the occur- rence of isomeric change in this series of compounds.

act’-Di-derivatives of Camphor.

It has already been mentioned tha t when the a’-hydlogen atom of a-bromocamphor is displaced by a nitro-group, the resulting a-bromo- a’-nitrocamphor gives no absorption bsnd, but a n almost linear absorption curve. The band of a-chlorocamphor, on the other hand, although destroyed by nitration, leaves behind in the a-chloro-a’-nitro- compound an extension of the spectrum at the same oscillation-

* The addition of one tliousandth of a11 equivalent of sodium ethoxide to a normal soliltion of a-broinocainplior is snfficiont t o establish D condition of equilibrium within an hour ; the proportiorid period for one equivalent would be about three seconds.

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818 LOWRY AND DESCH:

frequency and almost ah the same thickness of solution as the head of the chlorocamphor band ; there can, therefore, be little doubt tha t the extension represents an early stage in the development of the band. On account of the greater amount of material tha t mas avail- able, the idea suggested by this observation mas worked out with a-bromocamphor, the a'-hydrogen atom of which was displaced by the radicles *NO,, *CHO, *Br, *CI-I,Br, *C1, and *CH,. The bromonitro- compound proved to be an exceptional limiting case, all the other members of the series giving strongly inflected curves. Thus the bromoformyl and dibromo-compounds showed a n extension almost as

FIG. 4.

Bromo-derivatives of (1) Camp7~or ; ( 2 ) dfcthylcawzphor.

Absorption curges for N/lOO-soliitions in d c o h o l . No t i ce the gr t id i~al disrtppn.rtr,itco of the band in thc scrics (1) a-broneo-, as -d ibromo- , ~ - z 1 ~ o m o - , a-C1L10~'o-a'-brOmo-, and aa'-dibromo-cnnyhor ; ( 2 ) w-bromo-, a-bromo-, aw-clibromo-, f i-broneo-mdhyl- camphor.

pronounced as in the extreme curve for nitrocamphor ; in the next two members of the series, the extension was so rapid tha t the curves were brought right up to a horizontal position, and could be represented most accurately by showing the presence of a slight minimum ; finally, in the case of the a-bromo-a'-methylcarphor, the absorption curve showed an unmistakable band persisting over two exposures.

From these ohservations it is clear: (1) That no sharp line of demarcation ctin be drawn between corn-

pounds which show an absorption band and those tha t do not, and (2) That a definite band m a y be produced by R compound in which

all possibility of keto-enolic change h a s been renioved by tho displace ment of the whole of the mobile hydrogen.

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STUDIES OF DYNAMIC ISOMERISM. PART VIII. 819

Isomeric Bromomethytcarnphors.

If further evidence were needed to show tha t isomeric change is not the main factor in determining the appearance of the absorption band, it is provided in a particularly striking form by a comparison of the three isomeric bromomethylcamphors supplied to us by Dr. Glover :

w-Bromoniothylcamphor. a-Bromomethylcamphor. a-Bromoniethylcamphor.

Of these three compounds the first two are free to undergo keto- enolic change, whilst the last is fixed by the elimination of the a- and a’- hydrogen atoms. The absorption curve for the w-bromo-compound showed the deepest band tha t we have observed amongst the halogen derivatives of camphor, the persistence in N / 1 00-alcoholic solutions being six exposures as compared with four for a-bromocamphor, three for camphor and methylcamphor, and two for /3-bromocamphor ; the bromine in this position evidently exerts a maximum effect, possibly on account of the leverage which it acquires from the interposition of the methylene group. A comparison of the a- and P-compounds showed that, whilst the former, as has already been mentioned, gives a shallow bend, the latter merely gives a well-marked horizontal extension. This result is directly opposed to what might have been anticipated from the fixity of the a-compound and the mobility of the P-isomeride, but is fully in accord with the general tendency disclosed by the observations which have shown tha t the absorption band of camphor is strengthened by an a-bromine atom, but weakened by a bromine atom in the P-position; if this tendency be regarded as dominant in the case of methylcamphor, the observations now recorded are abnormal only in the average weakness of the band in these two bromo- derivatives.

IV.-Origin of the Absoyption Bands.

Whilst i t would obviously be premature to attempt to construct, from observations of a limited group of compounds, a general theory as to the origin of absorption bands, certain conclusions derived from the present investigation may usefully be stated as affording data from which a complete theory may a t some future date be developed.

