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# Organic Syntheses in the Plasma of Glow Discharges and Their Preparative Application.

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Organic Syntheses in the Plasma of Glow Discharges and Their
Preparative Application
By Harald Suhr[*]
It is only during the last few years that reactions of organic substances as a result of electron
collisions in the cold plasma of glow and corona discharges have been developed into a preparatively useful method with a wide range of possibilities. Its basic principles and development prospects are discussed in the present progress report on the basis of the research results
that are known at present.
1. Introduction
In the continuous search for new methods of syntheses, the
possibility of using the plasma[**]of spark, arc, and glow
discharges for this purpose was examined by chemists at
an early date[2][*"].However, many of the earlier efforts
met with only slight success. This is easy to understand
nowadays, since the early experimental techniques were
[*] Prof. Dr. H. Suhr
Chemisches Institut der Universitat
74 Tiibingen. Wilhelmstrdsse 33 (Germanv)
[**I
l'lx\in.i
=
parti? ioniied gas.
[***I A large number of publications (cf.
[lg]) report successful
attempts to synthesize fairly complicated systems from very simple
starting materials and to break down or isomerize organic compounds
by plasma reactions. However, the present review deals only with those
that are closely concerned with preparative applications.
Angew. Cliem. internat. Edit. 1 Vol. I 1 (1972) 1 N o 9
unfavorable, and gave only low yields and nonuniform
reaction products. The organic compounds investigated
in this way decomposed in the plasma, largely with
formation of tarry or polymeric products. Despite the
numerous efforts to make preparative use of reactions
in plasmas, therefore, the only processes of this type
that have achieved any industrial importance are the
formation of ozone in silent discharges and the formation
of nitrogen oxides and of acetylene in an arc. Apart
from these, a few syntheses of inorganic compounds
on the laboratory scaler3"]and the modification of oils[3b1
have also been developed. It is only in the last few years
that organic syntheses on the preparative scale have
been successfully carried out in plasma. These results and
the possible applications of plasma chemistry with organic
compounds form the subject of the present report.
78 1
2. Forms of Discharge
The various types of electric discharge are best illustrated
with the aid of a current-voltage diagram (Fig.
A current flows between two electrodes that project into a vessel
having a low gas pressure and that are connected to a source
of direct current. Various regions with different types of
discharge, depending on the current strength, are clearly
discernible. At currents of less than lo-” A, the current
flow is unsteady, and occurs only when charged particles
are produced in the vessel by cosmic radiation or light
quanta. At higher current strengths, the production of
electrons by external agencies is no longer necessary. The
self-supporting dark discharge occurs in the region of
10-12-10-5 A, and this is followed by a region in which
the passage of current is associated with luminous phenomena. The weakly luminous glow discharge appears first
between
and lo-’ A, and the bright arc occurs at
current intensities of more than 1 A. Whenever the current
is switched on, the discharge passes through all the stages
in a short time (approx. 1 ms) until the type determined by
the magnitude of the resistance is reached. If low-frequency
of direct current, a new discharge builds up in each half
cycle. The decay of a discharge takes approximately 0.5 ms.
At frequencies of more than 1000 Hz, therefore, the time
between two half cycles is no longer sufficient for the
ionization to decay completely. Numerous electrons are
therefore already available for the ignition process in the
next half cycle.
experimental set - up
with which they collide. Since a new electron is released in
each ionization, the number of electrons avalanches until
it reaches an equilibrium value determined by the rates of
the electron-supplying and electron-consuming processes.
The main electron-consuming process is recombination in
the gas phase or on the wall of the vessel.
The possibilities of using the various forms of discharge in
chemistry depend on the thermal stabilities of the starting
materials and of the final products, since extremely high
temperatures sometimes occur in the discharges. It is
necessary here to distinguish between the temperatures of
the electrons, of the ions, and of the neutral molecules.
A temperature can be formally assigned to each particle
on the basis of its velocity:
m
2
-v’=
3
-kT.
2
In a discharge, electrons and ions acquire additional velocity through the electric field, and hence a higher “temperature” than the neutral particles. Because of their high
mobility, the electrons reach a particularly high temperature. In a neon tube, for example, the temperature of the
electrons is about 25000°C, whereas that of the neutral
particles and ions differs only slightly from the ambient
temperature. Systems of this type in which the temperature
of the electrons is much higher than that of the neutral
particles and of the ions are known as cold or nonequilibrium plasmas; if the temperatures of the electrons, ions,
and neutral particles differ only slightly from one another,
one speaks of hot or equilibrium plasmas. Typical examples
of cold plasmas are glow discharges and corona discharges,
while arcs are typical of hot plasmas.
Arc discharges with gas temperatures above 1000°C can
be used for chemical syntheses only if the reaction products
are thermally very stable. Carbides, borides, and nitrides
have been prepared in arc discharges. Only a few organic
compounds, such as HCN, C,N,, C,H,, and CO, can
withstand such temperatures without decomposing. Larger
molecules are broken down under these condition~[~-’1.
In glow and corona discharges, the gas temperatures are
so low that thermal decomposition of organic substances
,I
I
I O - ~ 1~0 - l ~
lr29011]
,
..._.
10P
in-‘
I[Al
-
I O - ~ 10-1
I
10
Fig. 1. Plot of a current-voltage curve in a vessel SO cm long filled with
neon at 1 tom. The various types of discharge appear at various resistance
values.
At higher frequencies, discharges can be maintained even if
the electrodes are situated outside the low-pressure vessel
(capacitive coupling). Energy can be equally easily supplied
to the plasma by induction if a coil is wound round the
vessel and connected to a high-frequency generator.
