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OCR for page 65
HYDROCARBON FREE RADICALS IN PHOTOPROCESSES
HUGH S. TAYLOR
Department of Chemistry, Princeton University, Princeton, New Jersey
Received May 25, 1958
The communication by Leighton (10) on the mechanism of aldehyde
and ketone photolysis has discussed in detail one group of photoreactions
in which the primary process leads, at least in part, to the production of
hydrocarbon free radicals. In the present communication will be reviewed
a group of other photoprocesses in which the absorption of light also gives
rise to the production of hydrocarbon free radicals, from the secondary
reactions of which further data on the properties of such radicals and their
reactivities with various atomic and molecular species can be deduced.
From such studies are slowly accumulating a series of data, first qualita-
tive and later quantitative, with the aid of which a more certain interpreta-
tion of secondary reaction paths can be deduced.
The photoprocesses leading, in the primary process of absorption, to the
production of free radicals include the photolysis of alkyl iodides, the
photodecomposition of metal alkyls, the mercury-photosensitized hydro-
genations of unsaturated hydrocarbons, and the mercury-photosensitized
decompositions of saturated hydrocarbons. As yet, the majority of the
studies are confined to the simpler homologs of the several series of com-
pounds, and the radicals for which the studies are least equivocal are the
methyl and ethyl radicals.
THE PRIMARY PROCESSES
With the lighter alkyl iodides, which show regions of continuous absorp-
tion in the ultraviolet with well-developed band spectra of greater intensity
than the continua, beginning near 2000 A. and extending into the Schu-
mann region, it is quite generally postulated (2, 7, 8, 22, 24) that the
primary process leads to dissociation into a free radical and an iodine atom.
Thus, with methyl iodide we assume the formation of CH3 + I, and with
ethyl iodide the formation of C2Hs + I. There are no data which conflict
with this point of view. Differences in reaction product arising from such
photolyses are, as we shall see, to be ascribed to secondary processes rather
than to any other products of the primary absorption.
~ Contribution No. 6 to the Third Report of the Committee on Photochemistry,
National Research Council.
65
/
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66
HUGH S. TAYLOR
Of the metal alkyls the best investigated are those of lead and mercury.
The spectrum of mercury dimethyl shows diffuse bands below 2150 A.,
with an overlapping continuum which extends to about 2600 A. (12~.
Terenin and Prileshajeva (20) interpreted this absorption as leading to
primary dissociation into free radicals and detected such by their action
in removing metallic mirrors. Linnett and Thompson (12), after first
assuming that the major primary process was decomposition to mercury
and ethane, finally decided that the facts could be best interpreted by the
two possible free-radical decompositions
Hg(CH3~2 + he = Hg(CH3) + CH3
Hg(CH3~2 + he = Hg + 2CH3
Leighton and Mortensen (11) confirmed the observations of Terenin
(19) and of Duncan and Murray (5), that the absorption spectra of lead
tetramethyl, tetraethyl, and tetraphenyl were all continuous, the long
wave length limits of absorption by the vapors being 2800 A. for the methyl
and 3500 A. for the ethyl compound. Lead tetraphenyl in solution in
trimethylpentane gave an absorption limit around 2800 A. These con-
tinuous spectra point to dissociation in the primary absorption act, with
radicals or saturated molecules as possible products in addition to the metal
atoms or metal radical complexes.
In the mercury-photosensitized hydrogenation of unsaturated hydro-
carbons, when hydrogen is present in any marked amount, the primary
process occurs between excited mercury and molecular hydrogen genera-
ing atomic hydrogen. The free radical arises as a secondary reaction
between the atomic hydrogen and the unsaturated hydrocarbon. The
simplest radical so produced is the ethyl radical from ethylene. All the
recent evidence indicates that this is a process of good efficiency and that
the presence of ethylene serves to reduce the stationary state concentration
of atomic hydrogen to small values.
For the saturated hydrocarbons Bates (1) has shown that the quenching
efficiency of methane for excited mercury is very small, but that with the
higher homologs there is an increased quenching efficiency. The data of
Morikawa, Benedict, and Taylor (13) suggest that quenching of methane
even at room temperatures gives rise to CH3 + H either directly or by col-
lision with metastable mercury atoms. Indirect evidence suggests that
the dissociation process may require an activation energy of ~4.5 kg-car.
The efficiency increases with temperature.
