Factors controlling retention of amidines on silica gel TLC plates

R-F values were determined at 37 degrees C and with six eluents for over 10 0 amidines of general formula RR'N-CH= N-R-X, The compounds were divided in to four series depending on the substituents at the amino (N-1) nitrogen at om (RR'N group) and containing identical set of substituents R-X at the imi no (N-2) nitrogen atom. The R-F values of N-2-arylamidines correlate well w ith their pK(a) values or Hammett type constants of substituents on the phe nyl ring at the imino nitrogen atom. The differences between experimental a nd calculated R-F values are within acceptable agreement, The slopes of the correlation lines depend on substitution at the amino (N-1) nitrogen atom. For strongly basic compounds, such as amidines, the main factor determinin g their retention on silica gel is interaction of basic centers of the mole cule with silica acidic centers. The retardation factor (R-F) is predictabl e but is not an additive property. The changes of R-F caused by introductio n of a substituent into the molecule depend on the retention of the unsubst ituted compound.