Textbook Errors, 90
Richard F. Stockel American Cyanamid Company Bound Brook, N e w Jersey 08805
Dicyandiamide (~~anoguanidine)
T h e first proposed structure for dicyandiamide was made by Bamberger ( 1 ) in 1883. He assigned the erroneous structure I, which is still used today by virtually all authors of current popular textbooks. Fifteen years later the correct structure I1 was suggested by Pohl (Z),although it was based on questionable chemical evidence (3).
Table 1 .
Comparison of Bond Lengths in Dicyondiomide and in Model Compounds
Dicyandiamide CPN6
The first positive contribution to the long standing controversy was a complete X-ray examination of crystalline dicyandiamide (4). This study eliminated the ring structure I11 and IV and showed that the heavy atom skeletou was planar. Furthermore, the experimental bond lengths gave proof that considerable delocalization of ?r electrons extended over the whole six-atom system as illustrated in Table 1 (5). The posit,ions of the atoms indicate that the molecule is a resonance hybrid as illustrated below. N=C-N
-N=C=N
\\
C-NHI
/
o
\
+
C=NI12
/
-
Although the model compounds are in closc agreement with carbon-nitrogen bonds found in dicyandiamide, an exact correlation cannot be anticipated siuce bond leugths are dependent on resonance or hybridization factors (6, 7). Subsequent work on the dipole moment of dicyandiSuggestions of material suitable for this column and guest columns suitable for ~uhlieationdirectly should be sent with as many details ss po&ible, and partic"larly with reference to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia.Imtitute of Technology, Atlanta, Georgia 30332. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the sources of errors discussed will not he cited. I n order to he presented, an error must occur in a t least two independent recent standard books.
1.3.5 A
Model Compound C-N
1.47 A ICHaNH21
amidc (8) (S.16D) also concluded that the molecule is planar, because of t,he bigh value obtained, and can best be explained by assuming resonance as depicted above. I t was noted that the same types of resonance account for the large electric moment. found for urea. Dicyandiamide is an amphotcric compound, its acidic and basic dissociation constants are 6 X 10-l5 and 3 X respectively (9). From cousidcration of these dissociation constants (10) structure I1 was believed to be the best representation of dicyandiamide. If it had stmct,ure I, it would be a relatively strong acid similar t,o dicynnoguanidine, so its neutrality is unexplainable on the basis of this structure. On the other hand, the neut,ralit,y of structure I1 is undcrstaodable and an analonv -" can be drawn with urea where H2N\ C= is rendered neutral by attachment of an the H~N/ oxygen. With dicyandiamide t,he same thing happens by the attachment of a =N-CN group. Both of these functionalities should be equally effectivc in reducing the electron density of the amino groups. Table 2. Comparison of Vibrational Frequencies in Dicvondiomide and in Model Comoounds
va (NCX) va (NCN) vs (CXNZ) w (CXNd d (NHd r (NHd w (NH1) va, vs = asymmetric and symmetric stretching vibrations. w = in-phase wagging mode. d = out-of-phaie bending mode. r = in-phase rocking made. Volume 46, Number 6, June 1969
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391
Additional evidence in favor of structure I1 was provided by a thorough infrared study (11). The vibrations associated with the diamino-methylene grouping (Table 2) of dicyandiamide were compared with related molecules and found to be in excellent agreement. If dicyandiamide had structure I the grouping
would be isoelectronic with an
amide grouping
The interpretation of --
the spectrum was incongruous using this grouping and the possibility of anything other than a small quantity of the tautomer I is eliminated. These authors also augmented their evidence with an approximate molecular orbital treatment. The ultraviolet spectrum of dicyandiamide gives rise to a band at 2150 il (13, IS). This supports the conjugated structure 11, since structure I would not be expected to give rise to any absorption in this region. I n addition, no change of the spectrum was observed by varying pH indicating that dicyandiamide is an extremely weak acid or extremely weak base, or both. The application of nrnr spectroscopy to help resolve this problem was particularly appealing. Since there does not appear to be any report on dicyandiamide in the literature utilizing this technique, it was decided to investigate this approach.' By the judicious choice of an aprotic dipolar solvent like dimethylsulfoxide or dimethylformamide rapid exchange of the N-H's can 1 Varian A-60 operating st 60 M c was used with tetramethylsilane as an internal standard: 6 = (us - ums)/60.
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Journal of Chemical Education
be reduced to give a single sharp resonance peak a t 6.078,' which is virtually independent of concentration :*lid ttmp~~rtlfun~. 'I'l~isd c r n ( ~ ~ ~ i thc t r ~ equivalency te~ oi all the hydrogen in ~olurion1i11tl (WI hwf I,? expI:~intd by the conjugated structure 11. Thus, although the incorrect, nonconjugated stmcture I for dicyandiamide persists in many tests, a wide variety of data from different types of experiments indicate that the best representation is 11. Acknowledgment
The author wishes to express his indebtedness to Dr. J. E. Lancaster and Mrs. M. Neglia of the Central Research Laboratories of t,he American Cyanarnid Company for running the H 1 nmr. Literature Cited (1) BAMBERGER, E., Ber. 16, 1074 (1883). (2) POHL,F., J . Prakt. Chem. 121 77, 533 (1008). (3) HALE,W. J., AND VlBRANS, F. C., J . Am. Chen~.Soe., 4 0 , 1046 (1918). (4) HUGHES, E. W., J . Am. Chem. Soc., 62, 12.58 (1940). 15) , , "Tables of Interatomic Distances and Confinuration in Molecules and Ions," Special Publication 14, The Chemical Society, Burlington House, London, 1938. (6) DEWAR,11. J. S., AND SCHMEISING, A. N., Tetrahedron, 5,166 (1959); Tetrahedron 11,96 (1960). (7) MULLIKEN, R. S., Tetrahedron, 6 , 6 8 (19.59). (8) SCHNEIDER, W. C., J . Am. Chem. Soe., 7 2 , 761 (1950). (9) KIRK,R . E., AND OTHMER, D. F.,"Encyclopedis of Chemical Technology," John Wiley & Sons, Inc., New York, Vol. 6, 1965. (10) KUMLER, W. D., J . O P ~hem., . 20,700 (1955). (11) JONES,W. J., AND OIIVILLZ-THOM.~S, W. J., Trans. Faraday Soe., 55, 193 (1959). (12) SUKHORUKOV, B. I.,'ANDFINKISL'SHTKIN, A. I., Oplics and Speelroseopy, 9, 2.5 (1960). (13) TAKIMOTO, M., Nippon Kagaku Zasshi, 85, 159 (1964).
Nh.