The elementary processes of electric discharges141can be
most simply described as follows. Individual electrons,
formed e.g. by cosmic radiation, are accelerated by the
electric field, and can excite or ionize atoms or molecules
782
can be avoided. They should therefore be suitable for many
syntheses. Despite intensive efforts, however, only a few
syntheses (mainly inorganic) have been achieved with cold
discharges[’. *I. Apart from the formation of ozone, which
was mentioned earlier, the most important of these are the
syntheses of h y d r a ~ i n e [ ~ - ’sulfur
~],
and halides and hydridesl*’ of boron, silicon, germanium, phosphorus, and arsenic. Experiments with organic substances
did not lead to any preparatively useful syntheses. Though
it was possible to show that most organic compounds
undergo chemical changes in cold discharges, the products
were mainly polymeric material or low molecular weight
decomposition products. The reasons for the earlier failures
have been revealed only by recent investigations. Most
organic compounds have only a low stability in the plasma,
must not be exposed to excessively high energies, and must
be removed quickly from the reaction zone to avoid complete decomposition. These facts impose a very narrow
Angew. Chem. internat. Edit. 1 Vol. 11 (1972) / N o . 9
range of experimental conditions in which syntheses can
be carried out with preparatively useful yields.
3. Execution of Reactions in Cold Plasmas
Since glow discharges occur only under reduced pressure,
all arrangements that make use of this form of discharge
show some resemblance to vacuum distillation apparatus
(cf. Fig. 2).
A particularly simple arrangement is a glass apparatus with
metal electrodes, across which a DC or low-frequency AC
voltage of 2 L 5 0 V/cm is applied (Fig. 2a). Experimental
arrangements of this type are very suitable for the preparation of atomic hydrogen, nitrogen, or o ~ y g e n [ ~ For
~~'~].
the reaction of organic compounds, the electrodes must be
protected by an inert gas, since they otherwise rapidly
become coated with decomposition products['61. Apparatus of this type is cheap and robust, but is dangerous because of the high voltages required.
high-frequency energy in both cases can be easily carried
from the oscillator to the reaction vessel by coaxial cables.
This separation of the electronic from the chemical apparatus greatly faciIitates operation.
For corona or silent discharges, D C or low-frequency AC
current flows through a glass wall (Fig. 2e). In this way,
one obtains low-current discharges, which are uniformly
distributed over the entire surface. The advantage of this
form of discharge is that operation at normal pressure is
possible. Against this it has the disadvantage that high
voltages must be used, and that very large surface areas are
necessary for high conversion rates.
Experience gained so far in organic plasma chemistry shows
that the result of the reaction depends mainly on the field
strength, pressure, and vessel dimensions, so that similar
results can be obtained with various types of discharge.
Short-wave apparatus is therefore recommended for
laboratory work because of the ease of operation, while
the simpler high-voltage types of equipment are recommended for larger units.
4. Applications
4.1. Reactions of Atomic Gases with Organic Compounds
Glow discharges are excellently suited for the dissociation
of molecular hydrogen, oxygen, or nitrogen into atoms.
Since the atoms recombine mainly on the wall, high yields
are obtained only if the walls are "poisoned" with certain
substances such as water, phosphoric acid, Dryfilm, or
Teflon
221.
at
bl
Ci
0
magnetron
dl
electricaliy conducting
el
Fig. 2. Simple forms of apparatus for reactions in glow discharges
The atomic gases can react with many organic comp o u n d ~ [ ~ ~Atomic
- ~ ~ ]hydrogen
.
abstracts hydrogen atoms
from saturated compounds[261and adds to unsaturated
corn pound^^^^^. In both cases free radicals are formed, and
these can give a variety of further products by decomposition, disproportionation, combination, or rearrangement :
H-
+
H-
+ H,C=CHZ
H3C-CH:,
+
-
Hz
+ CH,-CH,-
CH3-CHz-
E
Products
No electrode problems arise in short-wave and microwave
units. The short-wave energy is coupled inductively or
capacitively to the plasma (Fig. 2b and 2c). For microwave
discharges (Fig. 2d), quartz"] reaction tubes are passed
through the wave guides or through resonance cavities.
Microwave and short-wave generators are commercially
available. Short-wave generators can also be constructed
in the laboratory at low cost. Even the simplest push-pull
oscillators give good results. Multistage oscillators have
the advantage of high stability of frequency and power output independent of load conditions. After matching the
Atomic oxygen abstracts hydrogen from saturated compounds. With unsaturated compounds, the first reaction
step is the formation of an epoxide, which then partly rearranges to form carbonyl c ~ m p o u n d s ~ Thus
~ ~ - ~2-~ ~ ,
pentene gives approximately 50% of cis-rmns-pentene
oxide, 25% of diethyl and methyl propyl ketones, and 20%
of 2-methylbutanal. A synthesis of particular industrial
interest is that of propylene oxide from propene and atomic
oxygen. Mixtures of oxygen and propene give good yields
of propylene oxide in silent discharges. With a conversion
of 20%, approximately 30% (corresponding to 100 g/kWh)
of propylene oxide was obtained[311.
I'[
Nitrogen atoms react very vigorously with organic comp o u n d ~ ' ~ ~ Irrespective
-~~!
of the type of compound, the
Because of the high dielectric losses, glass would heat up too much
and might soften.
Anyew Chem. internat. Edit. J Vol. I1 (1972) 1 N o . 9
783
reaction always yields large quantities of hydrogen cyanide
as well as some cyanogen and ammonia. In some cases the
yields are so high that the formation of hydrogen cyanide
can be used for the determination of atomic nitrogen[361.
