Martin Feldman
and J. W. Wheeler Howard Umvers~ty Wosh~ngton,D.C 20001
I
Nitration of Benzoic Acid Determination of isomer distribution
b y the isotope dilution technique W e have included an experiment in the isotope dilution technique in an advanced course in the laboratory methods of modern organic chemistry.' This sensitive technique makes it possible to determine quantitatively very small amounts of particular components of a mixture. We have chosen the analysis of the nitrobenzoic acids, obtained by direct nitration of C14-labeledhenzoic acid, to illustrate the method. The small amount of p-nitrobenzoic acid may be determined easily by t.he method, whose utility in determining nlinor products of aromatic substitution rraations hits heen elegantly demonst,rat,ed( I , @ . Renzoic acid serves as a good substrate for nitration in t,his experiment because the C14-labeled acid is relatively inexpensive, the reaction gives a high yield of mononitrated products, and the products are highmelting solids which may he easily recrystallized. We have used the reaction conditions of Holleman (3) in order to compare t,he results of t~heisot,ope dilution t,echnique wit.h his results, which have been accepted for almost sevent,yyears2 The t,echnique involves the dilut,ion of a portion of a mixture of radioactive compounds, whose specific activity is known (in this case, the three nitrobenzoic acids, labeled with CL4in the carboxyl group) with a large amount of a pure, nonradioactive. component of the mixture (a particular isomer). Recrystallization or other purification techniques are used to isolate the particular component, and when it is chemically pure and radiochemically pure (constant specific activity hefore and aft.er purifieat,ion),the specific act,ivity of t,hc final sample is used to c;tlc~~la,t,r t,he weight of t,htb iwmponent which was present in the original mixture. If x is the unlcnown weight of the isomer in the mixture, whose total specific activity is A , and w is the weight of the pure isomer which is added, then x ma? be determined from z = w[A'/(A - A')]
where A' is the final specific activity of the pure r~~ isolated i ~ o m e r . ~ The ratio of x to the total weight of the mixture
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Journal o f Chemical Education
gives the fractio~~ of the p~oductrepresented by thc particular isomer. The presence of labeled starting material in the mixture will give values of x which are too low (sum of the weights of the components will be less than the weight of the mixture), while contamination of one isomer with another will give values which are too high. Thus, there is an internal check on t,he cxperimental result,^. The Experiment
Slowly add C"-henzoic acid4 (about 1.0 g weighed itwurately) to fuming nitric acid5 (4 ml) at O T . Allow t,he mixture to stand 30 mi11 at O", then pour it onto 400 ml of ice water with stirring. Keep the precipitated nitrobenzoic acids at Oo for two hours, filter with vacuum, and wash thoroughly with small portions of ice water. Dry the acids in a vacuum desiccator. Extract the aqueous washes with three 20-ml portious of ether, wash the combined ethereal extracts with water, and dry them over anhydrous sodium sulfate. Add the residue obtained after removal of the ether to -~
p~
This course has taken the place of the traditional course in qualitative organic analysis, and includes experiments involving various methods of separation, purification, and analysis in order to provide experience in techniques which are valuable in organic chemical research. An oubline of the experilnent,~will he pnp vided on request. Holleman reporled the resrdts of three single experinrents, carried nut a t -30°, O', and +30". We have assigned s probable error to his data from the results of his control experiment, in which he analyzed a known mixture of nitrobenzoic acids whose vomposifio~> approximated 1 . h ~reactiou prnduets. Flollemsn's dat,a nw quoted in nu& w e n t , trxls as NOHM.~N, I{.0. C., AN^ TAYLIIK. R., "Electmphllic Suhetitutim i l l R e w ~ t ~ o i dcon^ pounds." Amwiesn Flsrvier Publishing Cw. Inc.. Npa Yo&, 1965, p. i 2 . This equation or its equivalent appears in many radiochemical texts. A detailed description including extensive experimental $ata is given in ref. ( I ) , which pertains to the nitration of halohensenes. " Rensni
