Calorific Value and Constitution - The Journal of Physical Chemistry

CALORIFIC VALUE AND CONSTITUTION" BY MATTHEW FELIX BARKER

The heat of combustion of an organic compound is the algebraic sum of a number of thermal changes accompanying the transformation of the carbon to gaseous carbon dioxide and the hydrogen to gaseous or liquid water, according to the experimental conditions. One of the simplest of combustions is that of solid carbon to gaseous carbon dioxide. The general chemical equation for this change is [C], n02 = nCOz (1) where [C], represents the solid carbon complex. According to general opinion, before the combustion can take place, the carbon and the oxygen must become dissociated. Both processes require a definite amount of heat, so that in the first instance the combustion is preceded by the two endothermic reactions [C], -+nC - nq, (2) no2 +2nO - nq, (3)

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If Qc denotes the absolute heat of combustion of one gramme atom of carbon to carbon dioxide, then nC znO = nC02 nQ, (4) and thus [C], n o 2 = nCOz n[Q, - q, - qo] = nC0z nQ where Q is the observed calorific value of 1 2 grammes of solid carbon. Further, if we consider the combustion of the hydrocarbon [CaHb],, where 71 indicates association of the simpler molecules to form a larger unit, it is seen that the process consists of a series of changes with specific thermal values. The first of these is the dissociation cf the complex into simpler molecules and then the atomisation of these elementary units. Both processes are endothermic and may be represented as in equations (6) and ( 7 ) . [CaHbIn = nC&b - nqi (6): nC,Hh = naC nbH - nqz (7) The combustion of (a) atoms of carbon and (h) atoms of hydrogen require za b/z atoms of oxygen. Thus a b/4 molecules of oxygen have to become dissociated (a b/4) 0 2 = (za b/z) 0 - (a b/4)qs (8) and if C 2 0 = coz q4 (9) O = H2O q, 2H (10) then the whole process of the combustion may be stated thus [CaHbI, = nC,& - nql nbH - n(b a - I ) q 2 nCaHh = nac n(a b/4)02 = n(za b/z)0 - n(a b/4)q3 Pan0 = anCOz anq4 anC bn/z 0 = I/Z bnHzO bnq, bnH

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1 i I

* Thesis approved for the degree of Doctor of Philosophy in the I'niversity of LondonExternal.

I346

MATTHEW FELIX BARKER

so that the thermal equation representing the complete combustion of the molecule [C,H,], is [C,H& n(a b/4)O2 = naCOz 1/2 bn HzO anq4 bnq6 - nql - n(b 4-a - I)qa - n(a+b/4)q3 (12) where ql is the heat necessary to release one simple molecule from the more complex unit; 92 the heat necessary to produce a scission of any two atoms in the molecule C,Hb, and q3 the heat required to dissociate one molecule of oxygen into its atoms. q4 and q, are respectively the thermal effects due to the combination of an atom of carbon with two atoms of oxygen and of two atoms of hydrogen with one atom of oxygen to form gaseous carbon dioxide and liquid water a t the temperature of observation. Since equation (12) is a true energy relationship, then the total amount of heat represented on the left hand side is the difference between the total energies of equal quantities of matter, and consequently it follows that the heat of combustion is an additive function of the chemical and physical constitution of the molecule undergoing the combustion. For simplification of the argument the value of the heats of combustion of like atoms has been taken as being equal to each other. Actual experimental results point to the contrary. This does not affect the above generalisation but rather emphasises the connection between the heat of combustion and the constitution of the molecule. From this consideration it appears that a much better interpretation of results may be effected, if, instead of endeavouring to obtain a mathematical formula to express results, the contributions of the various atoms are calculated and their variations correlated with differences in constitution. The study of the heats of combustion of members of homologous series has shown that the contribution of the additional CH2 group varies only between narrow limits. The variations are larger than those which would arise solely from experimental error, yet in spite of this, the contribution of bhe CH2group has been considered by various workers as a constant. Notably with regard to the aliphatic hydrocarbons various formulae have been put forward consisting of simple linear functions. Upon this basis, Gomed deduced the following expressions for the various series as indicated below

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Paraffin Hydrocarbons >, Olefine ” Acetylene Di-olefine ’’ Di-acetylene ” The number 157 is apparently the value assigned to the contribution of each CH2group. The purely numerical term may be calculated from the equation H = 157 n A where H is the molecular heat of combustion and n the number of carbon atoms in the hydrocarbon. According to these expressions the molecular calorific values are simple functions of the value for the CH2 group. If the hydrocarbon is unsaturated, H is not, however, entirely m func2nC where 2n is tion of the value for the CH2 group but of (x - 2n)CHZ

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1

Anal. Fis. Quim. 10, 153-166 (1912).

