David lodd
Worcester Polytechnic Institute Worcester, Massachusetts 01609
Several labelling experiments suitable for the elementary organic laboratory have been published in this Journal or in well-known laboratory manua1s.l Since the synthesis of labelled compounds has become of great importance, i t seems useful to work out more such syntheses to give the student a wider choice. We report here the synthesis of benzene-dl on a small scale. and its conversion to the solid meta-dinitrobenzene. In this experiment the student is exposed to the idea of an isotope effect. since the auestion of selectivitv of reaction obvidusly comes up in the nitration step. -while other methods can, and have been, used to convert small amounts of benzene to a solid d e r i v a t i ~ e ,dinitration ~ seems best from the point of view of the final ir analysis. The laboratory operations in this sequence involve the preparation of phenylmagnesium bromide, reaction of the latter with DzO, and a double distillation to concentrate the benzene. The benzene is then dinitrated and the mdinitrobenzene recrystallized. Infrared analysis is used to demonstrate the presence of deuterium in the final moduct. Procedure
Phen~lmagnesium bromide is prepared in the usual manner from 1.0 g of Mg and 2.2 ml of bromobenzene. T o the cold Grimard solution is added 1.0 ml of D20. and that forms is crushed well with a-flanged the rod to ensure comnlete reaction. After the usual dilute HCI hydrolysis, the ether solution, which contains some bromobenzene and diphenyl as well as the desired deuterated benzene, is distilled from the steam bath with the minimum of fractionation so as to force over the benzene along with the ether. Several small portions of ether are added, via a separatory funnel in the vertical neck of the T-adapter used, to the hot flask in order to force over all the benzene. The ether-benzene distillate is then fractionated, using a vertical unpacked condenser as a fractionating column, and slow steam.bath distillation (1-2 drops/sec). ~h~ dis. tillation is stopped when 2-3 ml is left in the pot, ~h~ ap. paratus is well cooled, then by manual pipet the benzene (which contains a little ether) is added dropwise to a cold mixture of 10 ml of conc. HNOJ and 30 ml of conc. HzSO* with good swirling. I t is prudent, when about half of the benzene has been added, to heat the nitration mixture
366 / Journalof Chemical Education
The Synthesis and Nitration of Benzene-dl briefly on the steam bath, then to recool before continuing the addition. After 10 min of steam bath heating, the nitration mixture is poured into 500-600 ml of cold water. The cmde precipitate is recrystallized from aqueous methanol and air-dried. Results
An infrared analysis is carried out in CHCk. The C-D stretching frequency appears a t 4 . 3 ~(2300 cm-l) as compared with the normal aromatic C-H value of 3.2-3.3~ (3100-3000 cm-l). It is instructive to have available-or for the student to run-the nmr spectra of both non-deuterated and deuterated m-dinitrobenzene. If a mass spectrometer is available, the mass spectrum of the above product can be used as the basis for a discussion of the isotope effect in aromatic nitration. The following questions have been found to be useful in assessing the students' understanding of this experiment 1) If unchanged hromabenzene were present in the liquid that is nitrated, what complication might result? 2) What products are formed in the dinitration of CsHsD, and in what relative molar amounts are they formed? (Assume the k,/k,ratio for aromatic nitration is 1.00, as indeed i t is.) 3) What is the e x ~ e c t e dratio of H to D in the total isolated dinitiobenzene you isolate in this experiment? 4) How many molecular ions (not satellite peaks) would you expect to see in the mass.speetrum of your product? 5) In a dinitration of impure C6H5D that contained 91.5% of CeHsD and 8.5% CsHe (i.e., i t was 91.5 male-% CsHsD), what is the expected mole-% of CeH3D(N02)~? 6) In the above experiment, inevitably some Grignard escapes reaction with D 2 0 and subsequently reacts with H20-HC1 (to g i w CRH*uf c u m e ) . When a mass sprc was run on one product sample 11 was found that the 169 IGX rntio was 1 71. Whnr ? of thp l h n n a r d failed to react wrth D 2 0 hut dtd react wnh H 2 0 ' 'The synthesis of 'kC-acetophenone and "C-henzaie acid: J. CHEM. EDUC., 40, 206 (1963); Mackenzie, "Experimental Organic Chemistry," 4th ed. 1971, P. 315; The synthesis of decane1-d: Rosenblatt and Davis, "Laboratory Course in Organic Chemistry," 1971, p, 73; -The Preparation of Naphthalene.a.d,.j J , CHEM. EDUC., 44,609 (1967), ZFor instance, in a study of the metabolism of tritiated ethylene by avocado skins ( J Bid Chem., 238, 1552 (1963)) FriedelCrafts suceinoylation was used for this purpose.
