Towards an Understanding of the Carbon‐Carbon Bond

Abstract

Dedicated to Professor Rorf Huisgen on the occasion of his 60th birthdayIn this essay, the classical question of "the influence of the number and kind of substituents on the strength of the C-C bond", is pursued with the modern tools of contemporary physical organic chemistry. Based on the work of Karl Ziegler, the products and kinetics of thermolysis of a large number of highly branched aliphatic hydrocarbons and phenyl-or cyano-substituted derivatives were investigated. For each class of compounds, a linear relationship was found between the free enthalpy of activation of the homolytic cleavage of the weakest C-C bond and the strain energy in the ground state. These relationships permitted a quantitative separation of steric and electronic effects on the cleavage of C-C bonds. The influence of the size of the substituent groups on bond angles, bond lengths, and the conformational behavior of model compounds was studied by means of experimental structure determinations and force field calculations. C-C bond lengths up to 164 pm, bond angles at tetracoordinate carbon as large as 126", and unusual eclipsed and gauche preferred conformations were found.

Homolytic Bond Dissociation Enthalpies of C–C and C–H Bonds in Highly Crowded Alkanes

“…12,13 Correlation between trend in C-C, C-H bond energies and steric effect has been firstly suggested by Eyring 14 in 1932. Later, Rüchardt [15][16][17] demonstrated the influence of substitutions on the strength of the carbon-carbon bond, and also emphasized polar and steric effect according to experimental data. Velevkin and Nölke 18 took steric repulsion into consideration, thus successfully used UV-irradiation with mercury in the gas phase to synthesize polycarbon hydrocarbons.…”

Mechanistic analysis and thermochemical kinetic simulation of the products from pyrolysis of poly(α-methylstyrene), especially the unrecognized role of phenyl shift

“…As reviewed above, there is disagreement whether s is zero (all backbone scission results from homolysis of its strained C-C bond [67]) or is small but finite (additional contribution from the transfer-b-scission sequence). To explore this question, we use the representative data point [6b,9a] discussed above that the time needed at 240 8C to reduce M n0 = 853,000 by half (t 50% ; breaking on average one backbone bond per molecule [9b]) was %17 h. We compare this to order-of-magnitude estimates of k i based on experimental values for small-molecule models [68].…”

“…Consider second the experimental k i = 10 17.6 exp(À75,600/ RT) s À1 (the rpd = 2 was already applied) for the internally strained 1 (n = 2) to give a tert and a prim radical [67]. To adjust for formation of a tert-benzylic radical, we decreased A by a half order-of-magnitude for formation of a resonance-stabilized radical [1] and decreased E by 12.5 kcal mol À1 , the difference in the recommended D8(C-H) values for i-butane and cumene [72] to obtain k i < 10 17.1 exp(63,100/RT) s À1 or t 50% > 115 h at 240 8C (the limit designations reflect the indication that PaMS is less internally strained than PIB as noted).…”

Structural dependence of heterolytic bond dissociation energy of carbon–carbon σ bonds in hydrocarbons

“…[37][38][39] Rüchardt and co-workers 39 investigated the products and kinetics of thermolysis for a large number of highly branched aliphatic hydrocarbons and phenyl-or cyano-substituted derivatives, and have shown quantitatively that, in addition to stability of radicals, the steric congestion in a dissociating molecule is a major enhancing factor in the homolytic cleavage of carbon-carbon s bonds.…”