NoteTaking_C5: Computational Organic Chemistry

CHAPTER 2: FUNDAMENTALS OF ORGANIC CHEMISTRY (2)

1. Origin of the Acidity of Carboxylic Acids: Resonance vs Electrostatic/Inductive Effect

Electrostatic/Inductive Effect: Carbonyl group polarized the O-H bond, making the hydrogen more positively charged than the hydrogen of an alcohol.
Resonance Effect: The electrons and the nuclei reorganized to reproduce the ground state of anion.

Two theoretical studies supported the notion of importance of inductive effect:

(1). Rablen, P. R., “Is the Acetate Anion Stabilized by Resonance or Electrostatics? A Systematic Structural Comparison,” J. Am. Chem. Soc., 122, 357–368 (2000).

(2).  Holt, J. and Karty, J. M., “Origin of the Acidity Enhancement of Formic Acid over Methanol: Resonance versus Inductive Effects,” J. Am. Chem. Soc., 125, 2797–2803 (2003).

Both works proposed a clever way to partition the contribution of resonance energy and electrostatic energy.

2. RSE of Cyclopropane and Cyclobutane 

Ring strain energies of cycloporpane (27.6 kcal/mol) and cyclobutane (26.2 kcal/mol) are surprisingly similar.
The reason is not clear yet.

Various proposed contritions to RSE:
a. Baeyer strain (C-C-C)
b. Pitzer strain  (C-H)
c. Dunitz-Shomaker strain (1,3-CC repulsions)
d. re-hybridization (CH strengthening in cyclopropane)
e. $\sigma$-aromaticity (not well defined)