Table summarizes the pKaH+ data of different substituted benzimidazolium cations. Different resonance structures of benzimidazolium cation are as shown in Scheme 1 (structures A, B, C). The most stable structure of benzimidazolium cation is as shown in D due to its C-2 symmetry. The deprotonation equilibrium is shown in Scheme 2. The pKaH+ data is available in 5% aqueous ethanol at 30 o C for six substituted benzimidazolium cations with five substituents at meta and one substituent at para position (columns viii, ix, and x, Table ). With this data an excellent Hammett plot is obtained (figure ). values in pure water at 25 o C (figure ) and in 5% aqueous ethanol at 30 o C (figure ), the value in pure water (1.91) is higher than in 5% aqueous ethanol (1.19). This may be since the proton on nitrogen in benzimidazolium cation (scheme 1, structure D) becomes less susceptible for deprotonation in less nucleophilic 5% aqueous ethanol than more nucleophilic pure water.