Abstract

The MP2 and DFT/B3LYP methods at 6-311++G(d,p) and aug-cc-pdz basis sets have been used to probe the origin of relative stability preference for eclipsed acetaldehyde over its bisected counterpart. A relative energy stability range of 1.02 to 1.20 kcal/mol, in favor of the eclipsed conformer, was found and discussed. An NBO study at these chemistry levels complemented these findings and assigned the eclipsed acetaldehyde preference mainly to the vicinal antiperiplanar hyperconjugative interactions. The tautomeric interconversion between the more stable eclipsed acetaldehyde and vinyl alcohol has been achieved through a four-membered ring transition state (TS). The obtained barrier heights and relative stabilities of eclipsed acetaldehyde and the two conformers of vinyl alchol at these model chemistries have been estimated and discussed.

Highlights

  • Acetaldehyde (CH3CHO) was first synthesized by Scheele in 1774 [1]

  • EL is the total electronic energy resulting from the localized “natural Lewis structure” wave functions in which the orbitals are doubly occupied i.e., it is the energy obtained by deleting all non-Lewis orbitals from the basis sets. It is useful in studying hyperconjugative interactions that are used in the quantitative study of relative stabilities and their origins

  • The energy differences (ΔET) between their full electronic energies (1.02–1.21 kcal/mol) using these model chemistries are in fair agreement with that of 1.162 kcal/mol obtained by experiment [3] in favor of the eclipsed conformer

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Summary

Introduction

Acetaldehyde (CH3CHO) was first synthesized by Scheele in 1774 [1]. The importance of acetaldehyde lies in its usage as an intermediate in many organic reactions [2]. The internal rotation of C–O bond in acetaldehyde produces two stable conformers; namely the eclipsed and bisected forms. Kilb et al [3] reported an experimental internal rotation barrier of 1.162 kcal/mol in favor of eclipsed conformer over that of a bisected one. Holmes and Lossing [11] estimated the relative energy of acetaldehyde to vinyl alcohol of 41 ± 8 kJ/mol in favor of the former. 67.38 kcal/mol for the lower energy route of converting symmetrical acetaldehyde to vinyl alcohol. Their suggestion is in conflict with other routes deduced from experiment [13,14].

Relative Energy
Hyperconjugative Interactions
Geometry
Activation Energies and Relative Stabilities
Computational Methods
Conclusion

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