page_title: Cheletropic reaction
section_title: Solvent effects
hierarchical_section_title: Cheletropic reaction / Cheletropic Reactions Involving SO2 / Solvent effects
caption_attribution_description: English: Picture for solvent effect section
context_page_description: Cheletropic reactions are a type of pericyclic reaction where one atom on one of the reagents gets two new bonds. A pericyclic reaction is one that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals. A reorganization of σ and π bonds occurs in this cyclic array.
  Cheletropic reactions are a subclass of cycloadditions. What sets cheletropic reactions apart is that on one of the reagents, both new bonds are being made to the same atom. A few examples are shown to the right in Figure 1. In the first case, the single atom is the carbon atom in the carbonyl group. That carbon atom ends up in carbon monoxide. The end result is making two new bonds to one atom. The first two examples are known as "cheletropic extrusions" because a small stable molecule is given off in the reaction. The driving force for these reactions is often the entropic benefit of releasing a gas.
context_section_description: The effect of the solvent of the cheletropic reaction of 3,4-dimethyl-2,5-dihydrothiophen-1,1-dioxide (shown at right) was kinetically investigated in 14 solvents. The reaction rate constants of the forward and reverse reaction in addition to the equilibrium constants were found to be linearly correlated with the ET(30) solvent polarity scale.
  Reactions were done at 120 °C and were studied by 1H-NMR spectroscopy of the reaction mixture. The forward rate k₁ was found to decrease by a factor of 4.5 going from cyclohexane to methanol. The reverse rate k₋₁ was found to increase by a factor of 53 going from cyclohexane to methanol, while the equilibrium constant Keq decreased by a factor of 140. It is suggested that there is a change of the polarity during the activation process as evidenced by relationships between the equilibrium and kinetic data. The authors say that the reaction appears to be influenced by the polarity of the solvent, and this can be shown by the change in the dipole moments when going from reactant to transition state to product. The authors also state that the cheletropic reaction doesn’t seem to be influenced by either solvent acidity or basicity.
  The results of this study lead the authors to expect the following behaviors:
  1. The change in the solvent polarity will influence the rate less than the equilibrium.
  2. The rate constants will be characterized by opposite effect on the polarity: k₁ will slightly decrease with the increase of ET(30), and k₋₁ will increase under the same conditions.
  3. The effect on k₋₁ will be larger than on k₁.
caption: Cheletropic Reaction Studied in Various Solvents
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