Communication
doi.org/10.1002/ejoc.202000181
EurJOC
European Journal of Organic Chemistry
Mineral acids with sodium nitrite give a similar product, al- well-formed peak at a detection wavelength of 260 nm. The
though at lower yield because the unbuffered solution is more DAD spectra are identical to UV data mentioned above, includ-
prone to decomposition. The same molecules are formed by ing the weak absorbance at 370 nm. The chromatographic peak
passing an equimolar mixture of NO and NO gas into an aque- consists of two overlapping signals with identical UV spectra,
2
ous cyanamide solution. Non-aqueous nitrosation, e.g. in di- always in a peak ratio of about 9:1. We interpret this as a partial
chloromethane or diethyl ether is also possible, using sodium separation of the interconverting E/Z-isomers 4a/b.
nitrite and acid or alkyl nitrites (see supplement S1.5 to S1.6).
The product 2a/b is less stable in organic solvents and can be
easily extracted into water.
Aqueous solutions of 2a/b are slightly yellowish to greenish
and show first gas bubbles through thermal decomposition
after about 15 minutes at 20 °C.
When heated to above 50 °C, a continuous and controllable
gas evolution occurs. The total gas volume is about the 30-fold
volume compared to that of the solution. We therefore alert all
readers, not to store a solution of 2a/b in a closed bottle, as it
might explode. During gas evolution, the pH raises from 3 to
about 6. Preliminary gas analysis gave a varying composition,
beginning with mainly nitrogen (N ) and ending up with mainly
2
nitrous oxide (N O). The remaining solution contains large
2
amounts of hydrogen cyanide (HCN, caution!). Ion chromatog-
raphy detected significant amounts of dicyanamide in solution.
GC/MS found small amounts of cyanogen (CN) in the evolved
2
gas mixture. We suppose, these products form by electrophilic
cyanation of cyanamide and HCN respectively.
Figure 1. Left: HPLC of 2a/b at pH 4 on Hydrosphere C18 with UV detection
at 260 nm. Right: LC/MS SIM traces in positive/negative mode with aqueous
formic acid as eluent on Hydrosphere C18 compared to a parallel UV trace
at 260 nm.
We determined the pH dependent UV spectra of a freshly
prepared solution of 2a/b (see supplement S2). An intense UV
2
–1
absorption at 260 nm (ε260 = 726 m mol at pH 4.0 or higher)
shows, that the cyanodiazotate anions 4a/b are present. Lower-
ing the pH, ε260 diminishes, indicating protonation to the neu-
HPLC was used to compare the assay of solutions of 2a/b
prepared by different methods (acids, solvents) and to monitor
decomposition. An absolute assay however was not possible,
because no stable reference standard could be defined. We
therefore took a freshly prepared acetate solution as the 100 %
reference and calculated concentrations of 2a/b based on that
reference. The double peak 2a/b was always integrated as a
whole.
LC/MS experiments were successful on Hydrosphere C18 us-
ing 50 mM aqueous formic acid as eluent, as ammonium acetate
at pH 4 did not give a good ionization. In mass spectra of posi-
tive mode, m/Z + 1 = 72 was found. In the negative mode, m/
Z – 1 = 70 could be detected as well. SIM mode traces in posi-
tive and negative mode correlated exactly to the UV trace at
tral species 2a/b with a pK of 1.8. A second, much weaker UV
a
2
–1
absorption band at 370 nm (ε370 = 2.96 m mol at pH 4.0),
responsible for the visual color, is almost independent of the
pH.
Titrating solutions of 2a/b with NaOH gives the expected
assay. Solutions prepared from sodium nitrite and mineral acids
+
–
are partly dissociated to the ion pair H O O–N=N–CN, even
3
in presence of excess mineral acid. Acetate-based solutions con-
sist of the diazotate anions 4a/b and equimolar acetic acid, thus
titration just measures the acetate content. Decomposition of
2
a/b, forming much less acidic products, can be monitored by
titration (see supplement S3).
2
2
60 nm. We therefore are confident, that the neutral molecules
a/b have a molecular mass of 71 and are interconverting iso-
Upon addition of silver nitrate a light yellow silver salt is
formed between pH 1 and pH 5. It decomposes slowly even
at 0 °C and is very impact sensitive, but not highly energetic.
Obviously, this silver salt was prepared and described years
mers, both absorbing at λmax = 260 nm.
For NMR experiments, we reacted equimolar amounts of so-
dium nitrite, cyanamide and acetic acid, all dissolved in D O at
2
[
4a,4b]
ago.
The FTIR spectrum of the silver salt shows a strong
13
0
°C. The sample solution was measured at 25 °C. C NMR
C≡ N band at 2177, a N=N band at 1441 and a N–O band at
spectra gave – besides the acetate signals – only two sharp
peaks at 122.0 and 114.4 ppm in an intensity ratio of about 2:1,
–
1
1
168 cm (see supplement S4.1).
FTIR measurement of the aqueous solution did not show the being distinctly different from traces of remaining cyanamide
C≡ N absorption. Raman measurements in solution (see supple- signaling at 117.9 ppm. We interpret those as C≡ N Signals of
ment S4.3) showed a sharp C≡ N band at 2185, a N=N band at the stereoisomers 4a and 4b (see Figure 2).
–1
Because 15N enriched materials were not available, we tried
N NMR experiments, despite the naturally very broad signals.
1
364 and a N–O band at 1201 cm . Those bands disappeared
1
4
upon thermal decomposition.
To our astonishment, solutions of 2a/b can easily be chroma- As chemical shifts in ppm scale are independent of the nucleus
1
4
15
tographed on a Hydrosphere C18 column at 20 °C (see Fig- measured, N data are comparable to N reference data. Peaks
ure 1). Aqueous phosphate or acetate buffers of pH 4.0 gave a at –172 and –160 ppm in the ratio 2:1 are interpreted as cyano
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
2
© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim