2,4-Dinitrophenylhydrazine: An Illustration of the Use of NMR Spectroscopy Recent articles in this Journal ( 1 3 1 have pointed out the need for experiments which show the application of high resolution nuclear magnetic resonance spectroscopy to organic chemistry. A common undergraduate preparation, originally suggested by Ault (4). is the two-step synthesis of 2,4-dinitraphenylhydrazine from bromobenzene as shown below.
Br
Br
NHNH,
rlro,
NO,
I
This preparation allows the student to synthesize a well-known reagent and to compare and contrast the effect of substituents on electrophilic and nucleophilie substitution. It is included in several recent textbooks (5-7). The nearly first-order 60 mHz nmr spectrum taken in DMSO-ds provides a confirmation of the structure. The spectrum illustrates the effects of intramolecular hydrogen bonding on chemical shift, the effect of aromatic substituents on the chemicals shifts of aromatic protons, aid the coupling constant differences for aromatic protons.
T
"-N\N/b..o Aromatic Proton Coupling Constants ortho meta para
Hd
*H
6.C-9.4 Hz 1.2-3.1 Hz 0.2-1.5 Hz
NO, A consideration of the deshielding effect of intramolecular hydrogen bonding an chemical shift (81 leads one to assign the resonance at 610.9 to the proton labeled HL, in the above structure. The absorption at 64.8 is due to the two protons labeled H, plus any water present in the sample. Students given the above table of representative coupling constants from the literature (9) can analyze the rest of spectrum as follows. The doublet a t 67.7 (J = 10 Hz) must result from artho splitting. I t cannot be due to Hd since it would be expected to exhibit further meta splitting by H,. I t is therefore assigned to He whose para splitting is too small to be observed. The doublet of doublets at 68.2 is due to the ortho and meto splittingof Hd; and the remeiningresonanceat 68.8 is assigned toH, (Jmat, = 2.8Hz). Alternatively, one can approach the analysis of the spectrum from the perspective of chemical shift. The effect of suhstituents of the chemical shift of aromatic protons is shown below (10). (A negative sign indicates a downfield shift relative to benzene.) NO* NH*
ortho -0.95 0.75
plwa -0.33 0.63
metn
-0.17 0.24
Although the additivity of these numbers for 1,2,4-trisubstituted benzenes must he regarded as very approximate, these data would lead one to expect He to be a t highest field of the aromatic protons, H, a t lowest and Hd in between. Copies of the spectrum shown here are available from the author. The spectrum may also he found in the Sadtler Catalog. It should also be noted that all portions of the spectrum shown here can be observed when non-labeled DMSO isused as the solvent.
Literature Cited (1) Glams, G.. andCromwel1, N . H . , J . CHEM.EUUC.,46,854(1969). (2) Glams, G., andCmmwell, N.H.. J.CHEM. EUUC..4R.202(1971). (3) Glsros, G.,andCromwell,N.H., J.CHEM.EDUC.,48,204(1~1). (4)Auk. A,. J . CHEM.EDUC.. 42.267(19ESI. 15) Rosenhlaft, D. H., and Davis, G . T.,"Laboretory Coune in Organic Chemislry..'Allyn and Bamn. Boston, 1911. p. 233. 161 M n r o . J.A.. andDalrvmole."EroerimentalMethdsin OreanieChornistrv." W.B.Saunderr Co..Philadelohia. 1971.o. 173
Jeffrey K. Keiser Coe College Cedar Rapids, Iowa 52402
Volume 51, Number 9, September 1974
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