Why Cyclohexatriene (C₆H₆, pK_a = 43) is Less Acidic than Cyclopentadiene (C₅H₆, pK_a = 15) and Cycloheptatriene (C₇H₈, pK_a = 36): A Freshmen Chemical Education Undergraduate Exercise

Sanjeev Rachuru¹, Devarakonda A. Padmavathi² 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:
Sanjeev Rachuru, Devarakonda A. Padmavathi and V. Jagannadham. Why Cyclohexatriene (C₆H₆, pK_a = 43) is Less Acidic than Cyclopentadiene (C₅H₆, pK_a = 15) and Cycloheptatriene (C₇H₈, pK_a = 36): A Freshmen Chemical Education Undergraduate Exercise. World Journal of Chemical Education. 2023; 11(1):1-3. doi: 10.12691/wjce-11-1-1

Abstract

The pK_a of Cyclohexatriene (Benzene) is 43. And that of Cyclopentadiene and Cycloheptatriene are 15 and 36 respectively. In the ascending order of number of carbon atoms of the three cyclic hydrocarbons, Cyclohexatriene lies between Cyclopentadiene and Cycloheptatriene. It is a tempting belief of undergraduate chemists who begin to pursue their undergraduate course that the pK_as will be in the same increasing order with increase in ring size. But surprisingly pK_a of Cyclohexatriene is more than that of either Cyclopentadiene and or Cycloheptatriene. Suitable explanations are given.

Keywords:
Cyclohexatriene Cyclopentadiene Cycloheptatriene and pK_a

This work is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Figures

References:

[1]	O. W. Webster, J. Am. Chem. Soc., 88, 1966, 3046
[2]	E. P. Serjeant and B. Dempsey (eds.), Ionization Constants of Organic Acids in Solution, IUPAC Chemical Data Series No. 23, Pergamon Press, Oxford, UK, 1979.
[3]	O. A. Reutov, I. P. Beletskaya, K. P. Butin, CH—Acids: A Guide to All Existing Problems of CH-Acidity with New Experimental Methods and Data, Including Indirect Electrochemical, Kinetic and Thermodynamic Studies, Pergamon Press LtD., First Edition 1978, Page 15.
[4]	Francis A. Carey and Richard J. Sundberg, Advanced Organic Chemistry: Part A: Structure and Mechanisms, 5th Edition, By Springer Science & Business Media, 2007, Chapter 1, page 12.
[5]	Roland Schmid and Arzu M. Miah, J. Chem. Edu. 2001, 78, page 116.
[6]	Erich Hückel, Zeitschrift für Physik, 1931, 70, 204-286.
[7]	R. Trachuk and C. C. Lee, Can. J. Chem., Vol. 37 (1050), 1644
[8]	Roberts, J. D., Streitwieser, A, JR., and Regan, C. M., J. Am. Chem. Soc., 74, 4579 (1952)
[9]	James Ashenhurst in the link https://www.masterorganic chemistry.com/2017/03/03/aromatic-antiaromatic-nonaromatic-some-practice-problems/#:~:text=The%20easiest%20example% 20to%20start,%2B%200%20%3D%206%20pi%20electrons.
[10]	Organic Chemistry Reagent Guide,  CreateSpace Independent Publishing Platform, 2013, 9781482523287 (ISBN10: 1482523280).
[11]	Ouellette, Robert J. (2015). Principles of Organic Chemistry, Amines, and Amides., 315-342.
[12]	Erich Hückel, “Grundziige der Theorie ungesattigter und aromatischer Verbindungen,” Verlag Chemie, Berlin, 1938, pp. 71-85.