Kinetics of the gas‐phase reaction of acetone with iodine: Heat of formation and stabilization energy of the acetonyl radical

The kinetics of the gas-phase reaction CH3COCH3 + I2 ⇄ CH3COCH2I + HI have been measured spectrophotometrically in a static system over the temperature range 340–430°. The pressure of CH3COCH3 was varied from 15 to 330 torr and of I2 from 4 to 48 torr, and the initial rate of the reaction was found to be consistent with as the rate-determining step. An Arrhenius plot of the variation of k1 with temperature showed considerable scatter of the points, depending on the conditioning of the reaction vessel. After allowance for surface catalysis, the best line drawn by inspection yielded the Arrhenius equation, log [k1/(M−1 sec−1)] = (11.2 ± 0.8) – (27.7 θ 2.3)/θ, where θ = 2.303 RT in kcal/mole. This activation energy yields an acetone CH bond strength of 98 kcal/mole and δH (CH3COĊH2) radical = −5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary CH bond strength in isopropyl alcohol, there is no resonance stabilization of the acetonyl radical due to delocalization of the radical site. By contrast, the isoelectronic allyl resonance energy is 10 kcal/mole, and reasons for the difference are discussed in terms of the π-bond energies of acetone and propene.

[1]  U. V. Rao.  Platinum group metals and compounds : a symposium sponsored by the Division of Inorganic Chemistry at the 158th meeting of the American Chemical Society, New York, N.Y., Sept. 8-9, 1969 , 1971 .

[2]  D. Golden,et al.  Kinetic study of the gas‐phase reaction c‐C5H10 + I2 ⇄ c‐C5H8 + 2HI the heat of formation of cyclopentyl radical , 1970 .

[3]  S. Benson,et al.  Carbon–hydrogen bond strengths in methyl formate and the kinetics of the reaction with iodine , 1969 .

[4]  D. Golden,et al.  Absolute rate constants for radical-molecule reactions over a wide temperature range. The heat of formation and entropy of the methyl radical , 1969 .

[5]  D. Golden,et al.  Free-radical and molecule thermochemistry from studies of gas-phase iodine-atom reactions , 1969 .

[6]  J. Knox,et al.  Iodination of alkanes : ethane, propane and isobutane , 1967 .

[7]  A. Trotman‐Dickenson,et al.  Reactions of difluoroamino-radicals. Part III. Addition to olefins , 1967 .

[8]  J. Kerr.  Bond Dissociation Energies by Kinetic Methods , 1966 .

[9]  D. Golden,et al.  The Kinetics and Mechanism of the Reaction I2 + C3H6 →← C3H5I + HI and the Heat of Formation of the Allyl Radical1 , 1966 .

[10]  S. Benson,et al.  Iodine and Nitric Oxide Catalyzed Isomerization of Olefins. VII. The Stabilization Energy in the Pentadienyl Radical and the Kinetics of the Positional Isomerization of 1,4-Pentadiene1 , 1966 .

[11]  D. Golden,et al.  The Thermochemistry of the Gas Phase Equilibrium I2 + CH4 [UNK] CH3I + HI and the Heat of Formation of the Methyl Radical1 , 1965 .

[12]  S. Benson,et al.  Iodine and Nitric Oxide Catalyzed Isomerization of Olefins. V. Kinetics of the Geometrical Isomerization of 1,3-Pentadiene, a Check on the Rate of Rotation about Single Bonds, and the Allylic Resonance Energy1a , 1965 .

[13]  W. D. Walters,et al.  The Thermal Decomposition of Ethyl Cyclobutyl Ketone1 , 1964 .

[14]  W. D. Walters,et al.  The Thermal Decomposition of Cyclobutanecarboxaldehyde , 1962 .

[15]  R. K. Brinton.  The High Temperature Photolysis of Acetone , 1961 .

[16]  N. Müller,et al.  C13 Splittings in Proton Magnetic Resonance Spectra. II. Bonding in Substituted Methanes , 1959 .

[17]  W. D. Walters,et al.  The Thermal Decomposition of Methyl Cyclobutyl Ketone1,2 , 1958 .