converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers. Moreover, redox reactions of alkanes involving free radical intermediates, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly reduced state; in the case of methane, carbon is in its lowest possible oxidation state (−4). Reaction with oxygen (if present in sufficient quantity to satisfy the reaction stoichiometry) leads to combustion without any smoke, producing carbon dioxide and water. Free radical halogenation reactions occur with halogens, leading to the production of haloalkanes. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes in C–H bond activation reactions. In highly branched alkanes, the bond angle may differ significantly from the optimal value (109.5°) to accommodate bulky groups. Such distortions introduce a tension in the molecule, known as steric hindrance or strain. Strain substantially increases reactivity. However, in general and perhaps surprisingly, when branching is not extensive enough to make highly disfavorable 1,2- and 1,3-alkyl–alkyl steric interactions (worth ~3.1 kcal/mol and ~3.7 kcal/mol in the case of the eclipsing conformations of butane and pentane, respectively) unavoidable, the branched alkanes are actually more thermodynamically stable than their linear (or less branched) isomers. For example, the highly branched 2,2,3,3-tetramethylbutane is about 1.9 kcal/mol more stable than its linear isomer, n-octane. Due to the subtlety of this effect, the exact reasons for this rule have been vigorously debated in the chemical literature and is yet unsettled. Several explanations, including stabilization of branched alkanes by electron correlation, destabilization of linear alkanes by steric repulsion, stabilization by neutral hyperconjugation, and/or electrostatic effects have been advanced as possibilities. The controversy is related to the question of whether the traditional explanation of hyperconjugation is the primary factor governing the stability of alkyl radicals. Reactions with oxygen (combustion reaction) All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is: CnH2n+2 + (n + ) O2 → (n + 1) H2O + n CO2 or CnH2n+2 + () O2 → (n + 1) H2O + n CO2 In the absence of sufficient oxygen, carbon monoxide or even soot can be formed, as shown below: CnH2n+2 + (n + ) O2 → (n + 1) H2O + n CO CnH2n+2 + (n + ) O2 → (n + 1)