Dissociation energies of π bonds in alkenes

Abstract

Most chemists agree that the bond dissociation energy of a pi bond (DJ is less than that of a corresponding sigma bond (D"). That is, of the two bonds in A=B the first is weaker than the second. By contrast, there are rather different views on how to estimate /)".Bond dissociation energies are defined for a process in which the broken bond(s), the involved molecules and their states are specified at 25°C (1). Now consider ethylene which has received the most varied-treatment. By definition, its bond dissociation energy or "double bond energy," i.e., D(H2C=CH2), refers to process (la).

Evidence for an Equilibrium between an N-heterocyclic Carbene and Its Dimer in Solution

“…Despite considerable steric bulk at the N-substituents the bridge leads to the immediate formation of the enetetramine 3 c. The analogous dimerization was not observed for 3 a or for a number of single-bridged imidazoline-2-ylidenes by Chen et al [11] The formation of enetetramine 3 c in solution was established by 13 C NMR spectroscopy (d(C2) 119.5) and in the solid state by X-ray diffraction analysis ( Figure 1). [19a) ] In the crystal the ene- (12), N1-C10 1.4640(12), N2-C2 1.4214(11), N2-C4 1.3981(12), C2-C2* 1.345(2), C4-C9 1.4001 (14); C2-N1-C9 107.13 (7), C2-N1-C10 121.40(8), C9-N1-C10 121.43(8), C2-N2-C4 107.17 (7), N1-C2-N2 107.64(8), N1-C2-C2* 125.81(5), N2-C2-C2* 125.94 (5). Symmetry (*) 1 À x, y, 1/2 À z. tetramine 3 c is twisted along the C2 ± C2* axis.…”

“…The dissociation of the double bond in (3 d) 2 at ambient temperature can be rationalized in terms of the binding model developed by Carter, Goddard, Malrieu, and Trinquier (CGMT Model [13a] ), which was also used by Chen [13b] to discuss the weakness of the double bond in electron-rich olefins. In formal dimers of singlet carbenes 1 ± 3 the strength of the CÀC double bond can be approximated by the bond strength of a regular double bond (usually the (sp 2 ± sp 2 )s/ (p ± p)p-double bond of ethylene of 172 kcal mol À1 [14] ) minus the sum of the singlet ± triplet energy gap for both carbene parts. The singlet ± triplet energy gap amounts to approximately 85 kcal mol À1 [15] per ªArduengoº carbene of type 1, which has been used to explain the weakness of the C C bond in molecules of type (1) 2 and the difficulties in their preparation.…”

“…In formal dimers of singlet carbenes 1 ± 3 the strength of the CÀC double bond can be approximated by the bond strength of a regular double bond (usually the (sp 2 ± sp 2 )s/ (p ± p)p-double bond of ethylene of 172 kcal mol À1 [14] ) minus the sum of the singlet ± triplet energy gap for both carbene parts. The singlet ± triplet energy gap amounts to approximately 85 kcal mol À1 [15] per ªArduengoº carbene of type 1, which has been used to explain the weakness of the C C bond in molecules of type (1) 2 and the difficulties in their preparation.…”

“…The (Im N,N'-dimethylimidazolin-2-ylidene). [17] The C 3 bridge between the two carbene donors in 5 allows a C(carbene)-Mo-C(carbene) bond angle of 95.77 (14)8 in the octahedral complex 5, whereas the C 1 bridge between two benzimidazolin-2-ylidene groups, at least in the PdI 2 complex, leads to an acute C-Pd-C-angle of 83.7(3)8. [18] The excellent donor ability of the carbene groups in 5 is also illustrated by the different MoÀCO bond lengths for the CO ligands which are cis and trans to the carbene ligands (2.035(3) and 1.963(3) ).…”

Nachweis des Gleichgewichts zwischen einem N-heterocyclischen Carben und seinem Dimer in Lösung

“…Molekülstruktur von 3 c im Kristall (SCHAKAL-Darstellung). Ausgewählte Bindungslängen [] und Winkel [8]: N1-C2 1.4212(11), N1-C9 1.3992(12), N1-C10 1.4640(12), N2-C2 1.4214(11), N2-C4 1.3981(12), C2-C2* 1.345(2), C4-C9 1.4001 (14); C2-N1-C9 107.13 (7), C2-N1-C10 121.40(8), C9-N1-C10 121.43(8), C2-N2-C4 107.17 (7), N1-C2-N2 107.64(8), N1-C2-C2* 125.81(5), N2-C2-C2* 125.94 (5). Symmetriecode (*): 1 À x, y, 1/2 À z.…”