I The Disproportionation of an David H. Kenny Michigan Technological University Houghton. MI 49931
Unsymmetrical Azine An nmr experiment for the undergraduate organic laboratory
This year's class in organic chemistry lahoratory a t Michigan Technological University has carried out an nrnr experiment based on the disproportionation reaction of the mixed azine of benzaldehyde and acetophenone. The above reaction
can he studied by nmr spectroscopy or by chromatography. This oaoer deals with the former. Half the students started with themixed azine, I, and the other half with an equimolar mixture of henzaldazine. 11. and acetoohenone azine. 111. Both groups reached the same &ilihrium starting from one side or the other. The startina materials were dissolved in an appropriate solvent, such as2euterated chloroform, and two n k spectra were taken, one shortly after mixing and the other during the next week's laboratory period after equilibrium had been reached. The students then calculated the equilibrium constant for the reaction, using data from the nmr spectra. The azines were synthesized from readily-available starting materials without difficulty. Although henzaldazine was easiest to prepare, the other two presented no serious problems in their syntheses. The recrystallized azines are beautiful yellow crystalline solids with sharp melting points. Benzaldazine melted a t 91-92", acetophenone azine had mp 120-121" and the mixed mine 59-60", considerably lower than either of the svmmetrical ones. Although the three azines used in this experiment are caoable of existing as differing stereoisomeric forms (Z, E isomers), only one stereoisomer is isolated, presumably the thermodynamically most stable.' Only one nmr tube was needed to perform this experiment and two nmr spectra had to he measured, one immediately after dissolution, the other after equilibrium was reached (after 120 hr). Both the methyl and methyne signals appeared as sharp singlets. Their chemical shifts were different enough in the unsymmetrical azine and in the two symmetrical azines to oermit their identification in the reaction mixture. A sharo singlet a t 62.55 in the nmr spectrum of the freshly-prepared solution of the mixed azine. I. is due to its three methvl . nro. tons. As the renrtim prweed.; toward equilil,riuy, another sharo sinelet de\'eIo~sat 52.3;. due t o the six rnethsl r~rotons in adetopienone azine, 111. he other sharp singlet at 68.41 proton. in the spectra of the mixed azine is due to its methvne . .
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o-Nitroacetophenoneazine has been shown toexist as threeZ, E isomers, all of which have been characterized. Fleming, I. and Harley-Mason, J., J. Chem. Soe., 5560 (1961). Szmant, H. H. and McGinnis. C.. J. Amer. Chem. Soc... 72.2890 . (1950). Smith, P. A. S., "Open-Chain Nitrogen Compounds," W. A. Benjamin Inc., New York, 1966, Vol. 2, p. 157. 462 1 Journal of Chemical Education
P PM. 161
NMR spectra taken an a Varlan Associates T-BO nmr spectophotometer of the mixed azine of benzaldehvde and acetoohenone I. taken immediatelv aner dlssal~non in deweraled chlomfam.aner standong far 12 hr, and alter staMmg lor 5 oays compared to m e nmr spectra ot benzadszme 11, ano acelophenone azine Ill.
Typical Student Results
Section
I 2 3
Number of Repms 11 8 12
Calculated Equilibrium Constant Range
Average
0.18-0.32 0.23-0.29 0.20-0.43
0.24 0.26 0.28
The singlet that develops with time at 68.75 is caused hy the two methyne protons of benzaldazine, I1 (see figure). The mechanism of the unsymmetrical azine disproportionation has not been studied. I t probably follows the same route as the related acid-catalyzed formation of hydrazones from a z i n e ~I.t~seems likely that the first step in both these reactions is hydrolysis of the azine by small amounts of water which would explain why these reactions are both catalyzed by traces of acid.3 The Experiment The instructor may have half of the students in the class synthesize the mixed azine and have the other half of the group approach the equilibrium from the other side, by synthesizing acetophenone azine and henzaldazine. Either eauimolar auantities (0.1 millimole is adeouate) of the two symmetricai azine are dissolved in 0.5 ml of suitable solvent like deuterated chloroform or a sample (0.2 millimole) of the unsymmetrical azines is dissolved in 0.5 ml of the same solvent in an nmr tube and nmr svectra can he taken as necessary. We found that if thr ~lncata~!'il.dnzinesstotd irom one 1aho;iltory ~ e r i o dto the nrxt week's laboratory period, the rt.act:on :ilmost always reached equilibrium.-k the few cases where equilibrium was not reached, a drop of acetic acid was added t d t h e azine mixture and equilibrium was reached in 2-3 hr. An equilibrium constant is calculated for the reaction by assuming that the area under the appropriate peaks is proportional to the number of protons and the concentration of the compound responsible for the signal. Thus, for the dis-
a
~ r o ~ o r t i o n a t i oofnthe mixed azine of benzaldehvde and acktophenone, the equilibrium constant is cal&lated hy multiplying one half the change in height of the integration line over the peak at 68.75 by one sixth the change in height of the integration line over the peak a t 62.37 and dividing this product by the square of either the change in height of the integration line over the peak at 68.41 or one third the change in height of the integration line over the peak at 62.55. The & reported by the students was the average of two values, one calculated using the integration line over the peak at 68.41 and the other using the integration line over the peak a t 62.55. Benzaldazlne Benzaldehyde (7.0 g) was added dropwise to a solution of hydrazine hydrate (1.5 g) in ethanol (10 ml). The reaction vessel was cooled in an &e water bath. The product was ohtained as yellow needles, m.p. 91-92' (96% yield) after recrystallization from 95% ethanol. Acetophenone Azine Acetophenone (8.0 e) was added drovwise to a solution of ~ wncentrated hydra& hydrate ( 1 . i p j in ethanol ( I O and hydrochloric arid r 2 mll. The reaction mixture was heated on a steam t~athfor 15 min. Arettrphenmea~inewas obtained as yell~~uf needles m.p. 120-121° t:6? yield, nftrr rerrystnllization from 95% ethanol. The Mixed Azine of Benzaldehyde and Acetophenone A mixture of aceto~henone(12.0 a ) and acetic acid (1 ml) in erhand r 2 mll wa-idded dropwisett, hydrazine hydrate ( 1 0 g1. The reaction mixture was hearcd fur 10 min on a steam bath. After co,lino, the reaction mixture wasdiisolved inether (20 m l ~i d the erhpr solution extracted with water. Afcer the ether layer was ieparnted, drird w r r magnriium suliate, and filtered through glass w