CH,-CH,-CH=CH-CH,
+0
4.2. Reactions of Organic Compounds
Several hundred organic compounds have already been
investigated in glow discharges. The findings are very
diverse. The number of results that have been reported so
far is not yet sufficient to show any clear patterns, but a
typical plasma behavior of certain classes of compounds is
already discernible. Some typical plasma reactions of this
type will be described in the following sections.
4.2.1. Isomerizations
CH, - CH, - CO- CH, - CH,
CH,-CH,-CH,-CO-CH,
L
]
Olefins can be isomerized even under very mild reaction
conditions. Thus trans-stilbene is converted into the cis
form[411.At high flow rates and a correspondingly low
conversion, 95% of the reaction product is cis-stilbene. If
the distillation rate is reduced to increase the conversion,
the content of cis-stilbene decreases, while the content of
phenanthrene increases. Similar isomerizations have been
observed for cinnamonitrile, crotononitrile, and stilba~oIe[~~].
25%
0
CH,-CH,-CH-C,
AH3
4
21%
Only a few cases are known at present in which active
nitrogen was incorporated into organic compounds. In the
quantity of hydrogen cyanide (about SO%), a mixture of
pyrrole, crotononitrile, and cyanobutadiene is formed137.
381. Isoprene similarly gives r n e t h y l p y r r ~ l e [in
~ ~yields
]
of
about 10%. A reaction was recently achieved between
atomic nitrogen and aqueous solutions of organic compounds. However, no products containing nitrogen are
formed; the atomic nitrogen merely has a dehydrogenating
action, and so converts e.g. alcohols into aldehydes[401.
H,C=CH-CH=CH, + N
H\
H
c=c’
L
Conversion
(I(J%:
cis-Stilbene 5 0 %
Another typical plasma reaction is the change in the position of substituents. In some cases this is only a side reaction. Xylene, for example, in addition to a large quantity
of the synthetic product bixylyl (cf. Table 4), gives about
1% of rearranged xylenes“]. The migration of the sub-
2 5%
\
CH,-CH=CH-CN
cis, trans
CeHsCN. C,H&N
~ 7 q o
R
-2%
=soqo
Table 1. Products and yields of rearrangement reactions [45-471
Yields of rearrangement products
[mol.-”/]
Starting compound
Anisole
Phenyl isopropyl ether
p-Tolyl methyl ether
1-Naphthyl methyl ether
2-Naphthyl methyl ether
Dlphenyl ether
N,N-Dimethylaniline
a-cresol
a-isopropylphenol
2,4-dimethylphenol
2-methyl-1-naphthol
1-methyl-2-naphthol
2-phenylphenol
N-methyl-a-toluidine
44
36
59
51
47
53
28
Cgik Whl
p-cresol
p-isopropylphenol
27
22
4-methyl-1-naphthol
31
4-phenylphenol
N-methyl-p-toluidine
18
15
50
25
58
55
58
180
23
~-
Whereas atomic hydrogen Often effects interesting reductions with inorganic
compounds, and many substrates can
be converted into nitrides with active nitrogen, no preparatively useful syntheses with atomic hydrogen Or nitrogen are known as yet for organic compounds.
784
-~
[*I In experiments wtth p-xylene that was free from the orfho isomer,
the xylene component recovered at conversions of 20-500/, contamed
about 1% of a-xylene; small proportions of m-xylene could not be
separated from the para isomer under these conditions. Simxlarly,
1% of 0-xylene is formed from m-xylene. o-Xyiene gives approximately equal quantities of the niefa and para isomers [43].
Angew. Chem. inzernat. Edit. 1 Vol. 1 I (1972) NO. 9
stituents often becomes the main reaction, as in the rearrangement of aryl alkyl ethers to a l k y l p h e n ~ l s [ ~Methyl
~].
ethers and ethers of primary alcohols isomerize as readily
as ethers of secondary and tertiary alcohols[451(Table 1).
Anisole gives a mixture containing 95% of o-cresol, p cresol, and phenol. m-Cresol, benzene, toluene, and
cyclopentadiene are also formed in small quantities. The
ratio of o-cresol to p-cresol can be varied only slightly,
whereas the content of phenol in the reaction products is
strongly dependent on the experimental conditions. It increases with increasing power, and can ultimately exceed
the content ofcresol. The rearrangements of other ethers[45,
461 are very similar to those of anisole, but cleavage of the
migrating alkyl group sometimes occurs as a side reaction. If
the ortho or para positions are blocked by other substituents
in these arrangements, the proportion of the corresponding
"phenol" always increases. In the case of the naphthyl
methyl ethers[461,the 1-naphthyl derivative gives a mixture
of 2- and 4-methyl-I -naphthol, while the 2-naphthyl
derivative gives only 1-methyl-2-naphthol.
OC H,
OH
stable aromatic compounds toluene and styrene[481. On
the other hand, ring expansions, which are frequently
assumed to occur in mass spectroscopy, are unusual in the
plasma. The only example observed so far is the rearrangement of 2- or 3-methylindole into q ~ i n o l i n e ~ " ~ ~ .
-l
t+
I
a
CH H
'
E
3
H
Interesting preparative possibilities are offered by ringchain isomerizations found for various heterocyclic syst e m ~ [ ~ ' ]In. glow discharges, pyrrole gives crotononitrile
and polymeric material having the same elemental composition. Similarly, indole decomposes to give phenylacetonitrile, and quinoline forms cinnamonitrile. Attempts to
OH
Q
H
H
Purely aromatic ethers exhibit similar rearrangements.
