As mentioned in the lesson, the separation process of crude oil provides insufficient yields for the desired products i.e. gasoline. To satisfy the demand for more desirable products, conversion processes are used to enhance the yield. Cracking is the process of breaking down large molecules into smaller ones. Free radicals are the active intermediate species in thermal cracking (as opposed to ions in catalytic cracking). Though radicals are more stable on ternary or secondary carbons, the weakest bond in a compound is broken and the radical is typically produced on a primary carbon. Additionally, due to the fact that beta-bond scission reactions proceed faster than isomerization in a radical during the thermal cracking process, the final product is primarily composed of straight-chain parrafins and negligible amounts of branched-chain paraffins. Straight-chain parrafins have a lower octane rating – or a higher tendency to knock (self ignite) – an undesirable effect in modern gasoline engines. Therefore, presently thermal cracking is rarely used to improve gasoline yields and instead used to convert heavy gas oils into light gas oils with some byproducts of gas, gasoline, and fuel oil. However, it is still used to improve diesel yields (where knocking is desirable) in countries that primarily rely on diesel fuel.