Reagents Organic Synthesis:

A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction or is added to see if a reaction occurs " . Such a reaction is used to confirm the presence of another substance. Examples of such analytical reagents include Fehling's reagent, Millon's reagent and Tollens' reagent.

Although the terms reactant and reagent are often used interchangeably, a reactant is more specifically a "substance that is consumed in the course of a chemical reaction".Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants. In organic chemistry, reagents are compounds or mixtures, usually composed of inorganic or small organic molecules, that are used to affect a transformation on an organic substrate. Examples of organic reagents include the Collins reagent, Fenton's reagent, and Grignard reagent.

In another use of the term, when purchasing or preparing chemicals, reagent-grade describes chemical substances of sufficient purity for use in chemical analysis, chemical reactions or physical testing. Purity standards for reagents are set by organizations such as ASTM International. For instance, reagent-quality water must have very low levels of impurities like sodium and chloride ions, silica, and bacteria, as well as a very high electrical resistivity.

Examples:

Collins reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. It is used to selectively oxidize primary alcohols to the aldehyde, and will tolerate many other functional groups within the molecule. It can be used as an alternative to the Jones reagent and pyridinium chlorochromate (PCC) when oxidising secondary alcohols to ketones. Moreover, the Collins reagent is especially useful for oxidations of acid sensitive compounds.

This complex is both difficult and dangerous to prepare, as it is very hygroscopic and can inflame during preparation. It is typically used in a sixfold excess in order to complete the reaction. Nowadays, PCC or PDC oxidation have largely supplanted Collins oxidation for these very reasons.

The Grignard reaction is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides (Grignard reagents) act as nucleophiles and attack electrophilic carbon atoms that are present within polar bonds (e.g. in a carbonyl group as in the example shown below) to yield a carbon-carbon bond, thus altering hybridization about the reaction center. The Grignard reaction is an important tool in the formation of carbon-carbon bonds and for the formation of carbon-phosphorus, carbon-tin, carbon-silicon, carbon-boron and other carbon-heteroatom bonds.

The disadvantage of Grignard reagents is that they readily react with protic solvents (such as water), or with functional groups with acidic protons, such as alcohols and amines. In fact, atmospheric humidity in the lab can dictate one's success when trying to synthesize a Grignard reagent from magnesium turnings and an alkyl halide. One of many methods used to exclude water from the reaction atmosphere is to flame-dry the reaction vessel to evaporate all moisture, which is then sealed to prevent moisture from returning. However, though the reagents still need to be dry, ultrasound can allow Grignard reagents to form with less stringent regard to the amount of water in the reaction mix by activating the surface of the magnesium such that it consumes any water present.

Fenton's reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or waste waters. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE).