Aviva Lapidot, Jack Reuben and David Samuel Weizrnonn Institute of Science Rehovoth.. h e 1
An NMR Demonstration of Isotopic Exchange
W e wish to describe a simple NMR experiment which demonstrates the isotopic exchange of deuterium using a retidily available compound. This experiment can be used as an extension to the original outline by Roberts1 for the teaching of NMR to undergraduates. It also provides a visual demonstration of the application of NMR spectra and integrated intensities to chemical rate processes. This subject has recently been comprehensively reviewed.2 Hydracrylonitrile (8-hydroxypropionitrile) NCC,H2CBHzOH (I) is a readily available compound which undergoes isotopic exchange of the a-protons in the presence of owing to the electron withdrawing effect of the neighhoring cyano group. I n the presence of sodium hydroxide, this exchange occurs a t room temperature, the rate being a function of the base concentration used. By choosing suitable conditions, the reaction can be followed easily between ten minutes to haIf an hour. The NMR spectrum of hydracrylonitrile (I) in D 2 0 is of the limiting AzBl type4 (J/6 0.1) (see Fig. 1). I n addition to the water line, containing the hydroxyl protons which undergo very rapid exchange with water, the spectrum consists of two triplets, a t 3.80 and 2.68 ppm. The spin-spin coupling constant is 6.5 cps. The triplet a t higher field (2.58 ppm) is due to the F;-
PPMi8)
Figure 1;
Hydrocrylonitrile lphydroxypmpionitrile) in DzO;
two pprotons split by the neighboring 8-protons, whiie the triplet a t lower field (3.80 ppm) due to the 8-protons is similarly solit by the a-~rotons. 'ROBERTS,J . D., J. CHEM.EDUC.,38,581 (1961). LOEWENSTEIN, A,, AND CONNOR, T.M., Ber. phys. C h m . , 67, 2 n ---~ IIPRR)~ \----,-
"APIDOT, A., REUBEN,J., AND SAMUEL, D., tO be published. 'POPLE,J. A., SCANEIDER, W. G. AND BERNSTEIN, H. J., "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Inc., New York, 1959, chap. 6.
570
/ Journol of Chemical Education
I n the presence of hydroxide ions, the m-hydrogen atoms undergo progressive exchange with solvent D1O
-
NCCHSHXOH (1)
-
NCCDHCHXOH (11)
NCCD&HgOH (IW
The NMR spectrnm of the monodeuterated con~pound (11) should consist of a doublet a t the low field position due to spin-spin coupling with the single proton remaining attached to the a-carbon atom. The latter proton should, however, still appear as a triplet (neglecting the coupling with deuterium) but with a halved intensity. The spectrum of the doubly deuterated compound (111) should give only a single peak due to the unexchanged ,%protons. I n actual practice a gradual transition from (I) through (11) to (111) is observed in the course of a single experiment. I n Figure 2, the NMR spectrum of hydracrylonitrile in the presence of 1.44M NaOH in D20 is shown as it changes with time. It is seen that in the presence of base there is a slight shift to low field, the a- and 8protons being at 2.72 and 3.93 ppm respectively. The protons close to the hydroxyl group are shifted more than those bound to the a-carbon. I n Figure 2A the spectmm a t zero time is shown within one or two min after mixing the solution (which is very similar to that shown in Fig. 1). After 3 min (Fig. ZB), as the reaction proceeds, compound (I) is partially converted to (11) and the spectrum of the 8-protons becomes more con~plex. I n between the peaks of the triplet due to (I), the doublet due to (11) appears. After six minutes (Fig. 2C) the amount of (I) has decreased considerably as is seen by the greatly diminished intensity of the outer component of the original triplet. At the same time the high fieId triplet due to the a-protons is also reduced in intensity. This triplet also becomes a little more complex due to a deuterium isotope effect on the chemical shift and to spin-spin coupling between protoni and the deuterium hound to the same carbon atom. As the exchange reaction continues, the amount of compound (11) decreases (Figs. 2D and 2E) while the single peak due to the deutero-compound (111) increases in intensity, until after 25 minutes (111) is the major speciespresent (Fig. 2F). By this time the triplet due to the a-protons has almost entirely disappeared. During the course of the same experiment, by offsetting the field a t intervals, the integrated intensities of both a- and &protons can be recorded on the same chart (see Fig. 2) together with the NMR spectra. It is evident that as the reaction proceeds, the integrated intensity due to the 8-protons (left side) remains constant, whereas that due to the exchanging a-protons (right side) decreases progressively with time. The half life of the exchange reaction can he r d i l y
1 min
1 mi"
2 mi"
4 min
4 min
5 min
5 min
7 8 9 13
6 min
11
2 min
7 min
min mi" min min
I 3 min
min
LEI I 5 mi"
1
I
.
,
,
,
l
l
l
l
1
l l l l l l l , , I , , I , I , , I , l , , . . I
. , . .
I
,
.
.
.
I
. . ,
0 40 1.0 0 PPM ( I Figure 2. Deuterium exchange of hydrocrylonitrile with D 2 0 in the presence of sodium hydroxide. lentitier of @-proton triplets. Right, Integrated ietensities of a-proton hiplets.
determined from a semi-log plot of the percentage of remaining protons (obtained directly from integrated intensity curves) versus time. The value obtained from the rnn shown here, was tl/, = 7 min. It should be noted that spectrum 2C was taken shortly before the calculated half life. The first-order rate constant for this exchange reaction was found to be k, = 1.62 X 10-8 sec-' at the sample temperature (about 36°C).
Left: Successive spectra.
Center: l n k g r o k d in-
The spectra presented here were obtained on a Varian A60 NMR spectrometer using tetramethyl-silane as an external reference. A solution (0.5 ml) of hydracrylonitrile (Eastman Kodak) (0.32 M) in DzO (99.7%) was used to which sufficientNaOH (0.3 rnl of a 3.85 N solution in D20) was added to make the solution 1.44 N in base. These conditions can be varied in order to increase or decrease the rate of exchange.
Division of Chemical Education: Election and Nominations Result of election: W. E. Momll, Chairman-elect, 1965. In 1965, the Division of Chemicd Education will eleot three officers-Chairman-elect, Secretary, and Member-at-largeto serve in 1966. Candidates for these officesare proposed by a nominating committee (Bylaw 111, section 4d) or may he nominated from the floor at the spring business meeting of the Division. Members of the Division who wish to suggest names for the consideration of the nominating committee should send them to the chairman by January 1, 1965. The current members of the committee are L. Carroll Kine. chairman. Northwestern Univemitv: Leallvn B. Clam. Brown University; and John F. ~ z x t e r~nivemity , of Florida.
-.
Volume 41, Number 10, Odober 1964
/
571