Diphenyl ether gives a mixture of o- and p-phenylphenol
together with some
The ortho compound partly
cyclizes to d i b e n z o f ~ r a n [ ~Finally,
~I.
the same reaction can
also be detected for vinyl ethers. Ethyl vinyl ether isomerizes
to 1-butenal or decomposes to form vinyl alcohol which is
isolated as the tautomeric aldehyde[461.
Behavior similar to that of the ethers is shown by the
analogous nitrogen compounds1471.N , N-Dimethylaniline
gives 0- and p-N-methyltoluidine as rearrangement products and N-methylaniline as a degradation product (cf.
Table 1). When N-methylaniline is used as the starting
material, o- and p-toluidine and aniline are found. Other
reactions take precedence over the rearrangement in the
cases of phenyl esters and acetanilides.
3 7%
Other types of isomerization in glow discharges are ring
contractions, ring expansions, and ring cleavages. Some
cycloolefins, such as cycloheptatriene and cyclooctatetraene, rearrange readily in the pIasma to form the more
Angeu,. Chem. internot. Edit. 1 Vol. 11 (1972) 1 N o . 9
a
isomerize nitriles to heterocycles, on the other hand, have
been unsu~cessful~~'~.
4.2.2. Eliminations
Another type of reaction that proceeds preferentially in
glow discharges is elimination. Single atoms or small
groups can be removed from many compounds in the
plasma without disruption of the rest of the molecule.
Eliminations of carbon monoxide, carbon dioxide, and
nitrogen proceed particularly smoothly. Thus isocyanates
are decarbonylated to nitrenes, which then dimerize or
undergo other nitrene reactions. Azobenzene and diphenylamine have been obtained in this way from phenyl iso~yanate'~~].
Aldehydes and ketones can also be readily decarbonylated
(Table 2). The resulting radicals are stabilized by hydrogen
abstraction or by dimerization. Thus benzene and biphenyl
are formed from benzaldehyde, thiophenecarbaldehyde is
converted almost entirely into thi~phene'~'], and 2pyridinecarbaIdehyde gives pyridine and 2,2'-bipyridy1142!
785
The mildness of this decarbonylation is evident in the case
of thiophenecarbaldehyde, since no desulfurization occurs
at iow conversions. Many ketones are decarbonylated in
high yields, and so offer interesting synthetic possibilities.
On decomposition of acid anhydrides, carbon dioxide is
eliminated first, followed by carbon monoxide. The intermediate carbonyl compounds can be detected in some
cases on the basis of the reaction product~[’’~.The main
Table 2. Products and yields of some decarbonylation reactions [42,49, SO].
_-~___
Starting compound
Conversion
Product
[%I
Benzaldehyde
63
Thiophenecarbaldehyde
2- Pyndinecarbaldehyde
4
40
25
Benzophenone
68
Benzil
Fluorenone
18
20
50
76
Camohor
benzene
biphenyl
thiophene
thiophene
pyridine
2.7’-bipyridyl
biphenyl
fluorenone
biphcnblene
biphenyl
biphenylene
biphenylene
trimethvlbicvclo, .
[?.l llhexane
Benzophenone gives biphenyl, and in the decomposition
of benzil, the formation of biphenyl is almost quantitat i ~ e [ ~Fluorenone
~].
undergoes ring contraction to form
biphenylene[”I. Aliphatic cycloketones react nonuniformly, and give only a small quantity of ring-contraction product. The decarbonylation of bicyclic ketones, such as
camphor and norcamphor, on the other hand, leads to
very good yields of bicyclic hydrocarbons resulting from
ring contraction[43.491.
&
0
81
16
99
86
70
20
27
36
36
98
99
99
16
93
18
24
46
18
24
24
21
43
63
24
25%
15%
+ Polymers (=400/0)
product of the reaction is an unsaturated compound. For
example, phthalic anhydride decomposes to give dehydrobenzene, which can react further in various ways. If no
other reactant is present, biphenylene, triphenylene, or
“polyphenyl” is formed[”. 531.
The decomposition of azo compounds in the plasma has
so far been investigated only for azobenzene, which is converted in high yields into biphenyl. The isoelectronic
compounds benzylidenaniline and stilbene behave differently ;they decompose mainly into benzonitrile or cyclize
to form phenanthrene, while the formation of biphenyl is
only of minor importance.
40 6 0%
-4
Yield
CgikWhl
Cmol-%l
i,a
-CHI
-CH::CO
10-20%
Decarbonylations are also observed for aromatic hydroxy
compounds. 10-15% of cyclopentadiene is formed from
phenol, while cresols give methylcyclopentadiene in a
similar
The yields are considerably higher in
the naphthalene series. 1-Naphthol gives up to 75%, and
2-naphthol up to 95%, of indenecS1].
Many plasma reactions result in the formation of atomic
hydrogen or free radical^"^. 5 4 * ’’I, which then usually
react with other molecules with hydrogen abstraction. For
Table 3. Typical dehydrogenation reactions in the plasma [43, 571
OH
The same reaction occurs for 1-nitr~naphthalene[~~l.
This
compound decomposes with loss of nitric oxide and subsequent decarbonylation to form the indenyl radical, which
is stabilized by formation of indene.
Yields similar to those obtained on decarbonylation are
also found on decarboxylation. Carboxylic acids are converted almost quantitatively into the corresponding hydrocarbons. The reaction course is less uniform in the case of
esters, but these also preferentially form hydrocarbons[’ ‘I.
786
Angew. Chem. internat. Edit. i Vol. I 1 (1972)
i No. 9
this reason, dehydrogenations in the plasma are very
common. They sometimes occur as side reactions, but can
also become preparatively useful principal reactionsc5'I.
