MARCH, 1950
IMPORTANCE OF FUNCTIONAL GROUP DETERMINATION IN ORGANIC QUANTITATIVE ANALYSIS SIDNEY SIGGIA General Aniline and Film Corporation, Central Research Laboratory, Easton, Pennsylvania
ATPRESENT much emphasis is being placed
on the need for more instruction in our colleges and universities in the field of organic analysis. Along this line, the importance of functional group analysis should he stressed as one of the most satisfactory and generally applicable methods of quantitatively determining organic compounds. This approach also lends itself very well toward identification of organic compounds. There are several reasons for mnning an analysis on a sample. These are: 1. To assay the sample for the desired component. 2. To follow a reaction to determine the reaction rates and the optimum reaction conditions. 3. To determine stability of a certain compound or mixturc under vnrious conditions. 4. To determine by-pro(lucts and unreacted starting materials. 5. To aid in identifying an unknown compound or proving the identity of a suspected compound.
tain percentage of amino group rather than just a certain percentage of nitrogen, or that it has a certain hydroxyl group content rather than a certain carhon and hydrogen content. There are interferences to functional group determinations as there are to any analytical procedure. However, these interferences are much more limited in number than those which exist in elemental analysis. The main reason for this is that there are just a few elements involved in organic chemistry, and these elements are common to a great number of organic compounds. However, in organic chemistry there are many functional groups that can he determined, and each functional group is characteristic of a small portion of all the organic compounds. Then also, each organic compound usually consists of more than one functional group so that often a functional group can be found which is common only to the desired component and which can be used to determine that component. An example of the above case is the analysis employed to follow the hydrogenation of 1,4-dihydroxybutyne-2.
Analysis for the elements can he applied only to case 5. It can be applied to the others if the component
being determined contains an element not contained in the impurities, but this situation is not common. Analysis for the functional group can be applied to all the cases stated above mainly because the analysis of mixtures is possible using this approach. Even in case 5 where elemental analysis is a t present primarily used, a functional group determination is much more meaningful. The proof of the identity of an organic unknown is much more definite if it is known that it contains a cer-
1 1L
. HI
HOCH&H&H&HO
-
Rearrangement
Rearrangement
HOCH,CHCH,C&
I
OH
A system of analysis was necessary for the determina-
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tion of the product, by-products, and unreacted starting anilme forms the nitroso compound. The diazonium materials in the final product. The unreacted starting compound will couple with phenols, amines, and pyramaterial was determined by attacking the acetylenic zalones, whereas the nitroso compound will not. By grouping using boron trifluoride and methanol.' The titrating the nitrosated sample with a standardized 1glycol by-product was determined via a periodic acid phenyl-3-methyl-pyrazalone-5solution, the aniline can o x i d a t i ~ n .The ~ aldehyde by-product was measured by be determined without being affected by the monoethyl making the bisulfite addition product.' The main aniline. product was measured by determining the total hyThe difficulty can also be avoided by first using a droxyl content of the sample and subtracting the method which determines the total of desired comamount of the different impurities contained in it. ponent and interference. Then the latter is determined If interferences exist which cannot be circumvented by a separate method, and the desired component is obby analyzing for another functional group, then it is tained by difference. An example of this technique is .possible to remove the impurity by changing it into a the determination of acetals which contain free aldeform in which it no longer interferes. Also, the whole hyde. The free aldehyde can be determined by bisulfite sample can be altered chemically, but in such a way that addition. Then the total acetal and free aldehyde can the altered form of the impurity will not interfere with be determined by acid hydrolysis and determination of the determination of the altered form of the desired total aldehyde. The acetal is then obtained by difcomponent. An example of this technique is the de- ference. termination of aniline in the presence of monoethyl Another factor which speaks for the utility of funcaniline. Both materials undergo the same analytical tional group analysis is the use of ordinary quantitative reactions, acetylate, consume nitrous acid, brominate. apparatus. For determining most functional groups, However, on reaction with nitrous acid, the aniline the balance, buret, pipet, flask, and condenser are all forms the diazonium compound and the monoethyl the apparatus that is required. Special apparatus is needed for a very few groups. 'WAGNER, D. C . , T. GOLDSTEIN, AND E. D. PETERS, Anal. Also, the accuracy and precision attainable by most Chem., 19, 103 (1947). of the present methods are of very high order. An POALE. W. D.. V. C. MEALENBACKER. AND J. H. COOK.Oil average accuracy and precision poorer than 1 1 per cent & S w p , 22; 1159 (1945). a SIGGIA, S., AND IV. J. MAXCY, Anal. Chem.,19, 1023 (1947). is not often found.
