Photooxidation of n-hexanal in air

Article abstract of DOI:10.3390/60400287

Dilute mixtures of hexanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) in air at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were butene, CO, vinylalcoho

Full text of DOI:10.3390/60400287

Molecules 2001, 6, 287–299
molecules
ISSN 1420-3049
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Photooxidation of n-Hexanal in Air
1
2
3
Jovan Tadić , Ivan Juranić and Geert K. Moortgat
1
Center for Chemistry IChTM, POB 815, Studentski trg 12-16, 11001 Belgrade, Yugoslavia, Tel.
+381-11-3236-603, Fax. ++381-11-639-827
+
2
3
Faculty of Chemistry, University of Belgrade, POB 158, 11001 Belgrade, Yugoslavia
Max-Planck-Institut für Chemie, Atmospheric Chemistry Department, Postfach 3060, 55020 Mainz,
Germany
*
Author to whom correspondence should be addressed; e-mail jtadic@Eunet.yu
Received: 21 November 2000; in revised form 11 December 2000 / Accepted: 12 December 2000 /
Published: 31 March 2001
Abstract: Dilute mixtures of hexanal in synthetic air (up to 100 ppm) were photolyzed with
fluorescent UV lamps (275-380 nm) in air at 298 K. The main photooxidation products,
identified and quantitatively analyzed by FTIR spectroscopy, were butene, CO, vinylalcohol
and ethanal. The photolysis rates and the absolute quantum yield Φ were found to be slightly
dependent on the total pressure. At 100 Torr, Φ100 = 0.43 ± 0.02, whereas at 700 Torr the
total quantum yield was Φ700 = 0.38 ± 0.02. These results may be explained by the
collisional deactivations of photoexcited molecules. Two decomposition channels were
identified: the radical channel C H CHO → C H + HCO, and the molecular channel
5
11
5
11
C H CHO → C H + CH =CHOH, having the relative yields of 27 and 73 % at 700 Torr.
5
11
4
8
2
The product CH =CHOH tautomerizes to ethanal.
2
Keywords: Photolysis, photooxidation, aldehydes.
Introduction
Aldehydes play a significant role in a variety of atmospheric reactions, such as formation of
photochemical smog, peroxyacetyl nitrate (PAN), and regional (tropospheric) ozone. Photodissociation
of aldehydes represents an important source of free radicals in the lower atmosphere, and thus may
significantly influence the atmospheric oxidation capacity [1-3]. Aldehydes are widely used in industry

Molecules 2001, 6
288
and are the products of an incomplete combustion of petroleum fuels. Smaller alkyl aldehydes are also
products of the atmospheric photooxidation of hydrocarbons, ethers, alcohols, and other organic
compounds. Vegetation, biomass and other living organisms emit many of these compounds: n-hexanal
and higher aldehydes have been observed in ambient air and in emissions of various plants, especially
grasses, with comparable emissions rates as the monoterpenes [4,5].
Aldehydes absorb in the near UV range, and dissociate upon absorption of light. Aliphatic
aldehydes exhibit a weak absorption band in the wavelength range 240-360 nm as a result of a
symmetry forbidden n - π* transition [6,7]. Photodissociation of aldehydes produces free radicals,
which are subsequently involved in other atmospheric reactions and reaction cycles. There have been a
number of studies devoted to the photodissociation of the simplest alkyl aldehydes, such as HCHO,
CH CHO, C H CHO [8-16], but a relatively small number devoted to longer chain aldehydes, such as
3
2
5
C H CHO, C H CHO [17,18] and iso-pentanal and tert-pentanal [32].
3
7
4
9
Photolysis of n-hexanal could theoretically occur according to the several suggested reaction
schemes for n-pentanal [18], where processes (1) and (3) are thought to be the most important:
CH CH CH CH CH CHO + hv
→
→
→
→
→
→
n-C H + HCO
(Norrish Type I)
(Norrish Type II)
(1)
(2)
(3)
(4)
(5)
(6)
3
2
2
2
2
5
11
n-C H + CO
5
12
C H + [CH =CHOH]
4
8
2
C H + CH CH CHO
3
7
2
2
C H + HCHO
5
10
CH CH CH CH CH CO + H
3
2
2
2
2
Process (1) represents the fragmentation into free radicals, with an enthalpy of around 350 kJ/mol
corresponding to a photochemical threshold of around 340 nm [18]. Process (3), which is common to
molecules with a γ-hydrogen atom, is an intramolecular rearrangement with enthalpy around 80 kJ/mol
(λ ≤ 1454 nm) [18]. The enthalpy for process (3) was calculated assuming that the keto-form of
acetaldehyde is formed in the primary step. As a matter of fact, this assumption is not correct, and,
therefore, enthalpy for these reactions should be changed for the difference between the heats of
formation of enol and keto forms of acetaldehyde (the pK value for keto-enol equilibrium is ~7 and the
corresponding ∆G value is ~18 kJ/mol) [33].
There is also a theoretical possibility for the n-hexanal molecule to undergo photocyclisation,
forming cis/trans isomers of 2-ethyl-cyclobutanol (Scheme 1). These compounds were not detected in
our experiments.
H
OH
.
O
hv
.
O
OH
.
.
both cis and
trans isomers
Scheme 1.

