Properties of C−C Bonds inn-Alkanes:  Relevance to Cracking Mechanisms

Abstract

The slight variations among the proton affinities and bond strengths of the C-C bonds in straight-chain n-alkanes have been determined to 1 kcal mol -1 accuracy for the first time, using computational quantum chemistry. Four computational methods (B3LYP, MP2, CCSD(T), and G2) were used to study n-alkanes (up to C 20 H 42 with B3LYP), including computations on the related alkyl radicals, carbenium ions, and carbonium ions. The proton affinities of the C-C bonds vary from 142 to over 166 kcal mol -1 , are highest for the center C-C bond, and decrease monotonically toward the end bonds. Bond strength, unlike proton affinity, is very constant (88 kcal mol -1 ), except for the R and bonds (89 and 87 kcal mol -1 , respectively). For thermal cracking, the results suggest that the most favored initiation step is the breaking of the bond of the alkane to create an ethyl radical. For Bronsted-acid-catalyzed cracking of straight-chain paraffins, if the initiation mechanism is via carbonium ions, then the results indicate that the central C-C bonds of n-alkanes will be most attractive to the Bronsted proton. However, for direct protolysis (Bronsted-mediated fission) of an n-alkane via a carbonium intermediate, the net exothermicities do not strongly discern among the C-C bonds. Trends in molecular geometry and infrared spectra features are also presented, and a signature IR band is predicted for carbonium ions that should aid in their identification.

Gamma-ray spectra of methane in the positron–electron annihilationprocess

“…Molecules without permanent dipole moment such as alkanes do not guarantee that the electron density is the same over the entire molecule. For example, it is a known fact that the proton affinity of the centre C-C bond in straight-chain n-alkanes, such as n-hexane is the highest (Hunter and East, 2002). In the case of methane, although it does not have a permanent dipole moment, all the C-H bonds are polar bonds.…”

Branching ratios for the dissociative recombination of hydrocarbon ions

“…The exact bond strengths in non-classical molecules are often unknown. Typical values for hydrocarbons in their ground states are 3.82 eV for a simple C-C bond in n-alkanes [59] (7.2 ± 0.3 eV) for a double bond and (8.8 ± 1.2 eV) for a triple one [60]. Reliable bond dissociation energies values are not available for strained cyclic compounds [61].…”

Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

“…A similar isotopic trend was observed by Bjorøy et al (1991) in the hydrous pyrolysis products from different kerogen types. In addition, a recent quantum chemical study on n-alkane bond strengths suggests preferential C-C bond fission at ␤-position of long carbon chains with the formation of C 1 and C 2 radicals (Hunter and East, 2002). This mechanism leads to isotopically heavier residuals in the longer section of the n-alkane moieties after thermal cracking.…”