NoteTaking_C6: Computational Organic Chemistry

CHAPTER 2: FUNDAMENTALS OF ORGANIC CHEMISTRY (2)

How to calculate ring strain energy (RSE)?

Take cyclobutane as an example, E(RSE)=E(cyclobutane)-E(CH₂CH₂CH₂CH₂).

Then what is E(CH₂CH₂CH₂CH₂)? 

E(CH₂CH₂CH₂CH₂)=4E(CH₂), where E(CH₂) is the energy of unstrained methylene group.

Using experimental gas-phase heats of formation:

E(CH₂)=$\Delta H_f(n{\rm{-hexane}})-\Delta H_f(n{\rm{-pentane}})$

Then E(RSE)=$\Delta H_f(\rm{cyclobutane})$-4E(CH$_2$)

Using quantum chemistry method:

"choose molecule(s) that can serve as reference(s) for the strained species"

(1) Isodesmic reaction:conserves the number of bonds of a given formal type.

(2) Homodesmotic reaction: conserves (1) the number of carbon (and other heavy) atoms and their state of hybridization and (2) the number of carbon (and other heavy) atoms bearing H atoms. 

(3) Group equivalent reaction: every equivalent group in the cyclic molecule must be paired with an equivalent group in a short acyclic molecule in the product.

Bachrach, S. M., “The Group Equivalent Reaction: An Improved Method for Determin- ing Ring Strain Energy,” J. Chem. Ed., 67, 907–908 (1990).