I347

CALORIFIC VALUE AND CONSTITUTION

the number of carbon atoms concerned in the formation of multiple bonds. Lemoult‘ deduces similar formulae for the calculation of the heats of combustion of saturated acids, their esters and anhydrides. He concludes from his calculations that the calorific values for the substances considered may be A where y = the heat of combustion connected by the equation y = I 57x of the compound; x = the number of carbon atoms present in the compound and A is a constant. This is obviously the same as H = 157n A previously uqoted in connection with the results arrived at by Gomez. In the second paper, by Lemoult, the heat of combustion of a hydrocarbon C,H,, is assumed By and from the heats of combustion to be given by the expression Ax of the saturated hydrocarbons A = 1 0 2 and B = 27.5. Hence the expression 2 7 . 5 ~and this equals the y of the equation y = 157x A. becomes 102x For saturated hydrocarbons y = 2x 2 , so that y = 102x 27.5(2x 2) = 156x 54 which is almost identical with the relationship put forward by Gomee. Although, these expressions give calculated values agreeing satisfactorily with the observed results, yet as regards the correlation between calorific value and constitution they are of little importance since they have been deduced mainly statistically. Thomsen2 puts forward an expression derived from more detailed considerations than those of Gomez and Lemoult. The different heat effects of the various carbon to carbon linkages are taken into account. He does not, however, consider the possibility that like atoms may have different contributions to the heat of combustion in consequence of their different relative positions in the molecule, as for instance in straight chain compounds. The constants in Thomsen’s relationship are arrived at by interpolation of the observed values of the molecular heats of combustion and the expression for the paraffin seiies reduces to 1 0 5 . 9 ~ 2 6 . 2 ~(cf. Lemoult: loc. cit.). Prior to these deductions by Gomee, Lemoult and Thomsen, there appeared, in a paper entitled “A Thermochemical Constant” by F. W. Clarke3 a deduction which a t first appears to be of importance but unfortunately it was arrived at by the assumption of a number of numerical and purely arbitrary constants. The relationship put forward was the following

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4K

1za+6b-c-8n

= constant

where K is the molecular heat of combustion; a is the number of molecules of carbon dioxide poduced; b is the number of molecules of water produced; c is the number of molecules of oxygen dissociated and n equals the number of atomic unions in the compound burnt. The average value of the constant for a number of hydro-carbons including several containing the double link is 13873. This number approximates to Compt. rend. 137, 656-658, 979-982 (1903). Z. physjk. Chem. 51, 657 (1905). 3 J. Am. Chem. SOC. 24, 882 (1902).

I348


the value for the heats of neutralisation of strong acids and bases and the round figure, 13800 is brought forward as a unit quantity of heat and termed the henotherm. The “law” has been severely criticised by Noyesl and Thomsen2. The scepticism regarding this relationship is well illustrated by the following extract from the review by Noyes; “Whether these empirical formulae are merely mathematical fictions or whether the author has succeeded in bringing to light real theoretical relationships can be fully established only by an exhaustive study of the question, whether the close agreement between calculated and observed heat effects could have arisen solely through the arbitrariness of the choice of even integral numbers of henotherms to represent the varicus elementary heat effects.” I n the above paper by Thomsen, it is shown that the relationship is by no means general and he emphatically declares that this empirical law is of no value. A similar conclusion may be arrived at by a detailed examination of the relationship as expressed in equation 13. The divisor Iza 6b c - 8n is obviously a simple linear function of composition, for if we consider the case of the saturated hydrocarbons C,Hz, t z then a = 4X b=4X+4 c=6X+z I n = 3X so that 4K Iza 6b c - 8n = 42X 14 and thus = const.

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42x

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For any series, X increases in arithmetical progression and the difference between any two consecutive values is one. If K1 and Kz be the calorific K2 KI values of two consecutive members of a series then 42 (x I ) I 4 42x f I 4 and therefore KZ - K1 = 4 2 X constant. Hence to fulfill Clarke’s “Law” Kz - K1 must be constant. Forany homologous series the expression formu4K Jated by Clarke may be written m ~ = constant, n from which it fol~

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lows that Kz - K, = {m(x I ) +:n - mx - n ] x const. = m X const. Whatever d u e s be given to, the coefficients in the original expression from which this is derived, .m will have a specific value. Since Kz - KI

approximates to constancy, theZquotient K2-K1 will also be correspondingly m constant but its value will be entirely dependent upon the arbitrary values given to the coefficients of a, b, c and n in the original expression. Thus the significance of the value 13800 for the constant disappears. Further, the relationship necessitates that Kz - K1 is constant for all series, which is contrary to experimental fact, 1 J. Am. Chem. SOC.25, 156r (1903). ZZ. physik. Chem. 47, 487-493 (1903).

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The formulae derived by Thomsen for the calculation of the heats of combustion are based upon more detailed considerations than those of the investigators previously considered. For the hydrocarbons the fundamental equation 2by - Z v where x is the contribution of each used is C,HZb = ax carbon atom, y the contribution of each hydrogen atom and Z v indicates the summation of the heat effects due to the carbon to carbon unions. When the carbon atoms are united by only single bonds, the number of these linkages is za - b and if the thermal effect due to one single bond is v1 then C,HZb = a(x - 2v1) b(zy v1) = aA bB In this equation the contribution for each carbon atom as well as for each hydrogen atom has been assumed to be constant. It will be shown later that the contribution of each hydrogen atom may be taken as constant, but from the following considerations this is not so with respect to the carbon atom. Ethane may be considered as the product obtained by the substitution of the CH3 group for one hydrogen atom in methane. This means that a group of relatively large atomic volume is introduced into the hitherto perfectly symmetrical methane molecule. This introduction will necessitate a rearrangement of the relative positions of the hydrogen atoms. Since for perfect symmetry the angle between the directions of the bonds in the normal tetrahedral position is a maximum, any distortion will result in the valencies being

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c1-81 represents Methane,

separated by a less angle than hitherto. Thus if

/I‘ then after substitution the three remaining valencies will be inclined to each other a t an angle \E thus

\E\ --.c-X \E/

where9is less than 0. The heat of com-

bustion of a carbon atom in an organic compound is partly dependent upon the angles 0’ and \E’ as designated below.

