We were taught that there are primarily 4 findfour kind of reactions -, synthesis, combustion, single displacement, and double displacement. When the reactants are given to you, the reaction can be predicted.
For example:
If the reactants are AB + CD, it will be a double displacement (2two pairs of compounds)
If the reactants are AB + C, it will be a single displacement (1one pair and 1one compound/element)
If the reactants are A + B, it will be synthesis.
Why do the "format" of the reactants determine the type of reaction? Why can't, for example, AB + CD synthesize to create ABCD? Or AB + C synthesize to create ABC? Why is it assumed that if the reactants are in a specific "format" a specific reaction will occur?