Thermal cracking is the chemical process of converting larger, long straight chain alkanes found in gas oils and other crude oil fractions into shorter straight chain alkanes. These shorter alkane chains are more desired because of their use in transportation fuels like gasoline. The thermal cracking reactions are governed by free radicals. The chain reaction of free radicals starts by breaking the C-C bond in the alkanes. This forms two free radicals. This step is called the initiation reaction. The next step, the propagation reaction produces a short chain alkane and one radial, which continues the chain. The final step is the termination reaction. This process started in the 1900’s as a way to increase the yield of motor gasoline from crude oils. These high-octane fuels were used in aircraft. Catalytic cracking came into use in the 1930’s and 1940’s. Because the catalytic cracking process produced higher yields of gasoline with high octane numbers, thermal cracking is no longer a method for breaking longer chains into shorter chains for gasoline production in modern refineries. In locations where diesel fuels are in high demand thermal cracking is still used. The use of thermal cracking in modern refineries is limited to naphtha cracking of residual fractions like vacuum distillation residue.