Electron-induced hydration of an alkene: Alternative reaction pathways

Article abstract of DOI:10.1002/anie.201412147

Electron-induced reactions in condensed mixtures of ethylene and water lead to the synthesis of ethanol, as shown by post-irradiation thermal desorption spectrometry (TDS). Interestingly, this synthesis is not only induced by soft electron impact ionization similar to a previously observed electron-induced hydroamination but also, at low electron energy, by electron attachment to ethylene and a subsequent acid/base reaction with water.

Full text of DOI:10.1002/anie.201412147

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201412147
German Edition: DOI: 10.1002/ange.201412147
Electron-Induced Ethanol Synthesis
Electron-Induced Hydration of an Alkene: Alternative Reaction
Pathways**
Jonas Warneke, Ziyan Wang, Petra Swiderek, and Jan Hendrik Bredehçft*
Abstract: Electron-induced reactions in condensed mixtures of
ethylene and water lead to the synthesis of ethanol, as shown by
post-irradiation thermal desorption spectrometry (TDS). Inter-
estingly, this synthesis is not only induced by soft electron
impact ionization similar to a previously observed electron-
induced hydroamination but also, at low electron energy, by
electron attachment to ethylene and a subsequent acid/base
Scheme 1. Proposed reaction mechanism for the hydration of ethylene
induced by soft electron impact ionization. After ionization of water or
ethylene, the attractive interaction between the radical cation and the
electron-rich double bond or free electron pair drives the reaction.
reaction with water.
C
hemical transformations induced by electron irradiation
are mostly associated with destructive effects such as radia-
tion damage in biological systems (e.g. DNA)[1] or the
decomposition of volatile precursors in focused electron
beam induced processing (FEBIP).[2] Depending on the
electron energy (E0), bonds can be selectively cleaved.[3]
However, irradiation of condensed matter also leads to the
formation of new bonds.[4] As an example, NH3, RNH2, and
R2NH add to the double bond of alkenes following electron
irradiation of frozen layers of the reactants.[5,6] These hydro-
amination reactions are induced by using E0 values near the
ionization threshold of the reactants to achieve a soft, that is,
nondissociative, electron impact ionization. While the elec-
tron-rich double bond of the alkene and the lone electron pair
of the nitrogen atom in the neutral reactants repel each other
and prevent a reaction, ionization of one reactant turns this
barrier into an attractive force. This leads to bond formation
between the reactants. Neutralization of the resulting adduct
by a thermalized electron then yields the neutral product
which contains all the atoms of the reactants. Electron
irradiation thus induces chemical synthesis through an
atom-efficient reaction mechanism. We show here that this
principle is transferable to other reactants. In fact, post-
irradiation thermal desorption spectrometry (TDS) reveals
that ethanol is produced in condensed mixtures of ethylene
(C2H4) and H2O. However, we also present evidence that
ethanol is not only formed by ionization (oxidation) of C2H4
(Scheme 1) but also following electron attachment (reduc-
tion).
Details of the experimental procedures can be found in
Ref. [7] and in the Supporting Information. Figure 1 shows
TDS data for multilayer films formed by depositing equal
quantities of C2H4 and H2O vapor on a gold surface held at
38 K. Prior to exposure, desorption signals of C2H4 (m/z = 28)
at 80 K and H2O (m/z = 18) at 150 K are observed. After
electron exposure of 500 mCcmÀ2 at E0 = 15 eV, new desorp-
tion signals with an intensity ratio of 2:1 appear at 160 K in
[*] J. Warneke, Z. Wang, Prof. Dr. P. Swiderek, Dr. J. H. Bredehçft
Institut für Angewandte und Physikalische Chemie
Universität Bremen
Leobener Strasse NW2,d-28359 Bremen (Germany)
E-mail: jhbredehoeft@uni-bremen.de
Homepage: http://www.iapc.uni-bremen.de/swiderek/
Figure 1. Thermal desorption spectra obtained from 20 monolayer
(ML) films of a 1:1 mixture of C2H4 (m/z=28) and H2O (m/z=18)
before (0 mC) and after electron exposure of 500 mCcmÀ2 at E0 =15 eV.
The signals at m/z=31 and m/z=45 with an intensity ratio of 2:1
illustrate the formation of ethanol.
[**] This work was funded in part by the Deutsche Forschungsgemein-
schaft (DFG) under project number SW26/15-1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201412147.
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the TDS spectra at m/z = 31 and 45. This ratio is characteristic
of ethanol, as verified by leaking the pure compound into the
chamber. The desorption temperature agrees with that of
pure ethanol (see Figure SI1 in the Supporting Information).
Together, this supports that ethanol is in fact produced.
