Third Report of the Committee on Photochemistry Reprint and Circular Series of the National Research Council (1938) / Chapter Skim
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The Photolysis of Aldehydes and Ketones
The Photolysis of Aldehydes and Ketones
Pages 51-64
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From page 51... ...
In the lighter aldehydes there appears to be a region of diQuse bands or predissociation preceding the continuum and, in addition, the bands themselves appear to be underlaid with a continuum which gradually becomes stronger as wave length is decreased. Fluorescence, where observed, is most intense for absorption in the long wave length part of the band, but the observed limits of fluorescence do not necessarily agree with the observed limits of structure in absorption (14, 18, 19, 53~.
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From page 52... ...
II. THE PRIMARY PROCESS IN DECOMPOSITION Discrete bands together with fluorescence indicate the production of a relatively long-lived molecule on absorption at longer wave lengths, while the appearance of diffuse bands and continua indicate a dissociation process at shorter wave lengths, although, as the different types of absorption overlap, the resulting processes must also overlap.
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From page 53... ...
, the predominance of the dissociation into finished molecules, particularly at longer wave lengths, is in accordance with the smaller number of methyl radicals produced in acetaldehyde as compared with acetone (43) , the small number of hydrogen atoms produced by absorption in the predissociation region as compared with the continuum in formaldehyde (39)
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From page 54... ...
, vie., RCHO +hv- >R + CHO >RH + CO In the former case the increase in B with decreasing wave length would arise from the changing potential energy of the excited molecule and the resultant change in relative probability of transition into different unstable states; in the latter case, it would arise from a more rapid separation of the radicals with increasing energy absorbed, with resultant change in the secondary reactions. By extending the latter point of view the data thus far discussed are capable of explanation entirely on the basis of a primary dissociation into free radicals, followed by appropriate secondary reactions (8, 20~.
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From page 55... ...
Evidence that type II dissociation results in finished molecules is furnished by the observation of Glazebrook and Pearson (15) that free radicals are apparently produced only by type I decomposition, and by the fact that, in all cases, type II decomposition occurs without modification in solution (29, 32~.
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From page 56... ...
Recent analyses of Blacet and Volman (8) show that at room temper tures the gaseous products of acetaldehyde photolysis consist entirely hydrogen, methane, and carbon monoxide, with no ethane or ethylen The Hz/CO ratio decreases toward zero with increasing wave length al: increasing temperature above 30°C.
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From page 57... ...
57 With increasing temperature the yield of diacetyl decreases, disappearing entirely above 60°C., although diacetyl itself is stable at this temperature, while in the gaseous products the C2H6/CO ratio approaches unity and considerable quantities of methane appear. For absorption in the continuum ~ < 2900 A.)
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From page 58... ...
(d) Deactivation by collision or fluorescence may be a contributing factor following absorption at longer wave lengths (e.g., acetone at 3130 A.)
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From page 59... ...
Independence between yield and pressure at shorter wave lengths indicates that these efl ects are related to the production of activated molecules. The decrease in yield with increasing pressure for acetaldehyde may be interpreted as indicating collisional deactivation or the removal of activated molecules by polymerization.
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From page 60... ...
LEIGHTON IV. POLYMERIZATION Relatively little attention has been given to the photopolymerization of aldehydes and ketones, but the data available indicate that two distinct processes are concerned.
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From page 61... ...
Gorin has studied the photolysis of acetaldehyde, acetone, formaldehyde and methyl ethyr ketone in the presence of iodine vapor. It has been shown that a few tenths of a millimeter pressure of iodine molecules is sufficient to react with all the free radicals formed in the primary process.
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From page 62... ...
In the photolysis of acetone in presence of traces of molecular iodine no appreciable carbon monoxide formation was observed by Gorin below 60°C. Methyl iodide is formed, with a quantum yield of unity, at all wave lengths.
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From page 63... ...
With methyl ethyl ketone and iodine, Gorin's results indicate that not more than a few per cent of the total primary process produces saturated hydrocarbon and carbon monoxide; the radical-producing processes are therefore overwhelmingly predominant. With acetaldehyde and iodine no polymerization was found by Gorin, indicating that it is the free radicals which are largely responsible for the polymerization observed during photolysis in the absence of iodine.
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