Molecules 2001, 6
289
In this paper, the results obtained from the photolysis of small quantities of n-hexanal in air, using
wide band emission lamps, are reported. We have investigated products and absolute quantum yields in
the pressure range 100 to 700 Torr.
The photooxidation experiments were carried out in a long-path quartz cell with detection of
precursors and products by FTIR spectroscopy. After identification and quantification of the products,
a mechanistic description of the photooxidation/photolysis was deduced. From the measured decay rate
of the starting material, and from the knowledge of the absorption spectrum, overall quantum yields for
the photolysis were calculated for various pressures.
Results and Discussion
Photolysis of n-Hexanal
Figure 1 shows the FTIR spectra of a mixture of 100 ppm n-hexanal in synthetic air, before and
after the photolysis, and the differential spectrum. Major products were found to be CO (ν
= 2037-
max
−1
−1
−1
2
235 cm ), 1-butene (911.5 cm ), ethenol (vinyl alcohol) being the enol form of ethanal (947.6 cm ,
−1
−1
−1
−1
−1
1
078 cm , 1118.3 cm , 1259.8 cm ), ethanal (1348.5-1355.5 cm ) and CO (2290-2384 cm ). The
2
latter compound is considered as an artifact, and is unavoidably produced from desorption from the
wall.
Figure 1. IR spectrum of 100 mTorr n-hexanal, before and after photolysis (6 TL/12
lamps, 100 Torr synthetic air). The differential spectrum is also shown below. Major
products CO, 1-butene, and ethanol, and CO as a by-product, are marked (exact positions
2
of the peaks are given in the main text).

Molecules 2001, 6
290
Figure 2 shows the concentration-time profiles for the various products.
Figure 2. Photolysis of n-hexanal: time profile - variation of the concentrations of
products. The CO curve shows that it is not only the primary product, but also a product
of the reactions following primary step. Ethenol partial pressure reaches maximum and
then starts to decrease by conversion to ethanal, which is in good agreement with
simulation of the process.