Thus the heat absorbed during the transformation

-c 18’ -+ -c I *’ e’,\

.I.!\\

will be a function of 0’-qf’ and since 8 is greater than \E, the heat lost from this cause in the case of methane is greater than that due to the same cause with respect to the calorific value of ethane. It follows therefore, that the contribution of each of the carbon atoms in ethane is greater than the calorific value of the carbon atom in methane. Further it would appear that the nearer 0’ approaches to *’ the greater is the calorific value of the carbon atom. This variability of the contribution of the carbon atom to the molecular calorific value is emphatically indicated by the observed values for the cyclic hydrocarbons of the paraffin series and also for the benzenoid hydrocarbons and it will be shown later that the calorific value of the carbon atom increases

I350


as the angle 0’diminishes. Obviously this is of the utmost importance regarding the correlation of the calculated values with the observed results and it is the neglect of such theoretical possibilities as these that has given rise to the numerous mathematical relationships that have been formulated from time to time purporting to connect calorific values and the constitution of organic compounds. The apparatus employed, was the Mahler-Kroeker modification of the Berthelot bomb. The combustions were carried out with pure oxygen a t pressures varying between twenty and thirty atmospheres. For comparison, the results of other observers are quoted side by side with those obtained for the purposes of this investigation. Determinations were made mainly for those compounds the calorific values of which are relevant to the subsequent discussion. Benzene 783 * 4 782.3 (Auwers and Roth.) 781. I (Davis and Richards.) Toluene 941.3 934.2 (Davis and Richards.) 935.2 (Auwers and Roth.) Xylene 1091.3 1084. o (Auwers and Roth.) 1089.5 ( ” ) Naphthalene 1238.0 1230.6 (Davis and Richards.) 1235.2 (Auwers and Roth.) Di-phenyl 1 5 0 0 . 0 1492. (Auwers, Roth and Eisenlohr.) Phenol 733 . o 732.3 (Stohmann and Langbein.) 734.6 (Berthelot and Luginin.) Cresol (ortho) 883.7 883.5 (Stohmann, Rodatz and Herzberg.) ,f 19 885.0 Cresol (para) ” ) 883.4 ( 1) >l Cresol (meta) 880.5 ( 883.0 1 ” 3) 1) 890.0 Benzyl Alcohol 1 ” 895.8 ( 618. I Pyrogallol Di-hydroxy benzene (ortho) 684.9 685 5 (Stohmann and Langbein.) Di-hydroxy benzene (meta) 683.9 683.7 Di-hydroxy benzene 684.7 685.9 (Berthelot and Luginin.) (para) 683.6 (Stohmann and Langbein.) Benzoin 1670.8 1672. o (Stohmann, Kleber and Langbein.) 7, 1, Benzil 1621.6 1625.3 ( 1 ” Furoin 1114.0 1 0 9 7 . 7 (Wrede.) Furil 1064.4 Lepidene 3288.4 p-Nitro acetyl Benzoin 1864.3 p-Nitro Benzil 1600.9 p-Nitro benzoyl Benzoin 2443 * 4 )’

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Calorific Values of the Carbonyl and Hydroxyl Groups The values of these groups were deduced from the determinations of the calorific values for ( I ) Di-phenyl, ( 2 ) Benzil and (3) Benzoin. The subtraction of the value for ( I ) from the value for ( 2 ) gives 1 ~ 1 4 0 0calories which should be equal to the heat given out during the oxidation of the portion of the benzil -CO molecule represented by 1 so that one half of this figure should approxi-

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mate to the thermal effect accompanying the oxidation of the carbonyl group in an organic compound. Thus CO 0 = COz 60.7 kilograhme calories. By using this result in conjunction with the calorific value for di-phenyl and substituting in the equation for the combustion of benzoin, the value for the hydroxyl group is deduced, taking into a.ccount that in this case one hydrogen atom will unite with one hydroxyl group. This gives the number 12.9 kilogramme for the thermal effect of the combination of one hydrogen atom with one hydroxyl group to form one molecule of water. This result is, approximately, numerically equal to the value obtained for the heat of neutralisation of strong acids and bases.