A recent study of Steacie and Phillips (17) is concerned with the inter-
action, in a circulatory process, of excited mercury and ethane. They
reached the conclusion that the products of the primary interaction be-
tween excited mercury and ethane are two methyl groups. The reaction
C2H6 + Hg' = C2H5 + H + Hg
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FREE RADICALS IN PHOTOPROCESSES
67
was ruled out because of the absence of molecular hydrogen in the reaction
products. In the corresponding experiments with butane, Steacie and
Phillips found abundant production of hydrogen. More recent experi-
ments by these authors, not yet published, indicate that, in single-pass
experiments in place of a circulatory process, the formation of hydrogen
from ethane definitely occurs. This means that, with circulation, the
hydrogen formed is converted into atomic hydrogen by photosensitization
and is consumed in further reaction processes the nature of which will later
be discussed.
THE QUANTUM YIELD
The alkyl iodides show remarkably low quantum yields, especially in the
vapor state. Methyl iodide has a yield of 0.02 in terms of iodine atoms per
absorbed quantum in the gaseous state (2), and 0.05 in hexane solution in
the region of continuous absorption (25~. With ethyl iodide, the yield is
of the order of 0.01 at 2600 A., increasing to 0.1 at 2026 A. In the con-
tinuous region, liquid ethyl iodide and its solution in hexane show yields of
about 0.6, whereas at 2026 A. the yield has decreased to 0.24 (24~.
The low yield in the continuum points to dissociation followed by re-
co~nbination. This view is supported by recent experiments of West (23),
in which photolysis in the presence of silver foil as a trap for iodine atoms
increased decomposition fortyfold, with a marked change in the composi-
tion of reaction products (see later discussion). The solvent molecules,
in the experiments in the continuum, also-should repress dissociation,
according to the Franck-Rabinowitch principle, so that the large influence
of solvent on quantum yield needs special consideration. In the short
wave banded region, West and Ginsburg assume the production of opti-
cally excited molecules and their interaction with normal iodide molecules.
With lead tetramethyl vapor Leighton and Mortensen ;~11) found quan-
tum yields at 25°C. somewhat more in excess of unity (1.01-1.13) than
could be attributed to experimental error. The authors therefore sus-
pected the existence of short chains. Linnett and Thompson (12), with
mercury dimethyl, found a quantum yield of unity at room temperature,
but at higher temperatures the yield increased gradually to 2.2 at 190°C.
Cunningham (4) found a more than threefold increase in mercury dimethyl
vapor decomposed between 50° and 300°C. These results point to the
better propagation of chains with increase in temperature.
In the mercury-photosensitized hydrogenation processes the quantita-
tive extinction of mercury fluorescence by hydrogen is well known, and
hence the quantum yield of the total process is dependent on the efficiency
of the secondary processes. In the hydrogenation of ethylene the evidence
points to a 100 per cent utilization of the primary products in these second-
ary processes. In the mercury-sensitized decomposition of ethane, Steacie
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.
.
68
HUGH S. TAYLOR
and Phillips found a quantum yield of approximately 0.2 in terms of
ethane disappearing. This they ascribed, at least in part, to the in-
efficiency of the primary process of quenching by ethane. With butane a
higher quantum yield, 0.55, was obtained.
THE SECONDARY PROCESSES
The primary processes in the photolysis of methyl iodide, lead tetra-
methyl, mercury dimethyl and, according to Steacie and Phillips, in the
photosensitized decomposition of ethane, all lead to the production of
methyl radicals. There are, however, conspicuous differences in the
products finally obtained.
In the case of methyl iodide, methane comprises 80 per cent of the prod-
uct at room temperatures with from 4 to 12 per cent each of ethylene and
ethane, for reactions in quartz vessels, packed or unpacked. With silver
foil present, the methane yield fell to 28 per cent, while the ethylene and
ethane yields rose to 18 and 54 per cent, respectively, ethane becoming in
this case the major product. As much as 36 per cent of the methyl iodide
disappearing is recoverable as CH2I2. Ethane predominates almost to
the exclusion of other products in the photolyses of lead tetramethyl and
mercury dimethyl. Linnett and Thompson found from 7 to 10 per cent
of methane and O to 5 per cent of ethylene with about 90 per cent of ethane.
They do not record any variation in product with temperature change.
Cunningham found ethane with negligible amounts of methane and eth-
ylene from room temperatures to 160°C. Beyond this temperature the
methane yield increased to about 20 per cent of the hydrocarbon product
at 300°C. In the photolysis of acetone similar results obtain. At 70°
and 160°C. the hydrocarbon is more than 90 per cent ethane; at 300°C.
equal volumes of ethane and methane are formed. In the Steacie-Phillips
experiments with excited mercury any methyl radicals are formed in the
presence of excess ethane. Hence no conclusion as to the amount of
ethane which is regenerated can be given. Of the other hydrocarbons,
nearly 60 per cent is methane, 23 per cent propane, and nearly 20 per cent
butane. The butane undoubtedly arises from recombination of two ethyl
radicals, since this gas is the predominant product in the mercury-photo-
sensitized hydrogenation of ethylene (9~.
