SOME PRACTICAL HELPS in ORGANIC ANALYSIS G. H. CHEESMAN Norwwd Technical Institute, London County Council, London, England
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HE following notes may prove useful in technical analysis and also for those teaching organic analysis. They have been incorporated into the standard scheme taught to organic chemistry students here and do not appear to have been described before. A TEST FOR PRESENCE OF ESTERS
On the other hand, the use of alcoholic alkali complicates the identification of the ester-alcohol. In such a case, the use of alkali dissolved in a mixture of pyridine and water greatly accelerates the process, and the pyridine can readily be removed a t a later stage. Thus, as an example, the liquid may be refluxed for two hours with five h e s its volume of 2 N aqueous potassium hydroxide and the same volume of pyridine. The nonacidic products of saponification and the pyridine are then removed by distilling down to small bulk, and separated from one another by redistillation after addition of sulfuric acid. This second distillate will contain the alcohol from the ester, together with the hydrocarbons, etc., originally mixed with it, and but little pyridine. The acid from the ester will remain in the residue from the first (alkaline) distillation. The action of the pyridine seems partly to be due to its promoting mixing of the layers, but experiments with other materials which dissolve readily in both layers seem to indicate that it does actually take part in the saponification process to some extent.
In examining organic materials for the presence of saponifiable matter it seems desirable to have a simple, rapid, and specific test which consumes little material. The ordinary method of boiling with alkali and examining the products to ascertain whether hydrolysis has occurred is wasteful of time and material if no ester be found. If almost any of the esters commonly met with in technical solvents, or offered to students for analysis, be refluxed with alcoholic alkali hydroxide, saponification proceeds rapidly and completely. By using a quantity of alkali less than sdicient to complete the saponification, the occurrence of the reaction may be detected by observing the disappearance of the alkalinity. SEPARATION OF ALCOHOLS FROM MIXTURES WITH The amount of alkali required for this purpose may ESTERS, HYDROCARBONS, ETC. be arrived at by considering the highest ester likely to w i d mixtures boiling below about 1700 may conbe encountered: thus, benzyl benzoate (mol. wt. 212) veniently be separated by conversion of the alcohols to will require 2.3 cc, of alkali per gram for its plete saponification. Assuming the density of esters their benzoic esters which boil considerably higher. A to be about 1, we may say that most esters will react convenient procedure is as follows. Dry the liquid for with more than twice their own volume of 2 N alkali, one hour over anhydrous sodium sulfate, filter and add an equal volume of pure dry pyridine, followed by a and hence we may apply this as a test as follows. To a portion of the liquid to be tested, neutralized if slight excess of benzoyl chloride added slowly in small necessary by addition of acid or alkali, add a quarter of portions with vigorous shaking, keeping the temperaits volume of 2 N caustic potash in methanol (this solu- ture below 20'; should the mass become solid during tion keeps indefinitely), reflux for five minutes, and di- the addition, dilute with more pyridine. When the lute with its own volume of cold water. The presence addition is complete (the cessation of heat evolution is of esters is indicated by the solution then remainingcol- a good indication of this), allow the mass to stand a while. Wash well with plenty of water, then with orless on the addition of phenolphthalein. I t is convenient to perform this test in an ordinary dilute hydrochloric acid, and finally with water. (These washings will contain any water-soluble substances, boiling tube with a very simple reflux condenser. other than alcohols, present in the original material.) Dry and transfer to a distilling flask. Place in an oil sAPoNIFICATIoN OF ESTERS IN THE PRESENCE bath and raise the temperature of the bath slowly to OF HYDROCARBONS, ETC. 200°. Collect the distillate till no more passes over When water-insoluble esters are being saponified at this temperature. The distillate contains the hywith aqueous alkali (with the object of recovering the drocarbons, etc., from the original material, while the products for recognition), and particularly if hydro- benzoates of the alcohols remain in the residue, which carbons or similar substances be present, the amount may be saponified to recover them, or fractionated to of organic material in the aqueous phase is so small identify them by their boiling points. In addition to that the saponification proceeds at a very slow rate. the benzoates, the following compounds may be 92
present: benzaldehyde (b. p. 179'); benzoyl chloride to so(b. p. 198'); benzoic acid (b. p. 250'-tends
lidify in the condenser) and benzoic anhydride (b. p. 360").
