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Why the Second Bond Exceptionally Effective on pKa Values of -ynoic (-C≡C-) Compounds? Chemical Education Perspective

R. Sanjeev, D. A. Padmavati, V. Jagannadham*

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

Abstract: Sudden decrease of nearly more than two orders of magnitude of pKa units of ynoic series of compounds is explained based on the electronegativity differences of sp, sp2 and sp3 carbons and on the subsequent stability of the corresponding anions. Both resonance and inductive effects operate in tandem to stabilize the ynoic anions.

Keywords: Ynoic compounds, chemical education, graduate research; pH and pKa

Received October 05, 2021; Revised November 07, 2021; Accepted November 15, 2021

Citation: R. Sanjeev, D. A. Padmavati, V. Jagannadham, Why the Second Bond Exceptionally Effective on pKa Values of -ynoic (-C≡C-) Compounds? Chemical Education Perspective. World Journal of Chemical Education, 9 (3). 94-95. January 2021. doi:10.12691/wjce-9-3-5

1. Introduction

Study of acidities of carboxylic acids, alcohols, hydrocarbons, amines, and any of the compounds that have dissociable hydrogen in terms of pH and pKa values is a common practice in an undergraduate and graduate research program. pH is nothing but the negative logarithm of hydrogen ion concentration introduced by Sørensen in 1909 [1]. pKa is a property that is used to quantify the strength of an acid.

2. Methods

The chemical structures are drawn using chemdraw. The collection of pKa values of the compounds are from references [2,3].

3. Discussion

Table 1 is the summary of the pKa and Ka data of series of oxygen, carbon and nitrogen acids and their corresponding unsaturated derivatives with a double and triple bond respectively. Looking at the pKa values of the carboxylic acids 1-6 (Table 1) immediately comes to the mind that the first two members of these acids 1 and 2; and 4 and 5 both the series have comparable pKas. They differ in their acidities (ratios of Ka) by a factor of only 1.66 and 3.16 respectively. But addition of the second π-bond in the acids make them stronger by a factor of 148 and 245 times, and 230 and 725 times than their corresponding saturated acid and the acid with a double bond respectively. This sudden increase in acidity of the acids 3 and 6 with additional π-bond is explained in terms of the different hybridizations of the carbons adjacent to the -COOH group i.e., the second blue carbon In acids 1 and 4 those carbons are sp3 hybridized, in acids 2 and 5 they are sp2 hybridized and in acids 3 and 6 they are sp hybridized. And the number of pi-electrons participating in delocalization in ynoic acids is double the number contributing extra stability of the corresponding carboxylate ion.

The ‘s’ character of carbon in sp3 hybridization is 25%, in sp2 hybridization it is 33.33% and sp hybridization it is 50% respectively. And it is known that the electronegativity of sp carbon > sp2 carbon > sp3 carbon [4]. The more is the electronegativity of a species higher would be its capacity to attract electrons. The increase in percentage ‘s’ character from sp3 carbon to sp2 carbon is 8.33 and that of sp2 carbon to sp carbon it is 16.67. It is to be observed that by addition of a second π-bond the increase in percentage ‘s’ character is twice due to the first π-bond. This makes the sp carbon (in blue) of structure C of scheme 1 more and more electronegative. This makes the carboxylate anions of ynoic acids more and more stable. Hence the acids 3 and 6 (Table 1) are much stronger than the acids 1 and 2; and 4 and 5 respectively. Looking at the carboxylate anion structures, it will be clear that the -COO⁻ group is more stabilized by a sp carbon because the linear structure and cylindrical geometry of the triple bond greatly helps the delocalization (scheme 2, structure C). Further both resonance and inductive effects operate in tandem to stabilize the 3-phenylpropiolic acid and propiolic acid carboxylate anions.

Table 1. pKa data of the compounds

Scheme 1.

Scheme 2.

Same trends are observed in alcohols, hydrocarbons, and amines in their acidities (Table 1).

4. Conclusions

The second π-bond of -ynoic (-C≡C-) compounds is exceptionally effective on pKa values.

Acknowledgements

We thank Prof. P S N Reddy (Retired) PhD FRSC, former Head and Chairman, Board of Studies in Chemistry, Osmania University for his valuable suggestions.

References

[1]   | Sørensen, S. P. L. (1909). Biochem. Z. 21: 131-304.
[2]   | pKa values are from Lange’s Handbook of Chemistry, Ed. by John A. Dean, 15th Edition, McGraw-Hill, Inc. New York, 1999.
[3]   | Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 5-88.
[4]   | J. March, Advanced Organic Chemistry, Reactions, Mechanisms, and Structure, 4th Edition, Wiley Interscience, 1992, page 269.