Chemical Interpretation of Octane Number

Anton Perdih,* Franc Perdih

Department of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia.
E-mail: anton.perdih@email.si

Absrtract
The empirical rules of Octane Number (ON) dependence on the structure of alkanes are amended. ON decreases with the number of CH₂ groups and increases with the number of CH₃ groups; the number of adjacent CH₂ groups has the highest but sigmoid influence; ON decreases with the separation between branches; it increases with the more central position of branches and with their bulkiness. Ethyl group causes apparently contradictory effects: If it increases the number of CH₂ groups, ON decreases; if not, ON increases. The use of structural features of alkanes, i.e. the size of the molecule, the number of branches, the position of branches, the separation between them, the type of branches, and the type of the branched structure enables a more thorough understanding of the relations between the structure of alkanes and their physicochemical properties. Comparison of kinetic and structural data of n-alkanes vs. branched alkanes indicates that in alkanes having a sufficient number (3 to 7) of adjacent CH₂ groups there exists the possibility of forming a limited number of gem-diperoxydiol groups, which may be responsible for knocking of low ON fuel - air mixtures.

Key words: gem-dihydroperoxide, hydroperoxide, octane number, polyperoxide, reaction mechanism, structural interpretation