The low quantum yield and the abnormal products in the methyl iodide
decomposition are accounted for by West with the following sequence of
reactions:
CH3I + he = CH3 + I
CH3 + I = CH3I
CH3 + CH3I = CH4 + CH2I
(1)
(2)
(3)
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FREE RADICALS IN PHOTOPROCESSES
CH2I + I = CH2I2
CH2I + CH3 = C2H~I
69
(4)
(4a)
CH2I + CH3I = CH2I2 + CH3 (5)
CH2I2 + he = CH2 + Is
CH2 + CH2 = C2H4
CH2 + CH3I = C2H5I
CH3 + CH3 = C2H6
I+I+X= I2+X
(6)
(7)
(8)
(9)
(10)
It is evident that the recombination reaction (reaction 2) must be ex-
ceedingly efficient to account for the low quantum yield. All other proc-
esses are minor in comparison with processes 3, 4, and 5, the most impor-
tant to account for the products found. West estimates that an activa-
tion energy of 10 kg-car. for reaction 3 in competition with reaction 2 would
account for the observed quantum yield. In agreement with this estimate
are some measurements of Ginsburg (6), indicating an increase of quantum
yield with temperature in the case of ethyl iodide photodecomposition.
Reaction 2 must also be rapid in comparison with reaction 9 which forms
ethane. A steric factor of the order of 10-4 or 10-5 has been ascribed to the
association of two methyls by Bawn (3~. When, however, the iodine
atoms are trapped by silver foil, reaction 9 appears to compete favorably
with the methane-producing reaction (reaction 4~.
A sequence of reactions to account for the photolyses of mercury and
lead methyls is suggested by the following scheme of Thompson and
Linnett:
Hg(CH3~2 + hv = Hg(CH3) + CH3
Hg(CH3) + X = Hg + CH3 + X (2)
Hg(CH3) + Hg(CH3~2 = Hg + Hg(CH3) + C2H6
CH3 + Hg(CH3~2
CH3 + Hg(CH3~2
CH3 + CH3
Hg(CH3) + C2H6
Hg + CH3 + C2H6
C2H6
Hg(CH3) + Hg(CH3) = 2Hg + C2H6
In these cases the recombination process
(3)
(4)
(5)
(6)
(7)
HgCH3 + CH3 = Hg(CH3~2
must be of lower probability than in the case of the iodide in view of the
quantum yield of ~1. That it occurs to some extent is known from
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HUGH S. TAYLOR
work of Paneth and others on the removal of mirrors by methyl radicals.
Taking this into account, the quantum yield of unity at room tempera-
tures signifies that some chain propagation is already occurring at these
temperatures. Reactions 3, 4, and 5 are chain-propagating stages. They
must possess an activation energy, otherwise a quantum yield of ~ 106
might be expected. Thompson and Linnett calculate that an activation
energy of ~11 kg-car. for the chain-propagating reactions is in accord
with the quantum yield of unity at ordinary temperatures and of 2.2 at
200°C. The presence of only traces of methane in the room temperature
product indicates that, owing to the weakness of the Hg C bond, this is
broken in preference to the C H bond in the mercury alkyls struck by
methyl radicals. Since about 20 per cent of the hydrocarbon is found as
methane at 300°C. this suggests that the breaking of the C H bond must
have an activation energy some 2 kg-car. higher than that required for the
breaking of the Hg C bond.
Molecular hydrogen does not readily react with methyl radicals until
temperatures of 160°C. and upwards are reached (13, 18, 21~. The activa-
tion energy of the process has a value of 9 at 2 kg-car. The interaction of
atomic hydrogen with methane is to be assigned a somewhat higher value
(16, 21) of 13 ~ 2 kg-car. and is insignificant below 250°C. Interaction
with ethane is much more easily obtained, some interaction occurring at
room temperatures. Trenner, Morikawa, and Taylor (21) ascribed this to
a reaction
H + C2H6 = CH4 + CH3
The argument developed by them to exclude a reaction sequence
H + C2H6 = C2H5 + H2
C2H5 + H = 2CH3
(1)
(2)
(3)
is not entirely compelling. It is well known that F. O. Rice's free-radical
mechanism (15) is incompatible with the ready occurrence of the first of
these interactions.