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Calorific Value of Aliphatic Hydrocarbons (a) Saturated Hydrocarbons. Referring to equation 1 2 , when n = I then q1 = 0, so that C,Hh (a+b/4)02 = aCOZ+b/z HzO+aq,+bqj- @+a- I)qz - (a+b/4)q3 where according to the view previously expressed with regard to the variability of the contribution of the carbon atom, q4 is the mean absolute heat of combustion of the carbon atoms set free from the compound. For the total of 1 1 a atonis the actual contribution will be aq4-aq3- 2aqz and for ‘b’ hydrogen

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atoms the contribution will be bq, - __ bqz - __ bq3. Thus if C is the mean 2

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nett heat given out by the combustion of a carbonatom in the above compound and H the heat given out by one hydrogen atom, then the heat of combustion may be written as a C bH. From X-ray determinations the structure of the free carbon molecule appears to be similar to that of the benzoid carbon complex hence the value of C will be quite different from that for the molecular calorific value of elementary carbon. On the other hand the mode of the combustion of diatomic hydrogen to water exhibits a similarity to the combustion of the hydrogen in a hydrocarbon. The process in both instances is preceded by the scission of an elementary bond equivalent to a unit of valency and hence should be thermally equal to each other. The value of H therefore bH = K is that of one gramme of gaseous hydrogen in the equation aC burning to water. Further, it implies that the calorific value of the hydrogen in an organic compound is constant and the same for all compounds. I n the above equation K (observed calorific value), H, a and b are known; thus the value of C may be calculated. This assumption regarding the contribution

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of the hydrogen atom is upheld by the figure for the molecular calorific value of formaldehyde, (130000 calories). The value already deduced for the carbonyl group is 60700, hence the value for the two hydrogen atoms in formaldehyde should be the difference of these two figures which equals69300. The usually accepted value is that of 68920 (by Thomsen). It would appear then that there is adequate foundation for the assumption that the heat of combustion of a hydrogen atom in situ is the same as, or very nearly so, as that of gaseous hydrogen and that it may be taken as a satisfactory basis for the calculation of the molecular heats of combustion of organic compounds or their component atoms. According to Thomsen, the molecular calorific value of methane is 210.9. This appears to be the generally accepted value and the one used by various investigators when considering methods for the calculation of molecular calorific values. Determinations carried out by Berthelot gave the figure 213.5. Putting these values in the equation aC b34460 = K the values deduced for C are 73.9 and 75.5 kilogramme calories per gramme molecule of carbon, respectively. The higher figure is, probably more correct, so that this latter value may be taken as the heat of combustion of a normal carbon atom the valencies of which are directed towards the corners of a regular tetrahedron. The substitution of the CH, group for one hydrogen atom in methane causes an increase in the molecular calorific value of 159.6 kilogramme calories. The theoretical contribution of the carbon atom of this group is therefore 159.6 - 68.9=go.7. Since there is no reason for doubting that ethane has a symmetrical structure, the two carbon atoms in the ethane molecule have the same calorific value. Thus the contribution of the original

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methane carbon has increased and is equal to (90*7 + 7 5 * 5 ) i. e., 83.1. Calcu2

lated direct from ethane it is 82.6. Allowing for the lower number of hydrogen atoms the molecule of a cyclic hydrocarbon of the paraffin series has a higher calorific value than that of the straight chain compound of an equal number of carbon atoms. It seems therefore highly probable that the angle between the valency bonds has a considerable effect upon the calorific value of the atom. A decrease in this angle causes, apparently, an increase in the calorific value of the atom. Thus in ascending from methane to ethane the bonds of the carbon have become displaced and do not point exactly towards the corners of a regular tetrahedron. As the number of carbon atoms increases, the effect of the additional CH3 group diminishes. This is shown by the following results for straight chain and cyclic paraffins. The calorific values for the carbon atoms have been calculated as already indicated. It is noteworthy that the two series of values appear to converge to the same figure. This would also follow from the fact that as the number of the carbon atoms in the cyclic molecule increases, the disposition of any two consecutive atoms approaches more nearly to that existing in a straight chain molecule.

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CALORIFIC VALUE AND CONSTITCTION

Hydrocarbon

Methane Ethane Propane Butane Hexane Heptane

Calorific value of the Carbon

75.5 82.6 84.2

85.5 85.8 85.8

Hydrocarbon

Cyclo-propane Cyclo-pentane Cyclo-hexane Cyclo-heptane

I353 Calorific value of t,he Carbon.

97.8 88.6 88.2 87.6

From the previous argument it should follow that when the four hydrogen atoms of methane have all become replaced by the same kind of group then the central atom should have the same contribution to the calorific value as the original methane carbon, for, presumably, the molecule is again perfectly symmetrical and hence the direction of the four bonds of the central carbon atom are the same as those indicated by the tetrahedral representation. That this is so is shown by the value for tetra-methyl methane (847.1 Thomsen.) The mean increment for each additional CH2 group is 158.8 so that the contributionof the central carbon atom is given by847.1-4X158.8-4X34.46= 7 5.1 (cf. calculated value in the preceding table). (b) Akphatic Hydrocarbons containzng doubly linked Carbon Atoms. Although the ethylene molecule contains two hydrogen atoms less than the ethane molecule, yet the calorific value is only diminished by approximately 37 kilo-gramme calories. The increase in the molecular calorific value for the additional CH2 group is of the same order of magnitude for this series as for the saturated hydrocarbons and for homologous series in general. These facts suggest that the carbon atoms concerned with the ethenoid linkage have a considerably higher contribution to the calorific value than the saturated atom. The molecular calorific value of ethylene has been determined by various investigators. The results are surprisingly discordant; there is nearly 4% difference between the highest and the lowest figures. The principle values are 333.4 (Thomsen), 341.1 (Berthelot) and 34 5.8 (Mixter). Merely upon the grounds that in any heat determination the higher results are most probably the more accurate, provided that the necessary precautions have been taken against radiation, the result of Mixter has the premier claim to correctness. On the other hand this high result and also the lower one of Berthelot causes an abnormality in the increment for the CH2 group for the series ethylene, propylene and iso-butylene and iso-amylene. Proceeding on similar lines as in the case of the saturated hydrocarbons, the contribution of the ‘ethylene’ carbon is obtained from the equation 2 C 4 H = 333.4. Thus (-C : C-)+C02 97.7 kilo-gramme calories. Using the highest value for ethylene the number is 103.9. It is noteworthy that these results are of the same order of magnitude as the value obtained for the combustion of elementary carbon. Further, considering the hydrocarbon trimethylene, which presumably consists of three carbon atoms in a ring and united by valency bonds inclined to each other a t an angle of 60°, it is significant that the value obtained for one carbon atom of this ring is 97.5.

