In which type of reaction do two molecules combine to form a single compound?

The type of reaction where two molecules combine to form a single compound is known as an addition reaction. In addition reactions, typically seen in organic chemistry involving alkenes or alkynes, the multiple bonds within a molecule are broken as new bonds are formed with additional atoms or groups, leading to a larger, single product.

For example, in the reaction between an alkene and a hydrogen halide, the alkene's double bond breaks, allowing the halide ion and hydrogen to add across the former double bond, resulting in the formation of a bromoalkane or chloroalkane. This process emphasizes the essence of addition reactions where new atoms are incorporated into the reacting species, reinforcing the idea of building up a more complex structure from simpler ones.

While displacement, elimination, and substitution reactions involve different mechanisms and outcomes—such as exchanging one group for another or removing parts of a molecule—the addition reaction distinctly focuses on the combination of two reactants into a singular, more complex product.