reagent
Summary
A reagent is a substance added to a starting material (the substrate) to cause a specific reaction, and it is usually consumed. The testable point: a reagent is used up, unlike a catalyst, and its identity (such as NaBH₄ versus LiAlH₄) decides which functional groups react.
Detail
Meaning: A reagent is a compound added to a starting material to bring about a chemical change, and it is usually consumed in the process. The molecule being transformed is the substrate. Reagents are often described by their role: oxidizing agents (PCC, KMnO₄), reducing agents (NaBH₄, LiAlH₄), nucleophiles, acids and bases. In reaction schemes the reagent is written over the arrow.
On the MCAT: Passages give a multistep synthesis and ask which reagent performs a step or what product forms. Know the selectivity: LiAlH₄ reduces esters and carboxylic acids to alcohols, while NaBH₄ reduces aldehydes and ketones but generally not esters or acids. PCC stops a primary alcohol at the aldehyde, whereas stronger oxidants such as chromic acid go on to the carboxylic acid.
Common confusion: A reagent is not a catalyst. A catalyst is regenerated, lowers the activation energy and is absent from the net equation; a reagent is consumed and appears in the stoichiometry. A reagent is also not the substrate, which is the molecule being changed, and not the solvent, which is the medium and is usually not consumed.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.