Homogeneous, unimolecular, gas-phase elimination of leaving groups at the alkoyl side of carboxylic acids

Article abstract of DOI:10.1002/(SICI)1099-1395(199606)9:6<348::AID-POC794>3.0.CO;2-C

The molecular gas-phase elimination kinetics of the series Cl(CH₂)_nCOOH (n = 1-4), show changes in mechanisms from polar five-centered intramolecular displacement of the Cl leaving group by the acidic hydrogen of the COOH to neighboring group participation of the oxygen carboyl of the COOH group. The mechanisms for the series 2-, 3- and 4-chlorobutyric acids are explained similarly as above. The leaving chloride at the 2-position of acetic, propionic, and butyric acids is displaced by the hydrogen of the COOH group through a prevaling path of a five- centered cyclic transition-state mechanism. This type of mechanism is also described for the pyrolysis of 2-hydroxy-, 2-alkoxy-, 2-phenoxy-, and 2-acetoxycarboxylic acids. The ease with which the groups at the 2-position of acetic and propionic acids are displaced by the H of COOH give rise the sequences AcO > OH > PhO > EtO > MeO > Cl and AcO > PhO > Br > EtO > MeO > MeO > OH > Cl, respectively. These two sequences differ only in the OH leaving group position. Additional work on glycolic acid pyrolysis is needed to explain the above differences.