Selective alpha-chlorination of propanoic acid to form 2-monochloropropanoi
c (MCA) and 2,2-dichloropropanoic acid (DCA) was investigated in a laborato
ry-scale, semibatch reactor at 90-130 degrees C at atmospheric total pressu
re and in the presence of chlorosulfonic acid (ClSO3H) and 2,2-dichloroetha
noic acid (DCA') as catalytic agents and oxygen as a radical scavenger. The
decomposition of the catalyst was investigated with sulfur analysis and UV
-spectrometry, The studies revealed that the majority of sulfur remains in
the reaction mixture, but is converted to an inactive form during the chlor
ination. The reasons may be the decomposition of ClSO3H and its reaction wi
th propanoic acid. The kinetic experiments revealed autocatalytic and paral
lel formation of MCA and DCA, the selectivity being independent of Cl-2 con
centration in the liquid phase. The experiments with DCA' also demonstrated
that DCA' has a catalytic effect on the chlorination The experiments confi
rmed the validity of a previously proposed reaction scheme for alpha-chlori
nation, which comprises the formation of the reaction intermediate (propano
yl chloride) from propanoic acid and ClSO3H, the acid-catalyzed enolization
of the acid and a hydroxyl-chlorine exchange reaction. The acid-catalyzed
enolization is the rate determining step in the reaction sequence, The kine
tic data were fitted to rate equations based on the reaction scheme. (C) 20
00 Society of Chemical Industry.