Researches on Cellulose Part 10

and generally the ketoses are distinguished from the aldoses by their susceptibility to condensation. Such condensation of laevulose has been effected by two methods: (a) by heating the concentrated aqueous solution with a small proportion of oxalic acid at 3 atm. pressure [Kiermayer, Chem. Ztg. 19, 100]; (b) by the action of hydrobromic acid (gas) in presence of anhydrous ether; the actual compound obtained being the omega-brommethyl derivative [Fenton, J. Chem. Soc. 1899, 423].

This latter method is being extended to the investigation of typical celluloses, and the results appear to confirm the view that cellulose may be of ketonic const.i.tution.

The evidence which is obtainable from the synthetical side of the question rests of course mainly upon the physiological basis. There are two points which may be noted. Since the researches of Brown and Morris (J. Chem. Soc. 1893, 604) have altered our views of the relations.h.i.+ps of starch and cane sugar to the a.s.similation process, and have placed the latter in the position of a primary product with starch as a species of overflow and reserve product, it appears that laevulose must play an important part in the elaboration of cellulose. Moreover, A. J. Brown, in studying the cellulosic cell-collecting envelope produced by the _Bacterium xylinum_, found that the proportion of this product to the carbohydrate disappearing under the action of the ferment was highest in the case of laevulose. These facts being also taken into consideration there is a concurrence of suggestion that the typical CO group in the celluloses is of ketonic character. That the typical cotton cellulose breaks down finally under the action of sulphuric acid to dextrose cannot be held to prove the aldehydic position of the carbonyls in the unit groups of the actual cellulose molecule or aggregate.

We again are confronted with the problem of the aggregate and as to how far it may affect the const.i.tution of the unit groups. That it modifies the functions or reactivity of the ultimate const.i.tuent groups we have seen from the study of the esters. Thus with the direct ester reactions the normal fibrous cellulose (C_{6}H_{16}O_{5}) yields a monoacetate, dibenzoate, and a trinitrate respectively under conditions which determine, with the simple hexoses and anhydrides, the maximum esterification, i.e. all the OH groups reacting. If the OH groups are of variable function, we should expect the CO groups _a fortiori_ to be susceptible of change of function, i.e. of position within the unit groups.

But as to how far this is a problem of the const.i.tution or phases of const.i.tution of the unit groups or of the aggregate under reaction we have as yet no grounds to determine.

The subjoined communication, appearing after the completion of the MS.

of the book, and belonging to a date subsequent to the period intended to be covered, is nevertheless included by reason of its exceptional importance and special bearing on the const.i.tutional problem above discussed.

~THE ACTION OF HYDROGEN BROMINE ON CARBOHYDRATES.~[4]

H. J. H. FENTON and MILDRED GOSTLING (J. Chem. Soc., 1901, 361).

The authors have shown in a previous communication (Trans., 1898, 73, 554) that certain cla.s.ses of carbohydrates when acted upon at the ordinary temperature with dry hydrogen bromide in ethereal solution give an intense and beautiful purple colour.[5] It was further shown (Trans., 1899, 75, 423) that this purple substance, when neutralised with sodium carbonate and extracted with ether, yields golden-yellow prisms of omega-brommethylfurfural,

CH:C.CH_{2}Br O CH:C.CHO.

This reaction is produced by laevulose, sorbose, cane sugar, and inulin, an intense colour being given within an hour or two. Dextrose, maltose, milk sugar, galactose, and the polyhydric alcohols give, if anything, only insignificant colours, and these only after long standing. The authors therefore suggested that the reaction might be employed as a means of distinguis.h.i.+ng these cla.s.ses of carbohydrates, the rapid production of the purple colour being indicative of _ketohexoses_, or of substances which produce these by hydrolysis.

By relying only on the production of the purple colour, however, a mistake might possibly arise, owing to the fact that _xylose_ gives a somewhat similar colour after standing for a few hours. Hence, the observations should be confirmed by isolation of the crystals of brommethylfurfural. No trace of this substance is obtained from the xylose product.

In order to identify the substance, the ether extract, after neutralisation, is allowed to evaporate to a syrup, and crystallisation promoted either by rubbing with a gla.s.s rod, or by the more certain and highly characteristic method of 'sowing' with the most minute trace of omega-brommethylfurfural, when crystals are almost instantly formed.

These are recrystallised from ether, or a mixture of ether and light petroleum, and further identified by the melting-point (59.5-60.5), and, if considered desirable, by estimation of the bromine.

It is now found, so reactive is the bromine atom in this compound, that the estimation may be accurately made by t.i.tration with silver nitrate according to Volhard's process, the crystals for this purpose being dissolved in dilute alcohol:

0.1970 gram required 10.5 c.c. _N_/10 AgNO_{3}. Br = 42.63 p.ct., calculated 42.32 p.ct.

This method of applying hydrogen bromide in ethereal solution is, of course, unsuitable for investigations where a higher temperature has to be employed, or where long standing is necessary, since, under such circ.u.mstances, the ether itself is attacked. Wis.h.i.+ng to make investigations under these conditions, the authors have tried several solvents, and, at present, find that chloroform is best suited to the purpose. In each of the following experiments, 10 grms. of the substance were covered with 250 c.c. of chloroform which had been saturated at 0 with dry hydrogen bromide. The mixture was contained in an accurately stoppered bottle, firmly secured with an iron clamp, and heated in a water-bath to about the boiling temperature for two hours.

