Side reactions negatively impact both the percentage yield and atom economy of chemical reactions by generating unwanted products and depleting reactants.

To elaborate, side reactions are secondary reactions that occur concurrently with the primary reaction. They pose challenges because they consume some of the reactants that would otherwise contribute to the formation of the desired product. Consequently, this results in a lower production of the target product, thereby reducing the percentage yield. The percentage yield is an important indicator of a reaction’s efficiency, defined as the ratio of the actual amount of product obtained to the theoretical maximum amount that could be produced. When side reactions take place, the actual yield is diminished, leading to a lower percentage yield.

Moreover, side reactions also influence atom economy. Atom economy quantifies how effectively atoms are utilized in a reaction, calculated by comparing the total mass of useful products against the total mass of all products generated. When side reactions occur, some atoms are diverted to form unwanted byproducts, which means the total mass of useful products is less than it could potentially be. As a result, atom economy decreases.

For instance, consider a scenario where reactants $A$ and $B$ react to yield product $C$, but there is also a side reaction in which $A$ reacts with $D$ to produce $E$. In this case, some of the $A$ that could have contributed to the formation of $C$ is instead consumed to produce $E$. This diversion results in a reduced quantity of $C$, thereby lowering the percentage yield. Additionally, it means that some atoms are utilized to form $E$ rather than $C$, further diminishing the atom economy.

In summary, side reactions can considerably reduce the efficiency of chemical processes by decreasing both the percentage yield and atom economy. For this reason, chemists strive to minimize side reactions to enhance the overall effectiveness of their reactions.