Rep:Transition States and Reactivity Exercise3 ZWL115

Exercise 3

Reaction Scheme

O-Xylylene can react with SO₂ via 2 different Diels-Alder cycloadditions and a chelotropic pathway. These pathways are illustrated in Figure. 5.

Figure 5. Reaction scheme showing the Exo, Endo Diels-Alder cycloadditions and the Chelotropic reaction mechanisms possible.

Reactants	Transition States	Products
SO2	Endo	Endo
Xylylene	Exo	Exo
	Chelotropic	Chelotropic

(Please check and understand the JMol code that you're using. You're asking for a particular MO number when you mean frame number Tam10 (talk) 12:10, 7 March 2018 (UTC))

Reaction Coordinate

IRC pathways

Endo	Exo	Chelotropic

The instability of Xylylene can be rationalised using Hückel's Molecular Orbital Theory. According to the theory, a compound is stable when all of its bonding orbitals are filled and no electrons fill the anti-bonding orbitals. This is especially true in aromatic compounds, where 2 electrons fill the lowest energy molecular orbital and 4 electrons fill the subsequent degenerate pair of orbitals (the number of pairs of orbitals is denoted by the by n). This gives rise to Hückel's rule where aromatic compounds have 4n+2 π electrons which are in a conjugated ring system and are highly stabilised. On the contrary, compounds with 4n π electrons show anti-aromatic behaviour and are highly unstable due to the filling of anti-bonding orbitals.⁹ O-Xylylene only has 4π electrons in the ring resulting in anti-aromatic instability due to filling of the anti-bonding orbitals. During the course of the reactions, the 6-membered ring of Xylylene becomes a stabilised benzene ring in all 3 pathways as seen in the IRCs. The benzene ring has 6π electrons in a conjugated ring system which obeys Hückel's rule (where n = 1), making it aromatic. Hence the resulting products are more stabilised and the reactions occur favourably.

Gibbs Free Energies

	Xylylene	Sulfur Dioxide	Exo Transition State	Endo Transition State	Chelotropic Transition State	Exo Product	Endo Product	Chelotropic Product
Energy/Hartrees	-0.119269	0.178119	0.092077	0.090559	0.099062	0.021452	0.021697	-0.000018
Energy/kJmol-1	-313.141	467.651	241.748	237.762	260.087	56.3222	56.9654	-0.04726

Energy Profile Diagram

Figure 6. Energy Profile Diagram showing the 3 different reaction pathways for the reaction between Xylylene and SO₂

Diels-Alder reaction with second cis-butadiene fragment

Reaction Scheme

Figure 7. Reaction scheme showing the Exo and Endo Diels-Alder cycloadditions mechanisms possible.

Jmol Files

Reactants	Transition States	Products
SO2	Endo	Endo
Xylylene	Exo	Exo

Reaction Profile Diagram

Figure 8. Energy Profile diagram showing the thermodynamics of the 2 Diels-Alder cycloaddition pathways

The Endo and Exo Diels-Alder reactions for this cis-butadiene fragment is thermodynamically and kinetically unfavourable. Comparing Figures 6 and 8, the activation energies for both the exo and endo Diels-Alder cycloadditions at this site are much greater than those of the terminal butadiene fragment. Furthermore, the products of the reactions for the non-terminal cis-butadiene fragment are much higher in energy than those for the terminal cis butadiene fragment. The overall reaction for both endo and exo pathways are endothermic in this case. The kinetic and thermodynamic consequences can be rationalised by the greater steric clash present in the transition state and the products when SO₂ reacts with the non-terminal cis-butadiene fragment which are absent with the terminal cis-butadiene fragment.

References

9. Aromaticity and the Hückel 4n + 2 Rule. Chemistry Libretexts.