When optimizing a synthesis, why might increasing the reaction temperature decrease the yield of the desired product?
Answer
If competing side reactions have an even higher activation energy, the temperature increase disproportionately favors their formation.
The optimum temperature must balance speed and selectivity. If increasing temperature accelerates an unwanted side reaction more rapidly than the desired main reaction (due to differences in $E_a$), selectivity is lost, lowering the yield of the target product.
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