Low-energy electrons can thus drive the formation of
ethanol in mixed films of C2H4 and H2O at cryogenic
temperature. As for the electron-induced hydroamination,[5,6]
ionization of one of the reactants removes the activation
barrier for the reaction. The formation of ethanol thus
proceeds under conditions that are fundamentally different
from those of technical processes for the production of
ethanol by hydration of C2H4. These processes require high
temperatures, high partial pressures, and a catalyst.[8] This
suggests that the described approach to control adduct
formation may also allow for an atom-efficient synthesis of
other carbon structures containing heteroatoms.
5 eV.[13] In clusters, EA is observed at slightly different
energies, but again not below about 6 eV.[14] However,
nondissociative EA to C2H4 proceeds around 1.5 eV.[15]
Therefore, EA to C2H4 to yield a radical anion C2H4CÀ must
trigger the formation of ethanol at low E0 values. This
provides, at the same time, the first example of an electron-
induced synthesis controlled by a nondissociative EA process.
The range of E0 values observed for this process compares
well with the broadening of a similar EA process in
condensed N2,[16,17] thus supporting our conclusion.
Neither in films of pure C2H4, nor in mixtures of C2H4 and
NH3,[6] did we observe formation of any products at such low
E0 values. This observation points to a key role of H2O in the
reaction observed here, which can be explained by its acidic
protons. Radical anions are typically basic species.[18] The
transient radical anion C2H4CÀ formed by EA to C2H4 can
accept a proton donated by H2O to form an ethyl radical
(C2H5C). In contrast, in aprotic media the radical anion can
only relax through autodetachment of the captured electron,
which is a relaxation channel in all EA processes.[3] We
propose that this proton transfer triggers the complete
sequence of reactions shown in Scheme 2.
Support for the proposed reaction mechanism can be
derived from a thorough analysis of the side products. At
E0 values above the ionization threshold, electron irradiation
of condensed C2H4 produces a variety of hydrocarbons,
including ethane (C2H6), butane (C4H10), butene (C4H8),
butadiene (C4H6),[6] and acetylene (C2H2, see Figure SI2).
Among the C4 hydrocarbons, the C2H4 dimer butene is the
dominant product at E0 = 15 eV.[6] This, together with elec-
tron-stimulated desorption (see Figure SI2), contributes to
the loss of C2H4 during irradiation, as seen in Figure 1. In
contrast, ethane and butane are the predominant products in
the EA regime. Significant contributions of butene and
butadiene can be excluded based on the mass peaks seen by
TDS (see Figure SI3). Furthermore, butane is only formed in
the presence of H2O (see Figure SI4). Together, these findings
strongly support the scenario depicted in Scheme 2.
The intermediate reactive species proposed in Schemes 1
and 2 cannot be monitored directly, although their contribu-
tion to the reactions can be inferred from the product yields as
a function of sample thickness (Figure 3). Anions resulting
from EA are short-lived species that can decay by loss of the
excess electron.[3] This process is enhanced in the vicinity of
a metal surface to which the electron can be transferred.[19]
The opposite process, namely transfer of an electron from the
surface can neutralize cationic intermediates. The probability
of such quenching processes depends on the distance between
the ions and the surface.[19]
In the case of ethylamine,[5,6] the reaction occurred only at
E0 values above the ionization threshold of C2H4 and NH3. To
confirm that electron impact ionization also drives the
formation of ethanol, we have measured the dependence of
ethanol yield on the E0 value. The ionization thresholds of
H2O and C2H4 in the gas phase are 12.6 eV and 10.5 eV,
respectively.[9] In the condensed phase, polarization forces can
lower such thresholds by up to 2 eV.[10] The continuous
increase in product yield with E0 above about 8 eV confirms
an ionization-driven reaction. Surprisingly, however, the
yields of ethanol also increase considerably as the E0 values
decrease below 6 eV (Figure 2).
At E0 values well below the ionization threshold, electron
attachment (EA) to molecules can occur. In the gas phase, the
resulting molecular radical anions usually decay into an anion
and a radical, a process named dissociative electron attach-
ment (DEA).[3,4] However, in condensed phase where polar-
ization forces stabilize the charge, the radical anion may also
survive.[3,4] As neutral excitation processes are absent below
roughly 4 eV in both C2H4 and H2O,[11] such anionic processes
must be responsible for the ethanol yield at low E0 values.
DEA processes for H2O in the gas phase occur around
7 eV, 9 eV, and 11 eV[12] and thus above the range of E0 values
concerned here. No production of OHC radicals occurs below
At E0 values above but near the ionization threshold,
a radical cation initiates product formation. In very thin
layers, quenching of this species by the nearby metal surface
inhibits product formation, as shown here for ethanol, ethane,
and butane (Figure 3, top). At E0 = 3 eV, the formation of
ethane and butane follows the same trend (Figure 3, bottom),
thereby pointing again to quenching by the metal surface. In
both cases, neutralization by quenching occurs before the
intermediate radical ion can react with an adjacent molecule.
