It may be urged that it is superfluous to import these questions of mass-aggregation into the problem of the chemical constitution of cellulose. But we shall find that the point again arises in attempting to define the reacting unit, which is another term for the molecule. In the majority of cases we rely for this upon physical measurements; and in fact the purely chemical determination of such quantities is inferential. Attempts have been made to determine the molecular weights of the cellulose esters in solution, by observations of depression of solidifying and boiling-points. But the numbers have little value. The only other well-defined compound is the sulphocarbonate. It has been pointed out that, by successive precipitations of this compound, there occurs a continual aggregation of the cellulose with dissociation of the alkali and CS residues and it has been found impossible to assign a limit to the dissociation, i.e. to fix a point at which the transition from soluble sulphocarbonate to insoluble cellulose takes place.
On these grounds it will be seen we are reduced to a somewhat speculative treatment of the hypothetical ultimate unit group, which is taken as of C6 dimensions.
As there has been no addition of experimental facts directly contributing to the solution of the problem, the material available for a discussion of the probabilities remains very much as stated in the first edition, pp. 75-77. It is now generally admitted that the tetracetate n [C6H6O.(OAc)4] is a normal cellulose ester; therefore that four of the five O atoms are hydroxylic. The fifth is undoubtedly carbonyl oxygen. The reactions of cellulose certainly indicate that the CO- group is ketonic rather than aldehydic. Even when attacked by strong sulphuric acid the resolution proceeds some considerable way before products are obtained reducing Fehling's solution. This is not easily reconcilable with any polyaldose formula. Nor is the resistance of cellulose to very severe alkaline treatments. The probability may be noted here that under the action of the alkaline hydrates there occurs a change of configuration. Lobry de Bruyn's researches on the change of position of the typical CO- group of the simple hexoses, in presence of alkalis, point very definitely in this direction. It is probable that in the formation of alkali cellulose there is a constitutional change of the cellulose, which may in effect be due to a migration of a CO- position within the unit group. Again also we have the interesting fact that structural changes accompany the chemical reaction. It is surprising that there should have been no investigation of these changes of external form and structure, otherwise than as mass effects. We cannot, therefore, say what may be the molecular interpretation of these effects. It has not yet been determined whether there are any intrinsic volume changes in the cellulose substance itself: and as regards what changes are determined in the reacting unit or molecule, we can only note a fruitful subject for future investigation. A priori our views of the probable changes depend upon the assumed constitution of the unit group. If of the ordinary carbohydrate type, formulated with an open chain, there is little to surmise beyond the change of position of a CO- group. But alternative formulæ have been proposed. Thus the tetracetate is a derivative to be reckoned with in the problem. It is formed under conditions which preclude constitutional changes within the unit groups. The temperature of the main reaction is 30°-40°, the reagents are used but little in excess of the quantitative proportions, and the yields are approximately quantitative. If now the derivative is formed entirely without the hydrolysis the empirical formula C6H6O.(OAc)4 justifies a closed-ring formula for the original viz. CO<[CHOH]4>CH2; and the preference for this formula depends upon the explanation it affords of the aggregation of the groups by way of CO-CH2 synthesis.
The exact relationship of the tetracetate to the original cellulose is somewhat difficult to determine. The starting-point is a cellulose hydrate, since it is the product obtained by decomposition of the sulphocarbonate. The degree of hydrolysis attending the cycle of reactions is indicated by the formula 4 C6H10O5.H2O. It has been already shown that this degree of hydrolysis does not produce molecular disaggregation. If this hydrate survived the acetylation it would of course affect the empirical composition, i.e. chiefly the carbon percentage, of the product. It may be here pointed out that the extreme variation of the carbon in this group of carbohydrate esters is as between C14H20O10 (C = 48.3 p.ct.) and C14H18O9 (C = 50.8 p.ct.) i.e. a tetracetate of C6H12O6 and C6H10O5 respectively. In the fractional intermediate terms it is clear that we come within the range of ordinary experimental errors, and to solve this critical point by way of ultimate analysis must involve an extended series of analyses with precautions for specially minimising and quantifying the error. The determination of the acetyl by saponification is also subject to an error sufficiently large to preclude the results being applied to solve the point. While, therefore, we must defer the final statement as to whether the tetracetate is produced from or contains a partly hydrolysed cellulose molecule, it is clear that at least a large proportion of the unit groups must be acetylated in the proportion C6H6O.(OAc)4.
It has been shown that by the method of Franchimont a higher proportion of acetyl groups can be introduced; but this result involves a destructive hydrolysis of the cellulose: the acetates are not derivatives of cellulose, but of products of hydrolytic decomposition.
It appears, therefore, that with the normal limit of acetylation at the tetracetate the aggregation of the unit groups must depend upon the CO- groups and a ring formula of the general form CO<[CHOH]4>CH2 is consistent with the facts.
Vignon has proposed for cellulose the constitutional formula
with reference to the highest nitrate, and the decomposition of the nitrate by alkalis with formation of hydroxypyruvic acid. While these reactions afford no very sure ground for deductions as to constitutional relationships, it certainly appears that, if the aldose view of the unit group is to be retained, this form of the anhydride contains suggestions of the general tendency of the celluloses on treatment with condensing acids to split off formic acid in relatively large quantity [Ber. 1895, 1940]; the condensation of the oxycelluloses to furfural; the non-formation of the normal hydroxy-dicarboxylic acids by nitric acid oxidations. Indirectly we may point out that any hypothesis which retains the polyaldose view of cellulose, and so fails to differentiate its constitution from that of starch, has little promise of progress. The above formula, moreover, concerns the assumed unit group, with no suggestion as to the mode of aggregation in the cellulose complex. Also there is no suggestion as to how far the formula is applicable to the celluloses considered as a group. In extending this view to the oxycelluloses, Vignon introduces the derived oxidised group