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This suggests that the valency directions in an unsaturated doubly linked atom are inclined to each other at an angle of 60’ also, or at any rate approximating thereto. Thus ethylene would be represented by H2C@,CHz which indicates that the unsaturated affinities form a closed triangle of force. The reactivity of such an arrangement is readily accounted for by the opening of this triangle thus H2C’‘CHz and hence the ability to combine with a further quantity of matter through the medium of these two freed valencies. For the series ethylene to iso-amylene the calculated value for the unsaturated carbon atom varies only between the following values 97.7 and 98.3 kilogramme calories per gramme molecule. The mean increment for the CH2 group for this series is 157.4. The calculated value for ethylene from the molecular calorific value of propylene is therefore 335.3, which is in good agreement with the value found by Thomsen. Since the increment for CH2 is, although not constant, yet always of the same order o l magnitude, the acceptance of the higher value found by Mixter would provide an interesting abnormality. This latter vaJue would give 146.9 for the increment ethylene -propylene, which is I I .o kilo-gramme calories below the mean value of the increment for thirteen different series. It is obvious that Thornsen’s value has much in its favour and must be credited with the greatei accuracy until substantial evidence is brought forward to the contrary. (c). Aliphatic Hydrocarbons containing trebly linked Carbon Atoms. The molecular calorific value of acetylene is 3 10.0(Thomsen). Here again the figure is higher than the value which would result if the allowance is made merely for the decrease in the hydrogen content of the molecule. Thus it appears that trebly linked carbon has a still higher molecular calorific value than even ‘ethenoid’ carbon. The value calculated from acetylene is 120.5, and from allylene it is 120.3. From di-propargyl the value deduced is somewhat higher; i. e., 124.5. Although the results for the contribution of trebly linked carbon are rather limited in numbers, yet there is sufficient evidence that such an atom has quite a large molecular calorific value. Further, with regard to the disposition of the three bonds forming the treble link, it is possible that it is made up of two closed triangles of valency, thus

/\

HC-CH.

v

The reactivity, would, by similar argument to that for the

ethylenes, be occasioned by the opening of the triangles of force giving successive pairs of freed valencies as indicated by the following (a) HC-\C , H ,’\

(b)

/ \

HC -CH I

x

I

x

Aromatic Hydrocarbons No Side Chains. The values for the calorific value of the carbon were obtained by subtracting the contribution cf the hydrogen atoms from the observed calorific values and dividing by the number of carbon atoms in the compound. The figures are satisfactorily constant and show a favourable agreement with the result (I)

CALORIFIC VALUE AND CONSTITCTION

I355

for elementary carbon. This indicates a similarity between the carbon skeletons of the hydrocarbons under discussion and that of the carbon complex in graphite. This similarity has been pointed out by Debye and Scherrer' and Hull2, who have shown by X-ray analysis that the structure of the graphite molecule is similar to that of the benzenoid carbon complex. The values of the carbon deduced by the method indicated above were benzene (96.1); naphthalene (96.2); anthracene (96.3); phenanthracene (96.3) and di-phenyl (96.3) from which it follows that the calorific values of these hydrocarbons may be expressed with fair accuracy by the simple formula 96.96 X 34.46 Y, where 96.96 is the molecular calorific value of carbon and 34.46 is the heat given out by the combustion of one gramme of hydrogen to liquid water. X is the number of carbon atoms in the molecule and Y the number of hydrogen atoms in the compound. The result will be given in kilo-gramme calories.