It is evident tha t the original suggestion, referred to at the beginning of this paper, that the absorption band observed amougst ketonic compounds is associated with the transfer of a labile hydrogen atom and that its persistence is a measure of the number of molecules in the solution tha t are actually undergoing isomeric chaage, cannot be maintained, as had, indeed, already become probable from the evidence

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820 LOWRY AND DESCH:

accumulated by Baly and his co-workers, The facts which tell most strongly against this view are :

1. The reversible isomeric change of nitrocamphor and of nitro- camphane, the occurrence of which is clearly demonstrated by independent methods, does not cause selective absorption, although the alkali salt of $-nitrocamphor gives rise to a very strong absorption band.

2. I n the simpler derivatives of camphor, the band is produced with equal facility whether the substance is undergoing isomeric change or not, and a weak band may even be produced by those deriv- atives in which isomeric chauge is impossible, owing to the complete displacement of the labile hydrogen at’oms,

I n the present investigation we are concerned with absorption bands of two types which must be carefully distinguished from one another, namely, (1) the bands which are produced by the alkali salts of nitrocamphor and of its p- and r-bromo-derivatives, but not by the parent substances or their ‘< fixed ” derivatives, and (2) the bands which are developed by the majority of the simpler derivatives of camphor in neutral and alkaline solutions indifferently.

Origin of the 2lTitrocamphor Bu,nd.--In the case of nitrocamphor our observations show clearly that neither the normal nitro-compound nor i ts $-nitro-isomeride gives rise to an absorption band, which first appears when the hydrogen atom of the $-nitro-group is displaced by an alkali metal. The view that the ionisation of the salt is the effective cause of its remarkable absorptive power is supported by few, if any, analogous cases, and by no direct experimental evidence. On the contrary, it has become a universal custom, when the band falls within the limits of the visible spectrum, to attribute the development of colour during neutralisation to a change of structure, as in the case of phenolphthalein or p-nitrophenol, and it is difficult to avoid the conclusion that some similar change takes place during the neutralisa- tion of nitrocamphor.

This view is rendered exceedingly probable by the analogous behaviour of the isonitroso-derivatives of malonic acid studied by Dr. Whiteley (Trans., 1903, 83, 34). These exist in colourless (iso-oxime) and in coloured (hydroxime) modifications, but invariably give rise to coloured (hydroxime) salts :

C,H;O~C:O Colourlese iso-oxiaie. Ycllow hydroxiitie. Yellow salt.

I n the case of nitrocamphor the possibility of two alternative formulae for the $-modification has long been recognised and discussed

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STUDIES OF DYNAMIC ISOMERISM. PART VIII. 821

(Lowry, Trans., 1898, 73, 995; Forster, Trans., 1900, 77, 254; Porkin, Trans., 1902, 8 1, 304). There is therefore nothing inherently improbable in the suggestion that the conversion of q-nitrocamphor into its salts is accompanied by a change of structure, in which the nitrogen atom passes from a tervalent to a quinquevalent condition :

Norninl nitrocamphor. +-Nitrocamphor. Sodium salt.

It will be noticed that the formula for the sodium salt contains a series of conjugated double bonds which is lacking, not only in both forms of the parent substance, but in all three modifications of nitrocamphane :

The explanation now suggested is, therefore, entirely in accord with our own observations, as well as with those of the other workers referred to above (p. 813).

Ovigigin of the Camphor Band,-The weaker absorption band developed by camphor and its simple halogen and methyl derivatives requires adiff erent explanation, and a t the present time constitutes amore difficult problem than that of the nitrocamphor band discussed above.

As borneol produces no selective absorption, the camphor band is evidentlyin some way a function of the ketonic group, but it is not a t all easy to say in what way the group acts in promoting the formation of the band. S o far as the chemical properties of the group are con- cerned, it must be recognised that the appearance of the band is not related in any direct way to tho property possessed by some members of the series of undergoing keto-enolic isomeric change. Nor bave we succeeded as yet in associating the band with the polymerisation of the molecule. Finally, the intensity of the band cannot-be correlated with the '' reactivity 'I of the carbonyl group towards agents such as hydroxylamine, since a-chlorocamphor and a-bromocamphor, which do not yield oximes, give deeper bands than camphor, methylcamphor, and P-bromocamphor, which readily react with this agent.

