Related Experiment Video
Updated: Jun 13, 2026

Original Experimental Approach for Assessing Transport Fuel Stability
Published on: October 21, 2016
Origin of stability in branched alkanes
Carl R Kemnitz1, Joel L Mackey, Mark J Loewen
1California State University, Bakersfield, 9001 Stockdale Highway, Bakersfield, CA 93311, USA. ckemnitz@csub.edu
Abstract:
The potential origins of stability in branched alkanes are investigated, paying close attention to two recent hypotheses: geminal steric repulsion and protobranching. All alkane isomers through C(6)H(14) along with heptane and octane were investigated at the MPW1B95/6-311++G(d,p) level. Their geminal steric repulsion, total steric repulsion, and orbital interactions were evaluated by using natural bond orbital analysis. All measures of steric repulsion fail to explain the stability of branched alkanes. The extra stability of branched alkanes and protobranching, in general, is tied to stabilizing geminal sigma-->sigma* delocalization, particularly of the type that involves adjacent C-C bonds and, thus, preferentially stabilizes branched alkanes. This picture is corroborated by valence bond calculations that attribute the effect to additional ionic structures (e.g., CH(3) (+) :CH(2) :CH(3) (-) and CH(3):(-) CH(2): CH(3) (+) for propane) that are not possible without protobranching.
Related Concept Videos
Relative Stabilities of Alkenes
Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes
Alkanes undergo combustion in the presence of excess oxygen and high-temperature conditions to give carbon dioxide and water. A combustion reaction is the energy source in natural gas, liquified petroleum gas (LPG), fuel oil, gasoline, diesel fuel, and...
Physical Properties of Alkanes
Nomenclature of Alkanes
The alkane nomenclature considers the length of the carbon chain, the number, and the location of the substituent to arrive at its systematic name. The IUPAC...
Mass Spectrometry: Branched Alkane Fragmentation
Constitutional Isomers of Alkanes
The linear isomer of an alkane is prefixed by the term “n”; hence a linear isomer of pentane is known as n-pentane. Based on the type of branching, some of the branched isomers are given...

