Attenuation Effect in Twenty One Different Proton Dissociation Equilibriums Brought on One Rope: A Chemical Education Tool for Evaluation of pK_a of Proton Dissociation Equilibrium of Any Substituted Benzene (XC₆H₅)

R. Sanjeev¹ and V. Jagannadham^2,

¹Department of Chemistry, Geethanjali College of Engineering and Technology, Cheeryal-501301, Telangana, India

²Department of Chemistry, Osmania University, Hyderabad-500007, India

Cite this paper

R. Sanjeev and V. Jagannadham. Attenuation Effect in Twenty One Different Proton Dissociation Equilibriums Brought on One Rope: A Chemical Education Tool for Evaluation of pK_a of Proton Dissociation Equilibrium of Any Substituted Benzene (XC₆H₅). World Journal of Chemical Education. 2020; 8(2):61-66. doi: 10.12691/WJCE-8-2-1

Abstract

The strong empirical relation, ρ = (2.4)^(2-i), between the Hammett ρ for proton dissociation of several acids and the number, “i”, of atoms between the ionizable hydrogen and the ring carbon (Andrew Williams, Free Energy Relationships in Organic and Bioorganic Chemistry, Royal Society of Chemistry, Cambridge, 2003, p. 75) is used to construct a graph for twenty one different proton dissociation equilibriums. The plot of Hammett ρ versus number of atoms i between ionizable hydrogen and the ring carbon atom is observed to be an excellent exponential-decay locus. A good average and intelligent value of Hammett ρ is obtained for the benzene dissociation equilibriums by interpolating the locus of the correlation on to Y-axis. Using this Hammett ρ value and the Hammett equation log [(K_a)_X / (K_a)_H] = ρσ, the pK_a value can be calculated for any substituted benzene knowing the pK_a value of benzene to be 43. The points for proton dissociation equilibriums of phenylethyl ammonium ions and benzyl alcohols deviated from the graph hence not included in the correlation. Possible explanations are given for deviation of these two equilibriums.

Keywords

attenuation effect, linear Free Energy Relationships, methylene group, acid dissociation equilibriums, pK_a of substituted benzenes, Andrew Williams’ empirical relation

Copyright

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References

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