JOURNAL OF T H E A M E R I C A N CHEMICAL S O C I E T Y (Registered in U. S. Patent Office)
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Copyright, 19b2, by the American Chemical Society)
SEl’TEMBER 28, 1962
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N U M B E 1s R
PHYSICAL A N D I N O R G A N I C C H E M I S T R Y [CO~VTRIBUTIOX FROM THEGIBBSCI~EMICAL LABORATORY, HARVARD UNIVERSITY, CAMBRIDGE, MASS.]
The Reactions of Methylene.
VI.
The Addition of Methylene to Hydrogen and Methane
BY JERRY A. BELLAND G. B. KISTIAKOWSKY RECEIVED FEBRUARY 2, 1962 The initial step in the reaction of rncthylene with hydrogen and methane is pritnarily- an addition to form, respectively, vibrationally excited mctliarie a i d ethane. These activated molecules are either stabilized by collisions or dissociate t o give free radical fragiricnts. T h e activated methane dissociates t o a methyl radical and a hydrogen atom and the ethane to two methyl radicals. T h e reaction with hydrogen is approximately one-sixth as rapid as reaction with carbon-carbon double bonds. Calculations based on the mode of decay of the excited methane molecule give D(CH2-H) 2 104 kcal./mole. Comparison of the lifetime of the ethane formed from methylene and methane with that produced by association of methyl radicals yields a second value, D( CHZ-H) = 103 =t6 kcal./mole. Combination of the present results with others recently obtained spectroscopically and thcrmocheniically suggest D(CH2-H) = 105 j=3 kcal./mole and, hence, D(CH-H) = 108 f 3 kcal. /niole.
Introduction The mechanism of the reaction of methylene with hydrogen is unsettled. Channiugam and Burton’ (C and B) photolyzed ketene and deuterium mixtures as well as ketene and hydrogen. On the basis of mass spectrometric analyses of the products they concluded that both the direct addition to give methane and the abstraction of a hydrogen atom to give a methyl radical occur. At room temperature the abstraction reaction is negligible compared to the addition reaction. Gesser and Steacie2 (G and S), however, observed no methane a t room temperature in the photolysis of ketene and hydrogen mixtures. On the other hand, they observed the formation of methyl ethyl ketone a t higher temperatures where methane was also observed. They proposed an abstraction mechanism involving methyl radicals
_
+ + + + + + + +
+ + + + +
CH?CO hvCH? CO CH? CH2CO +CZHd CO CH2 HP --f CHa €I €-I C H X O +CHa CO CH:, ----f C ~ H ~ CH3 H? +CHI €I CHI CH&O +CH2COCH3 CH2COCH3 +CzHjCOCHz CI33 _ ~
(la) (2a) (sa) (4a) (5a) (ea)
(7) (8)
Chanmugam and Burton objecteds that they did not observe the formation of ethane-dl in ketenedeuterium-hydrogen photolyses. They thus argued against a process involving methyl radicals exrlusively. In a similar state of flux is the evidence concerning the mechanism of the reaction of methylene with methane. Grard and VanpCe4 photolyzed ketene in the presence of methane and found carbon monoxide, ethylene and ethane as the major products of the reaction. They proposed the mechanism
+ hv +CH? + CO + CHLCO--+ C ~ H I+ CO CH2 + CHI J_ CzHe
CHzCO CH?
(la)
(2a) (13a)
Reaction 13a was thought to go to an excited ethane which is either stabilized by collision or dissociates to give methylene and methane. A reaction t o give methyl radicals was ruled out as too endothermic. C and B used methane-d4 and concluded that reaction 13a does not occur. A better understanding of the mechanisms of these methylene reactions would facilitate the determination of the energies of consecutive dissociation of the bonds in methane
(1) J. Chanmugam and &I. Burtoti, J . A m . Chem. SOC.,1 8 , 509 (1’25.6). ( 3 ) 1%. Gesser and E , W. I