However, the production of ethanol at low E0 values only
Figure 2. Dependence of ethanol yields on electron energy (E0) as
represented by the integrated TDS peak areas at m/z=31 obtained
from 30 monolayer (ML) films of a 1:1 mixture of C2H4 and H2O after
electron exposures of 600 mCcmÀ2. The increase above 8 eV is attrib-
uted to a reaction initiated by electron impact ionization, while the
ethanol formation below 6 eV is driven by an electron attachment
process to C2H4.
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Figure 3. Dependence of product yields for ethanol (filled circles),
C2H6 (open triangles), and butane (open squares) on film thickness as
stated in monolayers (ML) for electron exposure of 600 mCcmÀ2 at
E0 =15 eV (top) and 3 eV (bottom). All product yields are scaled to the
same height to facilitate comparison. All products follow the same
trend except for ethanol produced at 3 eV. The latter is attributed to
additional quenching of OHC radicals by thermalized electrons, while
formation of other products is impeded only by quenching of charged
intermediates near the metal surface. Error bars are comparable over
all sets of data, but omitted on all but one for better legibility.
Scheme 2. Proposed reaction pathways for the formation of ethanol by
electron irradiation in the EA regime. The first step (a) is initiated by
the formation of a radical anion C2H4CÀ, which is a basic species and
reacts with a proton to form the main reactive intermediate species in
the reaction sequence, the ethyl radical C2H5C. The resulting radical
may react with another C2H5C radical to form butane (C4H10), H2O to
afford ethane (C2H6) and an OHC radical, or C2H4, which would
ultimately result in either C4H10 or C2H6 and another OHC radical. The
formation of ethanol (b) is the result of a multistep mechanism with
these OHC radicals as key intermediates and may proceed in an
autocatalytic reaction. The OHC radicals can add to C2H4, and after
abstraction of a hydrogen atom from a neighboring molecule yield
ethanol. The direct reaction by a combination of OHC and C2H5C
radicals is in our opinion far less likely, since radical densities in the
film are presumably low.
thickness dependence of the product yield depends critically
on the reaction mechanism.
To conclude, the hydration of C2H4 in mixed condensed
layers of C2H4 and H2O proceeds through two different
electron-induced reaction routes. At E0 values above but near
the ionization threshold (soft ionization), ethanol formation is
driven by intact molecular radical cations, analogous to the
electron-induced hydroamination described before.[5,6] On
the other hand, we provide here the first evidence that
synthesis of the same product can also be initiated at E0 values
below the ionization threshold through EA, a regime where
degradation of the product ethanol is also considerably less
efficient than above the ionization threshold (see Figure SI1).
However, the acidity of water is essential for this newly
discovered reaction pathway. This is, to the best of our
knowledge, the first example of an electron-induced synthesis
that proceeds through nondissociative EA. The reaction can
be controlled by selecting between two different reaction
mechanisms, an oxidative and a reductive process with
regards to C2H4. Both mechanisms lead to the same product,
but the formation of side products is more selective through
the EA route. This finding is not only important from
a mechanistic point of view, but might also have interesting
applications in the field of electrosynthesis.
occurs at strikingly larger film thickness (Figure 3, bottom)
and is thus more visible in a thicker film (compare Figures 2
and SI5). OHC radicals are required for the formation of
ethanol in the EA regime (Scheme 2). As OHC radicals
possess a high electron affinity of about 1.8 eV[9] and energy is
easily dissipated in a condensed phase, the capture of
a thermalized electron to yield OHÀ is thermodynamically
favorable. We therefore propose that OHC radicals only
survive when they are formed at a depth where the electron
current is attenuated enough to render encounter of a second
electron with a given reaction site unlikely on the time scale of
the reaction of OHC radicals with adjacent molecules. Only
then can the production of ethanol occur and trigger the chain
reactions as proposed in Scheme 2. Similar radical quenching
is unlikely for C2H5C because of its negative electron affinity.[9]
Quenching of OHC radicals is only overcome at a thickness
far in the multilayer regime (Figure 3). This situation
compares roughly with the electron penetration depth for
mixed condensed layers of CH4 and H2O, as derived from
saturation of product formation.[20] However, this comparison
must be regarded with caution as our results show that the
Keywords: alkenes · electron-induced reactions · radical ions ·
reaction mechanisms · water
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Received: December 18, 2014
Published online: February 6, 2015
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