After standing for several hours, the mixture was treated with sodium carbonate (first anhydrous solid, and afterwards a few drops of strong solution), filtered, and the solution dried over calcium chloride. Most of the chloroform was then distilled off, and the remaining solution allowed to evaporate to a thick syrup in a weighed dish.

The product was then tested for omega-brommethylfurfural by 'sowing'

with the most minute trace of the substance, as described above. It was then warmed on a water-oven, kept in a vacuum desiccator over solid paraffin, and the weight estimated. When necessary, the product was recrystallised from ether, and further identified by the tests mentioned. The following results were obtained:

Weight of crude residue.

Swedish filter paper 3.0 crystallised at once by 'sowing.'

Ordinary cotton 3.3 " "

Mercerised cotton 2.1 " "

Straw cellulose[6] 2.3 " "

Laevulose 2.2 " "

Inulin 1.3 " "

Potato starch 0.37 " "

Cane sugar 0.85 " "

Dextrose 0.33 uncrystallisable.

Milk sugar 0.37 "

Glycogen 0.34 "

Galactose 0.34 "

The products from _dextrose_, _milk sugar_, and _galactose_ absolutely refused to crystallise even when extracted with ether and again evaporated, or by 'sowing,' stirring, &c.

The _glycogen_ product deposited a very small amount of crystalline matter on standing, but the quant.i.ty was too minute for examination; moreover, it refused altogether to crystallise in contact with the aldehyde. It may fairly be stated, therefore, that these last four substances give absolutely negative results as regards the formation of omega-brommethylfurfural; if any is formed, its quant.i.ty is altogether too small to be detected.

The specimen of _starch_ examined was freshly prepared from potato, and purified by digestion for twenty-four hours each with _N_/10 KOH, _N_/4 HCl, and strong alcohol; it was then washed with water and allowed to dry in the air. It will be seen that this substance gave a positive result, but that the yield was extremely small, and might yet be due to impurity. Considering the importance of the behaviour of starch, for the purpose of drawing general conclusions from these observations, it was thought advisable to make further experiments with specimens which could be relied upon, and also to investigate the behaviour of dextrin. This the authors have been enabled to do upon a series of specimens specially prepared by C. O'Sullivan, and thus described by him:

1. Rice starch, specially purified by the permanganate method.

2. Wheat starch " " "

3. Oat starch, contains traces of oil, washed with dilute KOH and dilute HCl.

4. Pea starch, first crop, washed with alkali, acid (HCl), and strong alcohol.

5. Natural dextrin, D = 3.87, alpha_{D} = 194.7; K = 0.95, (c 2.628).

6. alpha-Dextrin, C equation purified without fermentation, 30 precipitations with alcohol (Trans., 1879, 35, 772).

The examination of these specimens was conducted on a smaller scale, but under the same conditions as before, _one gram_ of the substance being treated with 12.5 c.c. of the saturated chloroform solution and heated in sealed tubes for two hours as above. The results were as follows:

Weight of crude residue.

1. Rice starch 0.046 crystallised at once by 'sowing.'

2. Wheat starch 0.044 " "

3. Oat starch 0.049 " "

4. Pea starch 0.064 " "

5. Natural dextrin 0.088 " "

6. alpha-Dextrin 0.055 " "

The results may therefore be summarised as follows:--Treated under these particular conditions all forms of cellulose give large yields of omega-brommethylfurfural, some varieties giving as much as 33 per cent.

Laevulose, inulin, and cane sugar give yields varying from 22 to 8.5 per cent.; various starches give small yields (average about 4.5 per cent.); and dextrins 5 to 8 per cent., whereas dextrose, milk sugar, and galactose give, apparently, none at all.

The yields represent the solid crystalline residue; this when purified by recrystallisation gives, probably, about three-quarters of its weight of pure crystals. (In the case of dextrose, &c., the yields represent the weight of syrup.)

These numbers, however, by no means represent the maximum yields obtainable, owing to the comparatively slight solubility of hydrogen bromide in chloroform. The process was conducted in the above manner only for the sake of uniform comparison. The ether method previously described gives much larger yields; for example, 12 grms. of inulin treated with only 60 c.c. of the saturated ether gave 2.5 grms. of substance. For the purpose of obtaining larger yields, other methods are being investigated.

The facts recorded above, taken in conjunction with those given in our previous communications, appear to point definitely to the following general conclusions. First, that the various forms of _cellulose_ contain one or more groups or nuclei identical with that contained in _laevulose_, and that such groups const.i.tute the main or essential part of the molecule. Secondly, that similar groupings are contained in _starches_ and _dextrins_, but that the proportion of such groupings represents a relatively small part of the whole structure.

The nature of this grouping is, according to the generally accepted const.i.tution of _laevulose_, the six-carbon chain with a ketonic group:

CCCCCC .

O

But the results might, on the other hand, be considered indicative of the anhydride or 'lacton' grouping, which Tollens suggested for laevulose:

CCCCCC / / .

O