Properties of C−C Bonds in<i>n</i>-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 α and β 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 β 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.