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Saturated Side Chains. Similar calculations for toluene and xylene gave large discrepancies. It is significant that the difference in the case of xylene, which has two side chains, is appoximately twice that in the case of toluene which has only one side chain. The values of these differences are 29.0 and 13.7 respectively. This suggests that the discrepancy is due to the contribution of the side chain carbon. It would appear that this carbon has a contribution of (13.7 29.0)/3 = 14.1 kilo-gramme calories less than that of the benzene carbon. Further, the results for these homologues afford evidence of the similarity of the carbon to carbon linkage, and the carbon to hydrogen linkage. The value for di-benzyl given by Auwers, Roth and Eisenlohr3 is 1809.3. Using the value for the side chain deduced above, (vie. 96.96-14.1 = 82.9) the calculated value is 1812.3. One half of this figure is 906.1. Thus the group CsHSCHz-has a calorific value of 906.1 kilo-gramme calories when united to carbon. If the linkage of carbon to carbon is equal to that of carbon to hydrogen, then the calorific value of toluene should be given by the summation of the heats of combustion of the benzyl group and one equivalent of hydrogen, that is 906.1 34.5 = 940.6 (observed value, 941.3). This equality is also shown by the values for benzene and di-phenyl. The calculated value for di-phenyl using the observed result for benzene is 2(783.4 - 34.5) = 1497.8 (observed value 1500.0). (2)

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Carboxylic Acids Considering the accepted constitution of the carboxylic acids it would appeal that the carboxyl group would burn in conjunction with an atom of hydrogen from the hydrocarbon residue.

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Physik. 2. 17, 277 (1916). a Phys. Rev. (2) 10, 661 (1917). Liebig's Ann. 378, 278 (1910).

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The molecular calorific value of various acids were calculated according to this scheme. To arrive a t the contribution of the carbon of the hydrocarbon residue, the value for the carbon in the hydro-carbon of an equal number of carbon atoms was used. Thus for acetic acid the value used for the carbon HS (H OH) (CO 0) is that for ethane. in the expression C The differences between the observed and the calculated are very large and suggest that the constitution of the molecules of these comFounds are not represented by the usual simple formulae.

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Acid

Formic Acetic Propionic Butyric Valeric Malonic Succinic Glutaric Adipic Benzoic Phthalic

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Molecular Calorific Vnlues Observed Calculated

Differences.

59.0 206.7 364.0

73.6 225.2 380.0

14.6 18.5 16.0

520.4 677.4 207.5 360.3

537.4 692.8

17.0

387.2 542.6

15.4 23.9 26.9 27.6

77= ’ 7

697.4 788.0

28.5 16.3

771.6

792.6

21

515.0 668.9

231.4

.o

These differences seem to have a definite significance. They are approximately twice the value for the di-carboxylic acids than for the mono-basic acids. It is likely therefore that these differences are due to some abnormality connected with the carboxyl group. It must be borne in mind that whereas the hydrocarbons have practically normal molecular weights the carboxylic acids show varying degrees of association. If we assume that such complexes depend for their existence upon some diverted valency energy, then a possible arrangement which suggests itself and which gives rise to the necessary conditions would result from a transference of hydrogen from the hydrocarbon residue to the carboxyl group thus

If a similar unit becomes so placed that these freed valencies may cooperate, then contact between molecule and molecule will be established. For acetic acid the association number given by Ramsay and Shields‘ is 3.6. Since any such figure is merely a mean complexity number for a mass of the substance under the conditions of observation we will consider the molecule of complexity 4. This will be made up of four units of the type shown above, J. Chem. SOC.63, 1089 (1893).

CALORIFIC VALCE AND CONSTITUTION

the carbons forming an eight membered ring. oscillatory system the next phase would be

I

HsC

I357

If this is considered.as an

! /OH :\OH

This is identical with that shown in (A), Any carbon atom in the above arrangements is attached to mono-valent groups and is to a great extent comparable to the carbons in the cyclo-paraffins. Thus the calorific value of such a carbon in the acetic acid complex would approximate to 87.6, and for one molecular proportion of acetic acid the calorific value calculated upon these assumptjons is 201.0. For the association of three molecules the calculated result is 202.3. These figures are 5.6 and 4.4 kilogramme calories less than the observed value, but nevertheless are much nearer the actual result than that obtained by the original method of calculation.

A fact that supports this view that the association of the molecules is responsible for the discrepancies between the observed and the calculated values obtained from the ordinary formulae, is, that the calorific values of the vapour of the acids agree fairly satisfactorily with the calculated results obtained in the first instance. Acid

Formic Acetic Propionic

Molecular Calorific Values Vapour at B. Pt. Gas at 18°C

70.75 227.6

390.1

69.4

225.3 386.5

Calculated

73.6 225.2

380.0

I n the case of propionic acid the association number is 1 . 7 7 . Thus suggests that a large proportion of the molecules consist of the di-complex


H C

CH3

/OH C\OH

1

I I

i

C . CH3 H The calculated value is therefore given by the expression 2C (CH2) z(H OH) which should be equal to 364.1, the observed value. The calculated value of the carbon atom in the four membered ring was calculated from the relationship between the calorific value of a carbon atom and the number of carbon atom in a cyclo-paraffin. This relationship is given by the equation Y = 166.45-39.45~ 6 . 6 5 ~ ~ - 0 . 3 7 where 5 ~ ~ Y is the calorific value of the carbon, and x the number of the carbon atoms in the cyclo-paraffin. When x = 4, Y = 91.0, so that the calculated value for heat of combustion of one gramme molecular quantity of propionic acid is 360.9 (c. f. with the observed value quoted above). Butyric acid has the association number 1.71. The calculated value for the double molecule is 516.6 and the observed value is

EO\& HO/

+

+

+

+

520.4.

