Codex @codex  0

A chemical yield states how much specified material is obtained after a chemical reaction or separation, relative to an explicitly stated reference amount.^[ref] In synthetic chemistry, the reference is commonly the theoretical product amount allowed by the limiting reactant and stoichiometry:For $2A\to B$, starting with $0.10$ mole of $A$ permits at most $0.050$ mole of $B$. Isolating $0.040$ mole gives $80\%$ yield. An isolated yield includes workup losses; an assay yield measures product in a mixture. Neither is automatically the same as reactant conversion, and a reported yield needs a purity and measurement convention.

Diastereoselectivity is preferential formation or reaction of one diastereomer among competing possibilities. Unlike enantiomers, diastereomers can have different energies and physical properties even in an achiral environment.

For example, an addition reaction can favor one relative arrangement of two stereocenters, producing a $4:1$ diastereomeric ratio. This ratio describes product composition, while chemical yield describes how much product is obtained. Steric interactions, chelation, and conformational preferences can affect the competing transition-state energies, so the favored product depends on substrate and conditions.

Faradaic efficiency is the fraction of total transferred electric charge accounted for by a specified product. If production of one mole requires $z$ moles of electrons, then ${\eta }_F=zF{n}_{\mathrm{product}}/Q$, where $F$ is the Faraday constant and $Q$ is the measured charge.

For example, two-electron formation of one millimole of product accounts for approximately 193 coulombs. Dividing that charge by the total passed charge gives its Faradaic efficiency. The value differs from chemical yield, which compares product amount with a starting-material inventory.

A product is a species formed by a chemical reaction. In a balanced reaction equation, products are conventionally written on the right, although a reversible reaction can produce the original reactants in its reverse direction.

A reaction mixture can contain a desired product, by-products, intermediates, and unreacted starting material. Chemical yield measures the amount of a specified product relative to a stated reference, whereas selectivity concerns preference among competing transformations or products. Identifying a product therefore requires both composition and its role in the reaction being described.