On the physical side, however, the gradual strengthening or weakening of the absorptive power of the molecule by various substituents, and especially by those which are contiguous to the carbonyl group, suggests a close analogy with other optical properties, such as refractive power and magnetic rotation, which are particularly sensitive to the influence of contiguous or conjugated groups. A t

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822 LOWItY AND DESCH:

present, therefore, the most hopeful method of attacking the problem con- sists in studying the relationship between the absorption and the other optical properties of the various compounds. Work on these lines is already in progress, and i t is anticipated that the results will prove to be of value in elucidating the various factors which regulate the power of selective absorption possessed by the various compounds of the camphor group,

V.-Methods of Observation. The methods used were essentially the same as those described

by Baly and Desch (Eoc. cit .) , and need not be set out in detail. The earlier observations were made on a ‘(No. 2 ” spectroscope by Hilger, with a single pair of quartz half-prisms and fully- achromatised quartz-calcite lenses of 13“ focus. The photographs were taken with a half-plate camera provided with zt graduated swing- back and a partly-achromatised quartz-calcite lens of 22” focus moving in a graduated tube. This gave a spectrum about 10 cm. long, and twenty-four exposures could be made on a half-plate placed vertically in the camera.

I n the later experiments two pairs of quartz half-prisms were used. This not only gave a much greater dispersion, but had the advantage of bringing the whole of the ultra-violet spectrum sharply into focus on a flat plate. With the new arrangemen6 it was necessary to arrange the half-plate horizontally, but by using a slightly shorter slit it was still possible to make twenty exposures on each plate. A t the same time it became necessary to exclude from the plate the greater part of the visible spectrum, but as none of the substances dealt with possessed any visible colour, this did not involve the loss of any part of the absorption; on the contrary, the useful part of the spectrum from X 4100 to X 2100 was spread out to the fullestl advantage over a distance of 15 cm. right across the plate, The increased dispersion thus secured was found to be of the utmost value when dealing with substances which gave only shallow bands of slight persistence.

The solutions under observation were placed in sliding-tubes of the type devised by Baly, but instead of using a millimetre scale, a direct logarithmic scale of thicknesses was provided. The tube was about 300 mm. long, and the graduations were as follows :

Mm .......... 251.2 199.5 158.5 125.9 100’0 79’4 63.2 50.1 39’8 31.6 {Logs ......... 2.4 2.3 2‘2 2-1 2-0 1.9 1.8 1.7 1 % 1.5

Mm .......... 25.1 20.0 15.9 12.6 10.0 7.9 6.3 5.0 4.0 3’2 2.5 -c Logs ......... 1-4 1’3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4

Those divisions were numbered consecutively from 21 t o 1, and as

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STUDIES OF DYNAMlC ISOMERISM. PAKT VIII. 823

the dark slide was also graduated and numbered to correspond with the tube, the risk of errors due to exposing a wrong thickness of solution was reduced to a minimum. Moreover, as each thickness exposed was made to correspond with a definite position of the plate, it was not necessary to make any record of the conditions of the observa- tions except to note on the negative the nature and concentration of the absorbing solution.

The logarithmic method of graduation has the advantage that successive exposures represent exactly equal increments or decrements of concentration. A change of thickness in the ratio 10 to 1 is effected in 10 equal stages, each of which, by a happy coincidence, is almost exactly 5 t o 4 (really 1.259 to 1) ; by a further coincidence the majority of the thicknesses approximate closely to round numbers of millimetres. One of the chief advantages of the logarithmic scale of thicknesses arises, however, from the fact that when the absorption curves are plotted (log-thickness against oscillation-frequency), the experimental points are equally spaced on the vertical axis and fall exactly on the main divisions of the curve paper. The appearance of the negative therefore approximates somewhat closely to the actual form of the absorption curve, the only deviation being due to the fact that, on the negative, the scale of oscillation-frequencies becomes more and more open on passing from tho visible to the far ultra-violet.

The materials used were for the most part those which had been specially purified for experiments on mutarotation and solubility, and need not be further described. We are, however, indebted to Dr. M. 0. Forster for the specimen of nitrocamphano used in the experi- ments and to Dr. W. €3. Glover €or tho methyl derivatives of camphor, and desire to take this opportunity of expressing our thanks. The alcohol used was Kahlbaum’s ‘‘ absolute,” distilled from a small amount of phosphoric oxide in order to remove all traces of basic impurity. The ether, ‘‘ distilled from sodium,” was redistilled from phosphoric oxide before being used. ‘The ethylene dichlo~ids was puri’fied as described by Perkin (Trans., 1902, 81, 308).

We desire to place on record our indebtedness to the Research Fund Committee of the Chemical Society for a grant which defrayed a portion of the cost of the investigation, to Prof. Armstrong and to Mr. E. C. C. Baly for valuable criticisms and advice, and to Mr. H. W. Southgate for assistance in the photographic work,

130, HORSEFERRY ROAD, WESTMINSTER.

Tim UNIVEXSITY, GLASGOW.

VOL. xcv. 3 H

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