Isotoluene

The Isotoluenes in organic chemistry are the non-aromatic toluene isomers with an exocyclic double bond. They are of some academic interest in relation to aromaticity and isomerization mechanisms ["Radical production from the interaction of closed-shell molecules. 10. Chemistry of methylenecyclohexadiene and the thermal polymerization of styrene" W. David Graham, John Glass Green, and William A. Pryor J. Org. Chem.; 1979; 44(6) pp 907 - 914; DOI|10.1021/jo01320a003.] ["Bimolecular reactions of 3-methylene-1,4-cyclohexadiene (p-isotoluene), 5-methylene-1,3-cyclohexadiene (o-isotoluene), 1-methylene-1,4-dihydronaphthalene (benzo-p-isotoluene), and 9-methylene-9,10-dihydroanthracene (dibenzo-p-isotoluene)" Joseph J. Gajewski and Andrea M. Gortva J. Org. Chem.; 1989; 54(2) pp 373 - 378; DOI|10.1021/jo00263a021]

The three basic isotoluenes are ortho-isotoluene or "5-methylene-1,3-cyclohexadiene." 1, para-isotoluene 2 and meta-isotoluene 3. One more isomer is the bicyclic compound "5-methylenebicyclo [2.2.0] hexene" 4.

The o- and p-isotoluenes easily isomerize to toluene, a reaction driven by aromatic stabilization. It is estimated that these compounds are 23 kcal/mol less stable.

The isomerization of p-isotoluene to toluene takes place at 100 °C in benzene with bimolecular reaction kinetics by an intermolecular free radical reaction. Other dimer radical reaction products are formed as well.

The ortho isomer is found to isomerize at 60°C also in a second order reaction in benzene. The proposed reaction mechanism is a concerted intermolecular ene reaction. The reaction product is either toluene or a mixture of dimerized ene reaction products depending on the exact reaction conditions. Ortho isotoluene has been researched in connection with the mechanism of initiator-free polymerization of polystyrene.

ee also

* Pentacene

References