Alkanes can be converted into alkenes and alkenes into
alkynes in this way. Cycloalkanes and cycloalkenes are
aromatized with dehydrogenation (Table 3).
Some aromatic systems form 5- or 6-membered rings in
the plasma with elimination of hydrogen[431.In favorable
cases, as in the formation of ~ a r b a z o l e [ ~
or~ 'dibenzof ~ r a n ' ~the
~ ' ,yields are so high that they offer interesting
preparative possibilities.
mostly require more elaborate apparatus, and have therefore been investigated in only a few cases.
A bimolecular reaction that very often occurs in the plasma
and makes no demands regarding equipment is the dimerization of a compound with removal of hydrogen or other
groups. Particularly good yields are obtained with aromatic
starting compounds. Thus biphenyl is formed from ben~ e n e [58-611,
~ ~ , and 1,2-diarylethanes are formed from
toluene and other methylaromatic corn pound^^^^^ 6 2 - 6 4 1
and from benzyl halides or benzyl alcohols'431 (Table 4).
Substituted ethanes are also formed from aromatic com-
Table 4.Products and yields of dimerization reactions
Starting compound
Product
Benzene
Toluene
p-Xylene
Durene
Isopropylbenzene
terl-Butylbenzene
1-Methylnaphthalene
p-Methylbenzonitrile
Biphenyl
Bibenzyl
Bixylyl
Bisdurene
2,3-Diphenylbutane
2,3-Dimethyl-2,3-diphenylbutane
1,2-Di(l-naphthyl)ethane
1,2-Di(4-cyanophenyl)ethane
The ease of removal of water or hydrogen halide can also
be used for the preparation of ole fin^[^^^. The elimination
of other groups has not yet been very extensively studied.
-b
3 0%
88%
1
71%
I
NH,
40
90
90
17
-
90
48
90
45
8
70
43
34
38
129
170
1581
1631
f44j
1431
1641
pounds having straight-chain or branched alkyl groups,
with C-C bond cleavage in the a position.
Trimerizations and tetramerizations occur only rarely in
the plasma. An exception is acetylene, which can also
trimerize and tetramerize. Because of its simplicity and its
industrial importance, the behavior of acetylene in plasmas
has often been in~estigated[~'-~~'.
In these experiments,
in addition to small quantities of vinylacetylene, diacetylene, and benzene, the main product isolated was always
a yellow polymer (cuprene). Under certain reaction conditions, however, entirely different results can be obt a i n e ~ i [ ~If~ ]the
. surface area of the vessel is greatly increased, the reaction can be stopped at an intermediate
stage. Polymer formation can be almost totally suppressed
in this way, and compounds containing eight carbon atoms
are formed instead, together with vinylacetylene and
diacetylene (Table 5). The reaction thus corresponds
formally to a tetramerization of acetylene, which can be
plausibly explained by the intermediacy of cyclobutadiene:
HO
The removal of halogen, hydroxyl, methoxy, or amino
groups has been observed in some cases. The yields are
mostly low, and the reactions are of little preparative interest.
4.3. Bimolecular Processes
The last section was concerned only with unimolecular
reactions, but chemistry in glow discharges is by no means
confined to such processes. However, bimolecular reactions
Angew. Chem. internal. Edit. 1 Vol. I 1 (1972) / N o . 9
If the reaction is not stopped at the tetramer, it proceeds
to yield high molecular weight compounds. The IR spectra
787
of the polymerization products from acetylene are very
similar to those of polystyrene[601.
d
0
0
Table 5. Liquld products of the plasma reachon of acetylene 1481.
~~
Product
~
(yo-+
0
1-
-a
Yield
Cmol-Xl
______
Styrene
Phenylacetylene
Cyclooctatetraene
Benzene
Naphthalene
30-70
15-50
I
5-10
10-20
Polymerizations are observed much more frequently than
oligomerizations in the plasma. In addition to the usual
monomers, such as styrene and butadiene, compounds
exhibiting no tendency to polymerize under normal conditions van also be polymerized in the p l a ~ m a [ ~ ~ - For
’~].
example, under drastic conditions, benzene gives a polymeric material, presumably “polyphenyl ”, as the only
product[59.601. A high molecular weight substance, presumably “polyphenyl”, is similarly obtained from phthalic
anhydride. A series of compounds containing nitrogen,
such as pyrrole, pyridine, and aniline, are converted in the
plasma into unsaturated nitriles, which can be readily
polymerized[421.
In bimolecular reactions of two different components, the
range of favorable operating conditions is very narrow.
Outside this range, at most only one of the components
reacts. Under suitable reaction conditions, propylene oxide
has been prepared from propene and oxygenc3’],and styrene
oxide from styrene and oxygen[481.Mixtures of benzene
and oxygen give phenol[”, 741, and mixtures of benzene
and ammonia can be converted into aniline[42.7s1. The
yields in such reactions are often unsatisfactory. Systematic
attempts to optimize the processes have so far been made
only for propylene oxideC3’].