This idea regarding wociation is not in opposition to the well known fact that substitution takes place initially in the carbon next to the ‘carboxylic’ group. Also the formation of a mono-halogenated derivative takes place much more readily than the formation of the higher halogenated acids. I n the case of acetic acid the attack of the halogen, for instance chlorine, on the complex, results in the formation of four molecules of the unstable di-chloro /OH compound C1.CH2 -C-OH From this, hydrochloric acid readily \Cl. splits off leaving mono-chloracetic acid. Similarly, in the case of propionic acid the chlorination would take place as indicted by the following scheme. CH3 CH

I

c1

c1

HO\C HO/

I

-.---CH.

/OH C -OH

I

c1 C1

I

---t

CH3

Z.CH3 CH .COOH

I c1

+

2

HC1

The existence of the di-hydroxylic molecule is also indicated by the production of a potassium derivative of ethyl acetate. In a recent communication, Scheibler, Zeigner and Pefferl show that the action of potassium on ethyl acetate in ethereal solution leads to the formation of

Ber. SSB,3921-3931

(1922).

I359


The Calorific Value of the Aldehyde Group The calorific values of the aldehydes show that the contribution of the aldehyde group to the calorific value is what would be expected from a group consisting of one hydrogen atom and one CO group. Thus the characteristic group of these compounds is, thermally, equivalent to a carbonyl group (value already deduced) and one hydrogen atom, and the combustion of this part of the molecule may be represented by the equation ( C 0 0) $(zH 0) = COZ $H20 60.7 34.5 Thus the complete combustion of an aldehyde group to carbon-dioxide and water (liquid) is accompanied by the evolution of 95.2 kilo-gramme calories per grarnme molecular weight.

+ +

+

+

Substance Found.

Formaldehyde Acetaldehyde Propionaldehyde Benzaldehyde Salicylaldehyde

+

+

Molecular Calorific Values. Calculated.

281.9

129.6 281.2

441.5 848.3 795.8

437.1 849.4 793.2

130.0

The Calorific Value of the Hydroxyl Group in Connection with the Simpler Hydroxy Aromatic Compounds With the exception of the di-hydroxybenzenes, there is satisfactory agreement between the calculated and the observed values. The phenol and cresols were the same as those employed for the work on freezing points' and were of an exceptionally high degree of purity. Substance Found.

Phenol Cresol (ortho-) Cresol (meta-) Cresol (para-) Pyrogallol Di-hydroxybenzene (ortho) Di-hydroxybensene (meta) Di-hydroxybenzene (para)

Molecular Calorific Values. Calculated.

733.0 883.7 883.0 885.0 618. I

732.7 884.0

684.9 683.9 684.7.

676.8

620.7

The calculated value for the cresols was arrived a t as follows; side chain carbon-82 .g, equivalent to that of toluene; benzenoid carbon-96.96; Hydroxyl-12.9 as deduced. 1

Fox and Barker: J. SOC. Chem. Ind. 38, 265-27211 (1918).

1360


The discrepancies in the case of the di-hydroxybenzenes may be due to the existence of tautomeric forms in equilibrium with each other as represented below for the ort,ho compound. CH

CH HC /%OH

,,

I! I

HC b)COH

IZc

I1

HC ()CO

I11

HC \/CO

(bICH2

The carbon atoms (a), (b) and (c) are to a great extent comparable with the carbon in a cyclohexane which has a calorific value of 88.2. Thus the calculated calorific values of I1 and I11 are 687.6 and 698.7 respectively. Since, according to the previous calculation I has a molecular calorific value of 676.8, mixtures of I, I1 and 111 may have the intermediate values found by observation. The Calorific Value and the Constitution of Benzene Earlier in this paper, it was shown that the carbon in benzene and naphthalene and the ring carbons in toluene and xylene had a molecular calorific value of 96.96. It was then pointed out that the heat given out by a carbon atom in cyclo-propane was also very nearly equal to this value. This fact appears to throw light upon the constitution of benzene and to afford an explanation of the results of Stohmann, Roth and Auwers concerning the calorific values of benzene and some of its derivatives and their products of hydrogenation. In 191 5, Roth and Auwers repeated the earlier observations of Stohmann relating to the increase in the calorific value due to hydrogenation, andalthough in some instances there were appreciable differences between the two sets of values, yet the same general conclusions were indicated by both. It is therefore quite beyond cavil that the successive addition of two hydrogen atoms to the benzene ring causes widely different incremenets in the molecular calorific values of the hydrogenated compounds. This is true not only for benzene but for the terephthalic acids and esters and naphthalene. The results necessary for a further discussion of this subject are given in the following table.