Molecules 2001, 6
291
From the observed products, it seems evident that only Norrish Type I and II processes occur, and
the proposed mechanism would be:
CH CH CH CH CH CHO → CH CH CH CH CH + HCO
(1)
(3)
3
2
2
2
2
3
2
2
2
2
→
CH =CHCH CH + CH =CHOH
2 2 3 2
1-Butene is purely primary product that can be expected in this reaction system. Ethanal is secondary
product arising from ethenol conversion, and undergoes further photolysis.
Reaction (2), also producing CO, does not take place, since a co-product of the reaction, n-C H
5
12,
was not detected during the photolysis. Reaction (1) gives two radicals, which immediately react with
oxygen. The formyl radical HCO is quantitatively converted to CO and HO [26-28], according to
2
HCO + O → HO + CO
2
2
allowing the usage of the primary produced CO as the indicator for process (1). n-Pentyl radicals also
subsequently react with oxygen, forming n-pentylperoxy radicals,
n-C H + O → n-C H O
2
5
11
2
5
11
The fate of the n-C H O radicals is to react with HO , or to disproportionate, to produce a variety of
5
11
2
2
products [29,30]
2
n-C H O
→ 2 n-C H O + O
2
5
11
2
5
11
→
n-C H OH + n-C H CHO + O
5 11 4 9 2
n-C H O + O
→ n-C H CHO + HO
4 9 2
5
11
2
n-C H O + HO → n-C H OOH + O
2
5
11
2
2
5
11
However due to the estimated low reaction rate constant for the disproportionation [30], the major
expected product would be n-pentyl-hydroperoxide (n-C H OOH). The latter compound (also n-
5
11
pentanal, and n-pentanol) was below the detection limit of the applied analytical method (less than 1
mTorr). Also no evidence was found for the presence of products of other possible channels.

Molecules 2001, 6
292
Figure 3. Photolysis of n-hexanal - products formation versus loss in n-hexanal. See
comments on Figure 2.
1-Butene is a stable molecular species and was used as the indicator for the process (3). The co-
product of reaction (3) is the formation of the ethenol CH =CHOH, which tautomerizes slowly to
2
ethanal. This is clearly seen in Figure 2, where the concentration-time profile of the enol product
displays the behavior of a primary product. Ethenol concentration reaches a maximum after 10 minutes
photolysis, and its subsequent decay is connected with its conversion to ethanal. Figure 3 shows
profiles of products concentrations (partial pressures) of CO, 1-butene and the sum of ethenol and
ethanal versus loss of n-hexanal. According to the suggested mechanism, the yield of 1-butene is
(within experimental error) nearly identical with the sum of yields of ethenol and ethanal, since both
compounds are products of the same decomposition channel. CO is mainly a primary product, but
small increases of the yield at long conversion times is observed, which can be attributed to the
photolysis of secondary products such as ethanal.
The relative probability for n-hexanal molecule to undergo two detected decomposition channels is
shown in the Table 1. The probability is deduced from the ratio of primary formed CO (Norrish Type
I), and 1-butene (Norrish Type II). This method is chosen to overcome the inability of measuring
absolute initial concentrations of the sticky substance, because of the loss of the substance on the walls
of gas-handling system before the substance is introduced to the reactor. This means that uncertainty is

Molecules 2001, 6
293
presented in determining initiall concentration of the n-hexanal after it is introduced in the main cell
(the rate is still clear, because there is no wall loss after introduction).
Table 1. Probabilities for n-hexanal molecule to undergo different decomposition
channels, TL12 lamps (errors are represented by experimental scatter).
Total pressure (Torr)
Norrish type I
0.35
Norrish type II
0.66
1
1
1
00.00
00.00
00.00
0.38
0.62
0.35
0.65
0
0
0
0
.36±0.02
0.28
0.64±0.02
0.72
3
3
3
00.00
00.00
00.00
0.26
0.74
0.28
0.72
.27±0.01
0.29
0.73±0.01
0.71
5
5
5
00.00
00.00
00.00
0.29
0.71
0.30
0.70
.29±0.01
0.27
0.71±0.01
0.73
7
7
7
00.00
00.00
00.00
0.28
0.72
0.27
0.73
.27±0.01
0.73±0.01
The relative yields for the different products observed are summarized in Table 2. Assuming that the
formation of CO is an artifact, the sum of both processes Norrish Type I and II is approximately 0.82
2
±
0.05, which would indicate that additional products might be formed by other processes.
Table 2. Relative yields of different products in typical n-hexanal photolysis
experiment, TL/12 lamps (done by quantifying n-hexanal using UV measurement of it)
∆
CO/ -∆ hexanal
.23
∆ 1-butene / -∆hexanal
∆ (ethanal+ethenol )/ -∆hexanal
0
0.61
0.57
Speculation about the formation of 2-ethyl-cyclobutanol is based on the mechanism of the formation
of an analogous product, 2-methyl-cyclobutanol, formed in the photooxidation of n-pentanal [25],
although we have found no evidence for it.

