3
8
BORNEMANN, SCHEIDT, AND SANDER
(
5
m/z = 43), C4H9+ (m/z = 57), and C4H10+ (m/z =
high concentrations of olefins with ethene as the major
product. Reduced FVP temperature results in a signif-
icant decrease of all bands assigned to the olefins, and
formaldehyde and NO2 are the main products at tem-
8) were observed as major ions, while signals above
m/z = 60 were not detectable. Pyrolysis temperatures
�
above 150 C result in a significant change of the
�
obtained mass spectra, indicating the thermal deco-
peratures below 500 C.
mposition of 2-EHN. The signal at m/z =57 strongly
+
decreases, whereas signals at m/z = 28 (C2H4 ),
+
+
+
CONCLUSION
m/z = 29 (C2H5 , CHO ), m/z = 42 (C3H6 ), m/z =
+
+
4
6 (NO2 ), m/z = 56 (C4H8 ), and particularly
+
+
2-EHN is an important diesel fuel additive that signif-
icantly increases the cetane number. Our kinetic mea-
surements clearly show that it only very slowly de-
composes under normal motor operation conditions
and thus the loss before reaching the ignition system
is negligible. Moreover, under the high-pressure condi-
tions inside the injection system the decomposition rate
is further reduced. The primary products of the ther-
mal decay is the (2-EHO) radical, which further breaks
down to formaldehyde, olefins, and smaller alkyl radi-
cals. This decomposition mode is in line with the frag-
m/z = 30 (CH2O , NO ) as major ions are increas-
ing in intensity (Fig. 2). Raising the temperature above
�
2
00 C does not lead to further significant changes in
the mass spectra.
The initial step of the fragmentation of 2-EHN (both
thermally and electron impact induced) is the loss of
NO2 (m/z = 46) to give the 2-ethylhexyloxy radical (2-
EHO, m/z = 129; Scheme 1). This radical is rather un-
stable and not directly observed. Subsequent loss of
formaldehyde (m/z = 30) produces the 3-heptyl radical
(
3-HEPT, m/z = 99) [2]. The latter radical rapidly de-
�
mentation path of alkoxy radicals RCH2O , which pro-
duce alkyl radicals R and formaldehyde in high yield
composes to give 1-butene (m/z = 56) and the 1-propyl
radical (m/z = 43).
�
[6]. The activation barrier for this process was deter-
The primarily formed 2-EHO radical can also re-
act by breaking the C2 C3 bond under formation of
a butoxy diradical (m/z = 72) and the 1-butyl radi-
cal (m/z = 57). The diradical decomposes immediately
into formaldehyde (m/z = 30) and propene (m/z = 42),
whereas the 1-butyl radical forms ethene (m/z = 28)
and the ethyl radical (m/z = 29). This decomposition
scheme rationalizes all of the major fragments formed
mined to about 10 kcal/mol (depending on R) [2,6] and
is thus much lower than that of the rate-determining
step (cleavage of the NO bond) of the decomposition
of 2-EHN.
We thank Prof. Dr. F. G. Kl a¨ rner for determining the de-
cay kinetics of 2-EHN under high pressure conditions.
This work was financially supported by ARALAG, the
Fonds der Chemischer Industrie, and Deutsche Fonschangs-
Semeinschaft.
�
at FVP temperatures higher than 150 C.
FVP of 2-EHN with subsequent trapping of the
products in argon at 10 K allowed to obtain IR spectra
of the thermolysis products. The FVP was performed at
�
temperatures between 250 and 625 C. At all tempera-
tures the decomposition of 2-EHN was quantitative and
the strong and characteristic vibrations of the alkyl ni-
trate moiety were absent in the IR spectra (Fig. 3). The
nonradical products observed by mass spectroscopy
BIBLIOGRAPHY
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. Clothier, P. Q. E.; Aguda, B. D.; Moise, A.; Pritchard,
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2
3
. Pritchard, H. O. Combust Flame 1989, 75, 415–416.
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were also identified in the matrix by comparison with
literature data (Table II). In addition, NO, two of its
van der Waals dimers, and traces of methane were ob-
served. Species like CO, CO2, H2O, and SiO are not
attributed to direct FVP products of 2-EHN, but rather
to impurities in the sample or in the pyrolysis tube.
The pyrolysis products were stable towards UV irra-
diation, while annealing at 35 K resulted in significant
changes in the spectra. Most of the changes are due to
aggregation of the small and mobile molecules like NO
and NO2.
9
3, 1583–1621.
4. Fraser, R. T. M.; Paul, N. C. J Chem Soc B 1968, 659–
663.
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The pyrolysis of 2-EHN produced always the same
set of products, irrespective of the pyrolysis tempera-
ture, however, the product ratio was strongly dependent
2
10. Louis, R. V. S.; Crawford, B. J Chem Phys 1965, 42,
857–864.
�
on the temperature. At 625 C the IR spectrum shows