Reagent reactions or reactants are substances or compounds that are consumed in the process of a chemical reaction. Catalysts and solvents are used in many chemical reactions, but they are not usually considered reactants. Reagent reactions are typically a test substance that is mixed in to a system for the purpose of causing a reaction, or to see whether or not an reaction will take place. These reactions are then used to confirm or deny the presence of another substance. Some examples of
analytical reagent reactions are; Toolen’s reagent and Fehling’s reagent. In organic chemistry the reagents are mixtures or compounds made up of inorganic or tiny organic molecules. These are used to affect a change in the substrate. Some examples of the organic reagents are Collins reagent, Grignard reagent and Fenton’s reagent.

Fehling’s reagent reactions is a solution used to make a distinction between water soluble aldehyde and ketone functional groups. Because ketose monosaccharides like fructose also tests positive. This is due to the fact that the conversion to aldoses by the based in the reagent. This is the reason why Fehling’s reagent is also known as the basic test for monosaccharides. In order to proceed forward with the test, the substance that is to be tested is heated along with Fehling’s solution. When a red precipitate is present it means that the aldehyde Ketones does not react. An important use for this reagent reaction is in the testing glucose in the urine. This was developed by Hermann von Fehling in 1849 and is used to screen for diabetes.

Toolen’s reagent was named after Bernard Tollen and usually consists of ammonial silver nitrate, although it can be other compounds so long as there is an aqueous diamminesilver(I) complex. The diaminesilver(I) complex is an oxidizing agent that is in turn reduced to silver. This will form a silver mirror in a clean glass reaction vessel. It is used as a test for aldehydes that are oxidized to carbonylic acids. Once it is seen that there is a carbonyl group on the organic molecule using Brady's reagent or 2,4-DNPH. This reagent reaction can determine if the compound is ketone or aldehyde. In certain special cases, Tollen’s reagent rations will show a positive test result for ketone. Only if the ketone is alpha-hydroxy ketone will the Tolen’s reagent produce reagent reactions.

In organic chemistry the Collin’s reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. It is only used on selected oxidized primary alcohols to the aldehyde. It is able to handle other functional groups within the molecule. The Jones’s reagent reaction can be used as an alternative. The Collin’s reagent reaction is especially beneficial for oxidations of acid sensitive compounds. This complex is not only hard to prepare, but can also be dangerous.