Basicity Constants (pKBH+) of 5-Substituted 2-Furaldehydes

The protonation equilibrium of 5-substituted 2-furaldehydes is investigated spectrophotometrically in aqueous sulfuric acid at 25 o C and the basicity constants(pK BH+) of the substrates is calculated by means of the excess acidity method. The basicity constant of 5-metyl-2-furaldehyde having electron donating group is larger than that of 5- nitro-2-furaldehyde having electron withdrawing group. Difference between the basicity constants(pK BH+) of these two compounds was about 3.25 pK unit. The m* value which is the degree of solvation of the protonated substrate is similar to that of acetophenone having same protonation site. The dependence of pK BH+ on m* value shows good linear cor- relation.