A reagent is a compound or mixture that is added to a system to initiate or test a chemical reaction. By causing a reaction with it, a reagent can be used to determine whether or not a specific chemical substance is present.
In organic chemistry, any general reaction can be written as:
Substrate + Reagent → Product
Where the substrate is an organic molecule to which the reagent is added. The reagents are classified as follows based on their ability to donate or abstract electrons:
- Electrophiles
- Nucleophiles
Electrophilic Reagents
At the valence shell, an electrophile has an electron-deficient atom, a vacant orbital, or an incomplete octet. These species contain molecules with a positive charge or those with an electron deficiency. An electrophile is a reagent that can accept an electron pair in a reaction. These typically have two fewer electrons than an octet. To complete the octet, these attack regions of high electron density in the substrate molecule. These are represented by the symbol E+. Lewis acid is an electrophile. (A Lewis acid is any substance that can accept a pair of nonbonding electrons, such as the H+ ion.) An electrophile is a type of organism that accepts a pair of electrons in order to form a new covalent bond.
Electrophiles are classified into two types.
- Neutral electrophile: These species have no positive or negative charge. For instance, AlCl3, BF3, Carbene, Nitrene, free radicals SO3, acid chlorides, and so on.
- Positive electrophiles: also known as positively charged electrophiles. Examples include H +, H3O+, NO2+, NO+, Carbonium ion, Nitrosonium ion, and Diazonium ion.
Nucleophilic reagents
- In a reaction, a nucleophile is a chemical species that donates an electron pair to an electrophile in order to form a chemical bond.
- Nucleophiles are any molecules or ions that have a free pair of electrons or at least one pi bond. Since nucleophiles donate electrons, they are Lewis bases by definition.
- They are negatively charged as they have electron-rich atoms and lone pairs of electrons.
- A nucleophile is a reactant that contributes two electrons to the formation of a new covalent bond.
Nucleophiles are classified into two types.
- Neutral nucleophiles include NH3, RNH2, H–O–H, R–OH, R–O–R, and R–S–R.
- Negative nucleophiles include Cl– (chloride ion), Br– ( Bromide ion), I– (Iodide ion), OH– , CN– , etc.
Examples of Reagents
Inorganic compounds or small organic molecules are more commonly used as reagents in organic chemistry. Monoclonal and polyclonal antibodies, oligomers, and cell lines are examples of biotechnology reagents. In analytical chemistry, they are frequently used as colour indicators. Some list of reagents includes Grignard’s reagent, Tollens’ reagent, Fehling’s reagent, Millon’s reagent, Collins reagent, and Fenton’s reagent. However, not all reagents have the word reagent in their name. Solvents, enzymes, and catalysts are also examples of reagents.
Reagents can also be limiting. When all of the limiting reagents are used up, the chemical reaction comes to a halt. The chemical reaction is reliant on the reagent to continue and stops when there is no more substance. As a result, limiting reagents determine when a chemical reaction can no longer proceed.