However, as noted by Alabugin and co-workers, an exception to this relationship is dibenzocyclooctyne A8. This compound was designed under the premise that an increased number of sp 2 centers within the cyclooctyne ring would enhance its strain, consequently increasing strain-release reactivity. In fact, A8 is more reactive than its strain release energy alone would suggest: The reaction enthalpy of A8 is 6 kcal mol -1 less exothermic than the parent cyclooctyne A4, which should in principle increase its activation barrier relative to A4 by around 3 kcal mol -1 if strain release alone were to govern reactivity. Upon dissection of the ∆∆H ‡ between A8 and A4 into strain release and delocalization components (using eq 3), it becomes evident that enhanced delocalization due to greater p-conjugation (∆(2-Nocc) = 0.05 e) in A8 accounts for a 6 kcal mol -1 barrier-lowering effect. Consequently, delocalization counteracts the effect of decreased strain release observed in A8, resulting in a net lowering of the activation barrier by 3 kcal mol -1an approximate 10 3 -fold rate acceleration at 298 K compared with A4. A similar analysis across a set of cycloalkynes (Figures and) reveals the importance of delocalization on the reactivity of monobenzocyclooctynes (A5), and to a smaller extent difluorinated cyclooctyne A9 and distal benzocyclooctyne A7, denoted by red asterisks in Figure . As with the small ring-opening reactions discussed above (Figure ), the negative sign of the delocalization coefficient 𝛽 for this cycloaddition reaction (-114 kcal mol -1 e -1 ) reflects the decrease in the intrinsic barrier due to delocalization. The smaller magnitude of 𝛽 for the cycloaddition reaction compared with the smallring opening (-114 vs -192 kcal mol -1 e -1 , respectively, for 𝜒 = 2-Nocc) reflects the lower sensitivity of the cycloaddition towards variation in bond delocalization. We suggest that this lower sensitivity may arise from smaller orbital overlap between the breaking p bond and the hyperconjugating group, such that the effect of this electron delocalization on the transition state is less pronounced.