Which of the following is not one of the four types of organic reactions?

Organic chemistry encompasses several types of reactions that are fundamental to understanding how organic compounds behave and interact. The four primary types of organic reactions are addition, substitution, elimination, and rearrangement.

In an addition reaction, two or more reactants combine to form a single product, which is commonly seen in reactions involving alkenes and alkynes where π bonds are broken to form new σ bonds. Substitution reactions involve the replacement of one functional group in a molecule with another, commonly occurring in reactions with alkanes and aromatic compounds. Elimination reactions involve the removal of a small molecule from a larger molecule, typically resulting in the formation of a double or triple bond, often observed with alcohols where water is eliminated.

Precipitation does not fall under the umbrella of organic reactions but rather describes a reaction that occurs in aqueous solutions, mainly in inorganic chemistry where a solid forms from two aqueous solutions, contributing to a different category of chemical reactivity. Thus, precipitation is the correct answer as it is not considered one of the main types of organic reactions.