Molecules 2001, 6
294
Absolute Quantum Yields
One of the objectives of the study was to determine the absolute quantum yield dependency on the
total pressure. Chlorine was used as the actinometer. From the decay of n-hexanal concentration, the
photolytic rate constants were deduced for the different pressures by plotting the natural logarithm of
concentration versus time (first order decay), and performing a least-square fit. From these results,
overall quantum yields were calculated according to the Equation (1) using the overlap integrals of n-
hexanal and chlorine spectra within the range of the TL/12 emission (280-360 nm, see Fig. 5). In all
cases the absolute quantum yield dependency on the total pressure was observed. The data are
summarized in Table 3 for experiments performed at a total pressure of 100, 300, 500 and 700 Torr
and also shown in form of a Stern-Volmer presentation in Figure 4. Since the intercept at zero pressure
is not equal to 1 (the intercept is 2.26), as should be expected if collisional deactivation is the only
relaxation process beside the photodecomposition, it seems very probably that there are other energy-
dissipating processes (the triplet state of n-hexanal could deactivate by phosphorescence etc.). The
interaction with the walls moving to the ground state is of minor importance becase of huge volume to
-4
surface ratio. The slope of the fit, described by 4.758x10 x P corresponds to the sensitivity of absolute
quantum yield on the total pressure P.
Figure 4. The pressure dependency of 1/(absolute quantum yields) in n-hexanal photolysis,
at different total pressures of synthetic air (Stern-Volmers plot). If collisional deactivation
is the only relaxation process, one should expect that the interception on the ordinate
should be one. This is not the case, indicating that one or more other energy dissipating
processes from photoexcited molecules are taking place. Possible explanations are given
in the main text.

Molecules 2001, 6
295
-
The total quantum yield Φ can be calculated from the following equation 1/Φ = 2.26 + (4.758 x10
P. Using the estimated relative quantum yield from Tables 1 and 2, it is possible to estimate the
4
)
absolute contribution at atmospheric conditions (700 Torr): for Norrish type I (radical) process, ϕ(I) =
.08 and Norrish Type II (molecular) ϕ(I) = 0.23 (total quantum yield being 0.38, and a contribution of
0
both decomposition channels being 82 %).
Table 3. Absolute quantum yield values n-hexanal photolysis at different pressures
of synthetic air (errors are represented by experimental scatter).
Pressure (Torr)
Abs. Quantum yield
1
1
1
00.00
00.00
00.00
0.43
0.41
0.44
0
0
0
0
.43 ± 0.02
0.41
3
3
3
00.00
00.00
00.00
0.42
0.43
.42 ± 0.01
0.43
5
5
5
00.00
00.00
00.00
0.38
0.41
.40 ± 0.03
0.40
700.00
700.00
700.00
0.37
0.37
.38 ± 0.02
Conclusions
In this work we achieved several goals:
·
·
Identification of the products of hexanal photolysis, detectable in IR region under applied conditions.
Quantification of the products, and deduction of the photodecomposition pattern, particularly of
primary step, exploiting the benefit of applied experimental system
·
Determination of the absolute quantum yields at different pressures, providing values necessary for
atmospheric modeling, at the same time examining the influence of the total pressure on the absolute
quantum yield, evaluating importance of collisional deactivations, and indicating other relaxation
channels

