The enthalpies of creating a cyclohexene radical isomers are: ORTHO: 444 kJ/mol META: 361 kJ/mol PARA: 401 kJ/mol The meta-isomer is most stable (that is the reason for the formation of meta product in radical addition). Para/meta isomers energies seem obvious - both carbons are equivalent, only the meta-carbon participates in multiple bond, para-carbon doesn't. I can't see why the ORTHO-isomer is the least stable one. The (.CR3) radical should rather be more stable than (.CHR2). What do you think?