The absence of methane, in the photosensitized hydrogenation of eth-
ylene, until all the ethylene is hydrogenated, even though ethyl radicals
and ethane are present, may be due to the low stationary state concentra-
tion of atomic hydrogen in presence of ethylene. Careful test by Jungers
and Taylor (9) failed to reveal any significant amounts of methane with
reaction in vessels kept carefully free from hydrocarbon deposits of higher
molecular weight. Under these conditions also, the predominant product
is butane, obviously by combination of ethyl radicals. The low relative
production of ethane or propane is additional evidence of low hydrogen
atom and methyl radical concentrations in such systems.
.
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FREE RADICALS IN PHOTOPROCESSES
71
In the photosensitized decomposition of ethane, Steacie and Phillips
now favor the C—H bond split by a reaction
Hg' + C2H6 = C2H5 + H + Hg
They favor the reaction
C2H6 + H = CH3 + CH4
(1)
(2)
to account for their observed methane formation. For the reasons just
discussed one is forced also to consider the possibility
C2H5 + H = 2CH3
(3)
as the source of methyl radicals, subsequently converted to methane and
propane by the recombination processes,
CH3 + H = CH4
CH3 + C2H5 = C3Hs
(4)
(5)
The recombination of ethyl radicals would produce the observed butane as
in the photosensitized hydrogenation of ethylene. Unless one accepts
reaction 3 as the mode of production of methyl radicals one is forced to
conclude, as Steacie has called to the attention of the writer, that reaction
2 must be at least four times faster than
H + C2H6 = C2Hs + H2 (6)
Otherwise hydrogen would not be consumed in the photosensitized inter-
action of hydrogen-ethane mixtures. The difficulties of the F. O. Rice
chain mechanisms (15) would once more be acute. Rice and Teller, from
a theoretical analysis, also strongly favor the mechanism. It is quite
evident that there is need for further study in this field.
Contrasting with the relatively large butane formation in the photo-
sensitization experiments are the products from the photolysis of ethyl
iodide vapor. Here, as West has shown, the products are predominantly
ethane and ethylene, with no butane and with minor amounts of hydrogen
and methane. As in the case of the methyl radicals from methyl iodide it
is the secondary processes which must account for the non-formation of
butane.
The sequence of reactions suggested by West is:
C2H5I + he = C2H5 + I
(1)
C2H5 + I = C2H5I (2)
C2H5 + C2H5I = C2H6 + C2H4I (3)
C2H4I + I = C2H4I2 (4)
C~H4 I = C2H4 + I (4a)
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HUGH S. TAYLOR
C2H4I + C2H5I = C2H4I2 + C2H5 (I)
C2H4I2 + he = C2H4 + I2
I+I= I2
(6)
(7)
Of these reactions 3, 4, 4a, and 6 are the essential secondary reactions which
account for the decomposition. The reactions must occur more readily
than the methyl reactions, since even in presence of silver foil no butane
was observed.
The increased yield in the photolyses of the alkyl iodides in the liquid
or dissolved state over that in the gaseous state is ascribed by West, at
any rate in part, to the action of the solvent molecules in providing third
bodies for the yield-increasing radical associations. This influence must
be exercised preferentially on the atom reaction I + I which certainly
requires a third body, whereas the radical recombinations and CH3 + I
and C2H5 + I probably occur, at least in part, as association reactions.
The interactions of the radical-iodine atom systems must be more efficient
than the radical recombinations as the quantum yield is so low. West
has shown that an inert gas such as carbon dioxide at a pressure of 10 to 40
atm. exercises an influence similar to solvent molecules in raising the
quantum yield, a pressure of 40 atm. exercising an eEect approximately
13.5 per cent of that obtaining in hexane solution.
In addition to the influence of solvent as third body in recombination
processes there is a possible influence due to secondary reaction with the
radicals. Recent work of Norrish and Bamford (14) has shown that
radicals from the photolysis of ketones may remove hydrogen atoms from
saturated hydrocarbon solvent molecules, becoming saturated thereby and
producing, ultimately, unsaturation in the solvent. This possible effect
has not yet been studied in alkyl iodide systems.
REFERENCES
(1) BATES: J. Am. Chem. Soc. 52, 3825 (1930~; 64, 569 (1932~.
(2) BATES AND SPENCE: J. Am. Chem. Soc. 63, 1689 (1931~.
(3) BAWN: Trans. Faraday Soc. 31, 1536 (1935~.
(4) CIJNNINGEAM: Ph.D. Thesis, Princeton University.