Another interesting possibility for carrying out bimolecular
reactions is to produce reactive intermediates in the plasma
and to intercept these with suitable reactants. This possibility has been investigated in the case of phthalic anhydride[s3! In the first reaction step, the compound loses
carbon dioxide, and carbon monoxide is then eliminated to
form dehydrobenzene. Two molecules of dehydrobenzene
can then combine to form a biphenyl diradical, which
cyclizes to biphenylene or reacts with another molecule of
dehydrobenzene to form triphenylene. The reaction can be
stopped at any of the intermediate stages by means of interceptor reagents. The addition of hydrogen reduces the
yields of biphenylene and triphenylene in favor of biphenyl
formation. Sulfur atoms also react with the biphenyl
diradical to form dibenzothiophene. Dehydrobenzene can
be intercepted by the addition of acetylene, and the main
reaction product in this case is phenylacetylene. Finally,
all the intermediates react with ammonia. The benzoyl
radical gives aminobenzaldehyde and benzamide, dehydrobenzene gives aniline, and the biphenyl diradical reacts to
form carbazole :
788
Preparative organic plasma chemistry is still at an early
stage of development. The results obtained so far have
already shown many possibilities for specific reactions and
preparative applications. Most of the investigations have
been carried out on simple model compounds, but the
results obtained can be readily applied to other, preparatively more attractive systems.
More complicated compounds can often react in several
ways, and in such cases the reaction with the lowest energy
requirements usually occurs preferentially. To be able to
predict the reaction course, the reactivities of the various
groups must be known. For this purpose, one still has to
rely largely on experience and estimates based on bond
energies. If the molecules contain only relatively strong
bonds, they react only with difficulty in the plasma. C-C
and C-H bonds in aromatic compounds and double and
triple bonds are therefore particularly stable. C-C bonds
in aliphatic systems are moderately stable, and C-C bonds
in benzyl or ally1 positions are particularly easily broken.
Most functional groups react readily in the plasma.
Aldehyde, carboxyl, nitro, and azo groups are most readily
attacked. Carbonyl, hydroxyl, alkoxy, amino, and alkyl
groups as well as halogens also react under rather more
drastic conditions. Only cyano and ethynyl groups and
fluorine have been found to be stable substituents.
5. Reaction Mechanisms
In a plasma reaction, the starting materials are known and
the end products can be determined by analysis. However,
the reaction path is usually unknown. The reaction course
can be influenced only by variation of pressure, electron
energy, and residence time. Interception reactions or other
means for the identification of intermediates are only rarely
possible. Views on the mechanisms of organic plasma reactions are therefore largely based on a discussion and
critical consideration of the various possible elementary
processes and on an interpretation based on the reaction
products.
Elementary processes of very different types can occur in
the plasma. 1 cm3 of the plasma zone normally contains
about 1010-1012 electrons and ions as well as
neutral
particles. Though the charged particles are present only in
a low concentration, it is the presence of these particles
that makes the plasma reactions possible. The electrons
AngeM?. Chem. infernal. Edit.
Vol. I 1 119721 I No. 9
absorb energy in the electric field and transfer it to the
neutral particles in elastic or inelastic collisions.
The transfer of energy by elastic collisions depends on the
relative masses of the colliding particles. On collision of an
electron with a molecule of mass 100, only ljlOO000 of the
energy of the electron is transferred to the larger molecule.
In the usual preparative plasma experiments, 0.1-1 mol
of starting material is passed through the plasma per hour.
Each molecule thus spends only about 0.1 s in the plasma,
and the quantity of energy that it receives through elastic
collisions during this time is so small that neither chemical
reaction nor appreciable heating of the neutral gas occurs.
The influence of the elastic collisions is therefore extremely
small.
The chemical reactions in the plasma are undoubtedly
results of inelastic collisions. Various processes can occur
in such collisions, depending on the energy of the colliding
electrons. Electrons with low energies may be trapped by
the molecules to form negative ions. Somewhat higher
electron energies lead to the formation of short-lived collision complexes, whose lifetime is sufficient in many cases
for part of the kinetic energy of the electrons to be transferred to the molecules. Vibrationally or electronically
excited molecules are formed in this way. If electrons with
still higher energies collide with the molecules, they may
produce positive ions, possibly in a vibrationally or electronically excited state. Since the kinetic energy in an
ionizing collision is distributed over three particles, the
probability of the formation of electronically excited
positive ions is small.
Vibrationally or electronically excited molecules are undoubtedly crucial intermediates in many plasma processes.
The role of the ions is less clear. Despite the low degree of
ionization, many of the molecules may be temporarily
ionized during their passage through the plasma, since the
exchange of charge via ion-molecule reactions takes place
extremely rapidly1771.However, it is necessary in every case
to investigate whether the structural changes in the molecule
take place in the ionic state.
The ions revert to the uncharged state by charge transfer
or by neutralization. The recombination of a positive ion
with an electron can take place only in a three-body collision or on the wall. Though the energy (approx. 10 eV)
is distributed between two particles here, the single molecule
receives so much energy that it may decompose. There is
less danger of decomposition for larger molecules, since
these can absorb more energy through their numerous
vibrational modes. Many positive ions form complexes
with neutral m ~ l e c u l e s ~ ~ If* -such
~ ~ ~a . complex recombines with an electron, or a positive ion with a negative ion,
the energy liberated can be distributed over several particles
and numerous bonds.
The negative and positive ions and the excited molecules
may rearrange or decompose. They or their further reaction
products, which are often free radicals, may form new
species by reaction with unexcited particles. Only by detailed investigations is it possible to establish which of
these numerous possibilities predominates in a given reaction. For this purpose, the electron density and energy
distribution and the nature and concentration of the
Anyew. Chem. internal. Edit.
/ Vol. 11 11972) / N o . 9
positive and negative ions, the excited molecules, and the
intermediates should be determined by sonde measurements or by mass, electron spin resonance, emission. or
absorption spectroscopy. Investigations of this nature
have already been carried out for simple systems, and
have demonstrated the possibilities of determining the
elementary processes in the plasma in this admittedly very
laborious way[8L-S71.
A much simpler method is the comparison of the plasma
reaction products with the products of related reactions.
