: An oxidation reaction in which two or more chemicals combine to form a single product is called a coupling reaction. For example, ferrous chloride combines with chlorine to produce ferric chloride. Again , hydrogen gas combines with nitrogen gas to produce ammonia gas. This is also an example of a coupling reaction. However, the synthesis reaction, in which only the basic substances are added to form compounds , is also called synthesis reaction. Thus the process of producing ammonia gas is considered as either a coupling or synthesis reaction.