(5) DUNCAN AND MURRAY: J. Chem. Phys. 2, 640 (1934~.
(6) GINSBURG: Thesis, New York University, 1934.
(7) GOODEVE AND PORRET: Trans. Faraday Soc. 33, 690 (1937~.
(8) IREDALE: J. Phys. Chem. 33, 690 (1929~.
(9) JUNGERS AND TAYLOR: J. Chem. Phys. 6, 325 (1938~.
(10) LEIGETON: J. Phys. Chem. 42, 749 (1938~.
(11) LEIGE[TON AND MORTENSEN: J. Am. Chem. Soc. 68, 448 (1936).
(12) LINNETT AND THOMPSON: J. Chem. Soc. 1934, 790; Proc. Roy. Soc. (London
A160, 603 (1935~; 166, 108 (1936~; Trans. Faraday Soc. 33, 501, 874 (1937~.
(13) MORIKAWA, BENEDICT, AND TAYLOR: J. Chem. Phys. 6, 212 (1937~.
OCR for page 73
4~:
FREE RADICALS IN PHOTOPROCESSES
73
(14) NOURISH AND BAMFORD: Trans. Faraday Soc. 33, 1521 (1937~.
(15) Rice, F. O., AND Rice, K. K.: The Aliphatic Free Radicals, p. 191. The
Johns Hopkins Press, Baltimore (1935~.
(16) STEACIE: Can. J. Research 16, 264 (1937~.
(17) STEACIE AND PHILLIPS: J. Chem. Phys. 6, 179 (1938~.
(18) TAYLOR AND ROSENBLDM: J. Chem. Phys. 6, 119 (1938~.
(19) TERENIN: J. Chem. Phys. 2, 441 (1934~.
(20) TERENIN AND PRILESEAJEVA: Trans. Faraday Soc. 31, 1483 (1935).
(21) TRENNER, MORIKAWA, AND TAYLOR: J. Chem. Phys. 6, 203 (1937~.
(22) WEST: J. Am. Chem. Soc. 67, 1931 (1935~.
(23) WEST: J. Am. Chem. Soc. 60, 961 (1938~.
(24) WEST AND GINSBURG: J. Am. Chem. Soc. 66, 2626 (1934~.
WEST AND PACL: Trans. Faraday Soc. 28, 688 (1932~.
Addendum
The previous discussion might also have included the recent work from
the New York University laboratories on the photolysis of azomethane as
the source of free methyl radicals (Burton, Davis, and H. A. Taylor: J. Am.
Chem. Soc. 59, 1038, 1989 (19379; Davis, Jahn, and Burton: J. Am. Chem.
Soc. 60, 10 (1938~. The quantum yield of this photolysis has been care-
fully studied by Forbes, Heidt, and Sickman (J. Am. Chem. Soc. 57, 1935
(1935~) with six monochromatic radiations at four pressures from 180 to
665 mm. The quantum yield approached unity as its upper limit for
initial decomposition and a temperature increase from 20° to 226°C. had
no effect on the quantum yield. The latter fell with increasing pressure.
These results led the authors to the conclusion that the photolysis was not
a chain reaction.
Burton, Davis, and Taylor have made a careful analytical study of
products of the photolysis in the temperature range—22.5° to 223°C. In
every case there is an excess of nitrogen formed, about 55 per cent at room
temperatures rising to a maximum of 69 per cent at about 220°C. Meth-
ane in the hydrocarbon product is about 7 per cent by volume at room
temperature and increases to 70 per cent by volume at 220°C. Ethane,
which represents more than 90 per cent at room temperature, decreases
to 15 per cent at 220°C. In the higher temperature range propane and
possibly butane, in small amounts, are increasingly produced. Hydrogen
and unsaturated hydrocarbons are not formed in measurable amounts.
There is no doubt that the majority of these results are consistent with a
primary act producing free methyl radicals with minor, if any, intramolec-
ular rearrangement to form stable molecules. The change in the char-
acter of the hydrocarbon products with temperature is consistent with the
data on the reactions already discussed. In the case of the azomethane
photolysis, however, there is quite evidently a more marked interaction
between the free radicals and the azomethane, presumably to yield hydra-
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74
HUGH S. TAYLOR
zincs, than is the case in several of the other substances described. The
overall reaction becomes correspondingly more complex, the authors
being of the opinion that the ethane formed results, not from the recom-
bination of the methyl radicals, but frown decomposition of the more
complex radicals and molecules. In view of the preceding discussion this
conclusion should be accepted with great reserve.
Representative terms from entire chapter:
methyl radicals