Some classes of substances, on reaction in the plasma,
exhibit similarities to pyrolysis, photolysis, or mass spectroscopy. If analogies are found in the products, it may be
assumed that the reaction mechanism is also analogous.
Aliphatic hydrocarbons are converted into a variety of
paraffins, olefins, or acetylenes in the plasma by decomposition and degradation reactionsEs8].A similar mixture
of products is formed on pyrolysis of hydrocarbons, and it
can therefore be concluded that the two processes have the
same (free-radical) mechanism for this class of compounds.
Similarities between glow discharges and pyrolyses occur
only occasionally, since the two methods differ in important
respects. In pyrolysis, many particles have a high energy,
while in the plasma only isolated molecules receive an
energy through electron collision that can appreciably
exceed the energy involved in pyrolysis. These hot molecules” react, but are then immediately deactivated again
by collisions with the less excited neighboring molecules.
‘I
Some plasma reactions yield the same products as photochemical reactions. It cannot be assumed that the reactions
in the plasma proceed indirectly via photons, since only a
small proportion of the photons produced in glow discharges are absorbed by the gas moIecules. The “photochemical reactions” in the plasma are due to the fact that
the same states can be excited by collision with electrons
as by absorption of a photon. Since there are no selection
rules and no forbidden transitions for collisions with slow
electrons, they can also lead to other states, which are not
directly accessible by photochemical processes.
In some classes of compounds, pronounced similarities are
found between the behavior in the plasma and in the mass
spectrometer. An impressive example is provided by anisole
(Table 6). For all the plasma products of a n i ~ o l e ~there
~~],
are decomposition paths leading to similar products in the
mass spectrum1891.The only difference is that reactions
requiring large amounts of energy, such as the elimination
of formaldehyde or of CH,+CO. are important in the
mass spectrometer, whereas the reaction with the lowest
energy requirement, i. e. the rearrangement to the cresols,
is particularly favored in the plasma. The analogy between
plasma chemistry and mass spectroscopy is not surprising,
since the reactions are induced in both cases by collisions
with electrons. However, there are differences with respect
to the electron energies and the pressure. Because of the
higher pressures in the plasma, bimolecular processes such
ion-molecule reactions can readily proceed, whereas these
reactions occur only rarely if at all in the mass spectrometer.
Reactions are occasionally found in the plasma for which
no analogies are known. One such case is that of nitro789
benzene[g01.In the plasma, this compound gives numerous
products, which are almost without exception derived
from three intermediates, i. e. the phenyl radical, the
phenoxyl radical, and dehydrobenzene. The decomposition
of nitrobenzene to form the phenyl radical requires 57
kcal/mol, and the elimination of nitric oxide presumably
opposed here by the decrease in the average electron energy
and hence in the number of chemically effective collis i o n ~ [ ~In~ many
].
reactions, the selectivity increases with
increasing pressure because of the lower electron energy.
I
,overali yield
Table 6. Comparison of the reaction products of anisole
(H,C6-O-CH,
= M) in the plasma with the fragmentation processes in the mass spectrometer.
Plasma products
[mol-%]
Phenol
Benzene
22-26
04 4
Cresols
1-3
62-70
Mass spectra
Rel. intensities
M+
M' - CH,
_.
CH,O
M ' - 11-CH20
M ' - CH,-CO
M- - H
M'-H
CO
100
MA
15
56
16
63
2 [a1
10 Cal
[a] Typical cresol band
requires rather less energy. Both reactions also occur in
p h o t o l y ~ i s [ ~pyrolysis[921,
~',
and mass ~ p e c t r o s c o p y ~It~ ~ ' .
reactions, the energetically much less favorable elimination
of HNO, (energy requirement approx. 200 kcal/mol)
plays such an important part in the plasma. This surprising
observation suggests that an appreciable proportion of the
nitrobenzene molecules have been ionized, and the energy
of approx. 10 eV liberated on recombination has allowed
aryne formation.
The mechanisms of plasma reactions have so far been
thoroughly investigated in only a few cases. The results
obtained are extremely interesting, but the picture of the
organic plasma that can be drawn on the basis of these
findings is still too incomplete for any pattern to emerge.
6. Possibilities and Limitations of Organic Plasma
Chemistry
The preparative use of a plasma reaction depends on the
conversion (in %) and the yield (in g/kWh). Other decisive
factors are whether the reaction leads uniformly to the
desired product and what trouble is involved in the separation of any by-products, whether the reaction product is
also obtainable by another route, and whether the value of
the product justifies the outlay on equipment.
The yield and the selectivity of a plasma reaction depend
on the experimental conditions, and can be optimized by
series of experiments. Figure 3 shows such a dependence
for the formation of biphenyl from benzeneL471.With increasing field strength, the total yield increases, but the
percentage of biphenyl in the reaction products decreases.
With increasing pressure, the yield of biphenyl passes
through a maximum. The first result is due to the fact that
secondary processes and fragmentations also occur to an
increasing extent with increasing energy. As to the pressure
dependence, an increase in the bimolecular reaction rate is
790
30 50 70
Transmitter energy[WI--,
05 1
15
Pressure [torrl +
Fig. 3. Yields of biphenyl in the plasma reaction of benzene [47]: a) at
constant pressure (0.8 torr) and with varying transmission energy;
b) at constant transmission energy (52 W) and with varying pressure.
The rules for the control of plasma reactions are not yet
sufficiently known, and it is therefore necessary to determine the optimum conditions experimentally for each new
reaction by variation of the pressure, field strength, tube
diameter, and flow rate. Moreover, the transfer of experimental data from one plasma apparatus to another is
often not directly possible, and a number of further
optimization experiments are then necessary. A direct
transfer of data will become possibly only when the plasma
properties required for a given reaction have been establi~hed[~~].
