Effect of the replacement of a methyl by a trifluoromethyl group on the acid-base properties of pyrazoles

Experimental determinations have been carried out of the thermodynamic basicities and acidities in the gas phase and in aqueous solution of pyrazoles substituted at positions 3 and 5 with methyl and trifluoromethyl groups. Corresponding theoretical calculations at the STO-3G level have also been done. The results are highly consistent both between relative gas-phase properties and theoretical calculations (including STO-3G calculations of anions) and between gas-phase and aqueous solution values. The trifluoromethyl group is found to considerably decrease the basicity (more in position 3 than in position 5) and increase the acidity (more in position 5 than in position 3)