Molecules 2001, 6
296
There is evidence that up to C4 aldehydes the decomposition upon absorption of light mainly
follows the free radical channel (Norrish type I), forming formyl radical and alkyl radical [25,31]. The
higher than C4 aldehydes, starting with pentanal, mainly decompose by internal rearrangement of the
molecule (Norrish type II), forming vinyl alcohol, and the corresponding 1-alkene. Our results strongly
support this pattern. There is no conclusive evidence for the formation of cyclobutanol derivatives, and
further examinations on the subject should be done.
Experimental
The apparatus employed in this work has been described elsewhere [19-20] and will be briefly
discussed here. The central part of the apparatus is a 44.2 liter (1.40 m length and 20 cm diameter)
quartz cell equipped with two independent sets of White-optic mirror arrangements. Sapphire-coated
aluminum mirrors were used in the infrared region (l = 33.6 m) for the measurements of the educts and
products. A set of MgF -coated mirrors was used in the UV/visible region (l = 9.82 m), for measuring
2
−1
.−1
the photolysis rates of the actinometer, Cl . Infrared spectra at 0.5 cm resolution (450-4000 cm )
2
were measured with a Bomem DA8-FTIR spectrometer. For the UV measurements the same diode
array detector as previously described [20-22] was used. This method provides the possibility of
simultaneous detection and monitoring of all the IR-active products and the starting material.
Photolysis was achieved with six radially mounted lamps, TL/12-sunlamps (Philips 40W TL/12 lamps
(275-380 nm).
Spectra were taken every 5 -10 min. with total irradiation time of 30-50 min. Extent of the
conversion of initial compound was approximately 30 %.
The use of a continuous broad band light source allows only the determination of an integral,
int
effective quantum yield Φ for the photoactive spectral region. Quantum yields were calculated
2
0
according to the following equation (for carbonyl compound, C, and actinometer, Act):
Equation 1.
For all TL/12-experiments, chlorine was used as actinometer (with φ = 1), using ethane as Cl-atom
scavenger. The quantum yield is the single unknown parameter in the equation. The photolysis rates
for both compound Kphot(C) and actinometer K t(Act) could be directly measured, and the terms
pho
ΣOV(C) and ΣOV(Act) represent the calculated overlap of lamp emission and absorption spectrum of
substrate. Figure 5 displays the emission spectra of the lamps and the spectrum of Cl2.

Molecules 2001, 6
297
The knowledge of the UV absorption spectra of hexanal was a basic prerequisite for these experiments.
The absorption spectrum has been recently measured [23], and is very similar to the homologous
aldehydes, butyraldehyde and pentanal [24,25]. The spectrum displays a broad absorption band
-20
2
-1
between 250 and 350 nm, with a maximum at 295 nm with cross section of 6.0 x 10 cm molecule .
For comparison the spectra butanal and pentanal are also given in Figure 5.
Figure 5. Emission spectrum of photolysis lamps (TL12/Philips), absorption spectrum
of Cl and cross section of n-hexanal [23]. Cross sections of butanal and pentanal are
2
also given for comparison. Figure explains why chlorine was used as actinometer, and
why TL12 lamps were chosen for photolysis, since there is a good overlap of all three
regions.
Experiments were carried out at room temperature (298 K), at pressures between 100 to 700 Torr (1
Torr = {101325/760}Pa), with an initial concentrations of approx. 100 ppm. Qualitative and
quantitative data evaluation was carried out by comparing the products spectra with reference spectra
obtained in the same cell and using calibration curves at corresponding pressures and resolution.
Carbonyl compounds were obtained from Sigma-Aldrich Company with purity higher than 98%.
Before use, all samples were degassed by several freeze-pump-thaw cycles. Purity of the compounds
was checked by FTIR spectral measurements and no impurities were found. The calibration of vinyl

Molecules 2001, 6
298
alcohol was done indirectly, from the amount of formed ethanal and the converted vinyl alcohol (peak
decrease), since the conversion of these compounds is 1:1 (keto-enol tautomerism) according to:
CH =CH-OH = CH -CH=O
2
3
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