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[摘要]：The C-H bond dissocn. enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New detns. are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chem. methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the a-C-H BDE in cyclopentene were also calcd. The s-homodesmotic model was used to det. SEs for both the parent mols. and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title mols. are explained by the calcd. SEs in the parent mols. and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 KJ mol-1 lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a sm aller eclipsing strain in the five-membered radicals relative to the parent mols. (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent mol., due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol-1 higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chem. insight afforded by group additivity methods in choosing the correct model for SE estn. is highlighted. |
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