What is one chemical reaction that begins with petroleum as a starting material

Thermal decomposition or cracking

Explanation:

Petroleum is a mixture of hydrocarbons which are usually formed naturally. Petroleum undergo a host of chemical reactions. One of such is thermal decomposition or cracking.

Cracking is used in the petroleum industry to covert heavy fractions to more useful lighter ones.

When petroleum is subjected to high temperature and pressure, and in the presence of catalyst, the long chain type of petroleum will decompose into more useful smaller and lighter molecules.

Example is given below:

                  C₁₅H₃₂ → C₈H₁₈ + C₃H₆ + 2C₂H₄

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