1361


Stohmann

Benzene

Roth and Auwers 782.3

779.8 68.2

Di-hydrobenzene

848.0

Tetra-hydrobenzene

892.0

44.0 893 * 7 41.2

Hexa-hydrobenzene Naphthalene Di-hydronaphthalene

Tetra-hydronaphthalene Terephthalic Acid Di-hydro ” ” I 4 I 5 2 5

Tetra-hydro

770.9 836.8 842.7 845.8

44.8 938.5 1235.2 1297.8 1299.8 1302.7 1341.2

933.2

62.6 64.6 67.5 41.1

70.6 41.3

”

882.5 45.8

Hexa-hydro

”

928.6

According to the Kekulh formula, as hydrogenation proceeds, the carbon approaches more nearly to the condition of the carbon atom in a fully hydrogenised six membered ring. The calorific value, in consequence, drops from 96.96 to 87.4 so that the increase in the heat of combustion due to the addition of two hydrogen atoms, can never be equal to the heat I 3 of combustion of two hydrogen atoms. For the Kekul6 CH formula the increment for the first two hydrogen HC atoms should be given by 2 X34.5-~(96.96-87.4) = 49.9. This is not in agreement with the observed results. Assume now that the carbon atoms are arranged in two sets of three, and each carbon atom is BC CH situated at, a corner of an equilateral-triangular prism. Since the hexa hydrogenated molecule consists of six fully hydrogenated carbon atoms united together in I a six membered ring, it is necessary to assume that the di-hydrogenation may occur by addition either to I , 3 ; I 3 ’ or 2 , 2 ‘. The addition of the first two hydrogen atoms causes an increment very nearly equal to that of the calorific value of two hydrogen atoms. From this it appears that the calorific value of the carbon atoms in the di-hydro compound are the same as in the parent substance and also that the original molecule has undergone no appreciable change in configuration during this process. Di-hydrobenzene may therefore be considered as in 11, in which the two three membered rings are still intact. The increase in the molecular calorific ’)

1362


value due to di-hydrogenation, is therefore, theoretically equal to 68.9 kilogramme calories. Further hydrogenation would give 111. Here the carbon atoms I , 2 , 3, I ‘, 3 ’ form a five membered ring. The calorific values of I ) 2 ) 3 will be appreciably diminished since they are now fully hydrogenised and have much greater freedom than 1’, 2’, 3’ which are still maintained in the three membered ring. The calorific value of the carbon in cyclo-pentane is 88.6 so that the calorific value of tetra-hydrobenzene should be given by3 XC96.1+3X C S s a 6 + XH34.46 ~o = 898.7. The observed values are 892.0 (Stohmann) and 893.7 (Roth and Auwers). The calculated value for the Di-hydrobeneene is 85 7 -4 and therefore the increment for the step di-hydrobenzene to the tetra-hydrobenzene is 41.5 or 43.9 according as we use the actual value for benzenoid carbon calcu-

I1

I11

lated from benzene or the observed value for elementary carbon. The increment found by Stohmann is 44.0. The conversion of the tetra-hydro to the hexa-hydrobenzene results in the scission of the remaining cyclo-propane ring. From the results for the hexa compound (Stohmann 933.2; Roth and Auwers 938.5) the calorific value for one carbon atom is 86.6 and 87.4 respectively. The calculated increment for the transformation of tetra-hydrobenzene to hexa-hydrobenzene will be therefore 48.2 and 49.8. If instead of the calorific value for elementary carbon the value deduzed from the molecular calorific value for cyclo-propane is used, the calculated increments are 46.5 and 48.1. The actual increments from the observed results are 41.2 (Stohmann) and 44.8 (Roth and Auwers) for the hydro-benzenes and 45.8 for the respective terephthalic acid derivatives. Thus this argument expla,ins in a satisfactory manner the series of values obtained for the molecular heats of combustion of these compounds. The subject, however, may be viewed from another standpoint. I n dealing with the calorific value of the doubly linked carbon atom the idea was put forward that the two doubly linked carbon atoms are really members of a closed chain of force. If three pairs of such atoms are connected in ring formation we have the following system when the apices are coincident. This is what is implied in the centric formula for benzene. Di-hydrogenation would cause the destruction of three of these triangles and consequently a comparatively large decrease in the calorific value of the two carbon atoms common to both since they are now fully hydrogenated. If the


I363

system 3 4 5 6 is sufficiently stable to remain intact after this has taken place, then the hydrogenised carbon atoms I and 2 a.re comparable to those in cyclohexane. Thus the increment for the di-hydrogenation would be only 53.1. This result is not in accordance with observed facts. Similarly the increments for the remaining two stages are identical with that for the first, and are appreciably higher than the experimental figures. Taking all the evidence, the prism formula for benzene is strongly supported by the thermochemical results obtained for benzene and the hydrogenated compounds under discussion.

Conclusions ( I ) The molecular calorific value of an organic compounds is a function of its composition. ( 2 ) The calorific value of a carbonyl group is 60.7 kilo-gramme calories. (3) The calorific value of the hydroxyl group is 12.9 kilo-giamme calories. (4) The contribution of the carboxyl group is not what would be expected from the ordinary COOH representation; that is equivalent to one carbonyl group and one hydroxyl group. Thermochemical results point to the mobility of a hydrogen atom giving rise thereby to an additional, potential hydroxyl group.

( 5 ) The value for benzenoid carbon deduced from various benzene hydio-

carbons agrees closely with the values for amorphous carbon and not very different from the value for graphitic carbon. (6) The molecular calorific value of the carbon atom in situ varies and has the lesser values when the disposition of the bonds approaches nearer to that in the symmetrical tetrahedral positions. (7) The results obtained by various observers in connection with hydrogenated benzene compounds indicate that the benzene molecule is best represented by the prism formula. April 21, 19M.

Calorific Value and Constitution - The Journal of Physical Chemistry

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