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Author(s):   Al-Awadi, N.; Kaul, K.; El-Dusouqui, O. M. E.
Title:   Kinetics and mechanism of thermal gas-phase elimination of alpha-substituted carboxylic acids: role of relative basicity of alpha-substituents and acidity of incipient proton
Journal:   J. Phys. Org. Chem.
Volume:   13
Page(s):   499 - 504
Year:   2000
Reference type:   Journal article
Squib:   2000ALA/KAU499-504

Reaction:   2-(4-methylphenoxy)propanoic acid → CH3CHO + 4-Methylphenol + CO
Reaction order:   1
Temperature:   515 - 566 K
Rate expression:   2.75x1010 [s-1] e-140700 [±20.00 J/mole]/RT
Category:  Experiment
Data type:   Absolute value measured directly
Pressure dependence:   None reported
Experimental procedure:   Static or low flow - Data taken vs time
Excitation technique:   Thermal
Time resolution:   By end product analysis
Analytical technique:   Other
Comments:  
Results are from sealed tube experiments using ppm level substrate in solvent. Typical stated conditions are 0.2 ml acetonitrile solvent in 9 ml sealed tube. Authors do not indicate pressures, but the maximum calculated pressure (Ideal Gas Law) is about 19 bar. This is high enough that rates should be close to their high pressure limits.

Rate constants were determined by following disappearance of the starting substrate. Analytical techniques were said to involve GC-MS, FT-IR, and HPLC, but procedures are not clearly delineated. The arene product was identified in the pyrolysate.

The uncertainties are from the statistical analyses, but the degree of confidence or number of standard uncertainties is not specified.

The authors cite computational work from the literature indicating the overall reaction is a two step process, initially leading to a (substituted) phenol and alpha-propanoic acid lactone, followed by rapid decomposition of the latter compound to give CO and CH3CHO.


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Rate constant values calculated from the Arrhenius expression:

T (K)k(T) [s-1]
515 1.48E-4
525 2.76E-4
550 1.19E-3
566 2.82E-3