The yields of plasma products vary widely. Rearrangements give approx. 1 mol/kWh and dimerizations approx.
1-2 mol/kWh; eliminations usually proceed with yields
of 0.1-0.5 mol/kWh. These data relate to experiments in
which the field strength, the pressure, and the flow rate
have been optimized. If the tube diameter is also varied and
the high-frequency coupling optimized, it should be possible to increase the yields further.
Plasma chemistry is confined to the gas phase, and for
preparatively useful mass throughputs it requires a vapor
pressure of 1 torr or more. In principle, therefore, all compounds that can be distilled without decomposing at pressures of at least 1 torr should be suitable for use in plasma
reactions. However, the number of preparatively useful
reactions is much smaller, since product mixtures can occur
with molecules for which several energetically equivalent
reactions are possible. Only if one of the possible reactions
has a much lower energy requirement than any of the
others can the reaction be controlled in such a way that
only a single product is formed. In the case of isopropylbenzene, in which the bond energies differ by almost
30 kcal/m01'~~~
the reaction can be carried out in such a way
as to obtain 2,3-diphenylbutane almost quantitatively by
the use of mild conditions. Reactions that require the
cleavage of a stronger bond become important only at
higher electron energies, but are then accompanied by the
formation of 2,3-diphenylbutane.
Angew. Chem. internal. Edit. 1 Vol. 11 (1972) 1 No. 9
The mass throughput is decisive to the economy of plasma
reactions. The laboratory equipment developed so far
operates at pressures of 1-5 torr and distillation rates of
about 0.1 mol/h. If the pressure is increased to raise the
throughput, the field strength must be simultaneously increased, since the ratio of the two quantities should be kept
approximately constant. However, this often leads to such
high gas temperatures that starting compounds or final
products undergo thermal decomposition. It may be possible to overcome these difficulties with pulsed plasmas, in
which there is no excessive heating of the neutral gas up to
the region of about 50 torri9’I.
7. Future Developments
The transformation of organic substances by collision with
electrons, which has been known for some time, has been
developed in recent years into a preparative method
offering a wide range of possibilities. Many technological
problems still have to be solved before the processes elaborated so far on the laboratory scale can be applied to industrial production. Owing to the considerable cost of
plasma equipment, plasma chemistry is not very likely to
be considered for large-scale syntheses. Its future use will
presumably lie in the production of compounds that are
unobtainable or obtainable only with difficulty by other
methods. Though organic plasma chemistry is still in the
early stages of development, it is likely, from the very
recent preparative successes, that plasma-chemical methods of synthesis will be introduced into laboratory chemistry and industrial production in the near future.
Received: December 28, 1971 [A 901 IE]
German version: Angew. Chem. 84,876 (1972)
Translated by Express Translation Service, London
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Energy Metabolism in Mitochondria
By Hans Walter Heldt"]
Mitochondria are separate metabolic compartments within the cell. The functional boundary
of the mitochondrial compartment is the inner membrane. This membrane contains the
enzymatic apparatus for the electron transport and oxidative phosphorylation. The substrate
breakdown cycles are localized in the mitochondrial matrix space. Specific carriers are
responsible for the exchange of ADP, ATP, phosphate, and intermediates of the citric acid
cycle between the matrix space and the extramitochondrial space. The particular importance
of the adenine nucleotide transport to the regulation of the energy metabolism of the cell is
discussed in detail.
Mitochondria occur in the cytoplasm of all aerobic eukaryotic cells. They are the site of cell respiration, and have
therefore been referred to as the "power plant of the cell".
The energy liberated on oxidation of the substrate hydrogen
drives the endergonic synthesis of adenosine triphosphate
(ATP) from adenosine diphosphate (ADP) and orthophosphate (P). This process is known as oxidative phosphorylation[**].The energy liberated on hydrolysis of ATP
to ADP and phosphate covers the energy-consuming processes of the cell ( e . g . mechanical, chemical, osmotic, and
electric work).
There are two important metabolic compartments in the
animal cell, i. P . the extramitochondrial compartment,
which comprises the ground cytoplasm and the nuciear
space, and the mitochondrial compartment. Coordination
[*] Priv.-Doz. Dr. H . W. Heidt
Institut fur Physiologische Chemie und Physikalische
Biochemie der Universitat
8 Munchen 2, Pettenkoferstrasse 12-14a (Germany)
The author's own work in this field was supported by the Deutsche
Forschungsgemeinschaft.
[**I
A comprehensive article on the mechanism of oxidative phosphorylation has been published in this journal by Schatz [l].
792
of the metabolic processes requires the specific transport of
substrates and products through the membranes that
separate the two compartments. The present report is
concerned with the structure of the mitochondrial compartment. Details of specific transport processes between
the two compartments should provide an insight into the
regulation of the oxidative metabolism.
1. Morphological Structure of Mitochondria
Mitochondria were observed with the aid of the optical
microscope in the middle of the last centurylZ-sl, but a
preciseelucidation oftheirstructure(Fig. 1) became possible
only with the advent of electron microscopyr6-']. A particularly striking feature of mitochondria is the high membrane fraction. They have two types of membrane (Fig. 2),
which differ in their fine structure, lipid composition, and
protein content. The outer membrane surrounds the mitochondrion. The inner membrane extends more or less
densely packed in an endlessly folded or in a tubular
arrangement in the interior of the mitochondria. It has a
very large surface. The mitochondrial matrix that it surAngew. Chem. internat Edit
1 Vol. I1
(1972) / N o . 9

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