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 isomerisation mechanisms. The three basic isotoluenes are "ortho"-isotoluene or "5-methylene-1,3-cyclohexadiene" (here labelled 1); "para"-isotoluene (2); and "meta"-isotoluene (3). Another structural isomer is the bicyclic compound "5-methylenebicyclo[2.2.0] hexene" (4). The "o"- and "p"-isotoluenes isomerise to toluene, a reaction driven by aromatic stabilisation. It is estimated that these compounds are 96 kJ mol less stable. The isomerisation of "p"-isotoluene to toluene takes place at 100 °C in benzene with bimolecular  reaction kinetics by an intermolecular free radical reaction. The intramolecular isomerisation, a 1,3-sigmatropic reaction, is unfavorable because an antarafacial mode is enforced. Other dimer radical reaction products are formed as well. The "ortho"-isomer is found to isomerise at 60 °C in benzene, also in a second order reaction. 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.