Some of the chemical reactions are very slow and moderately slow in nature. For increasing the rate or speed of reactions, suitable attacking reagents are used. There are mainly two types of attacking reagent:
1. Electrophiles: An electrophiles are those species which can accept electron pair from the electron rich species. They are also called electron loving species. Electrophiles act as lewis acids. Electrophiles are of two types:
- Positive electrophiles: The positive electrophiles are those species which carry positive charge. They include proton, cations and carbonium etc. Examples: H+, Br+, NO2+ etc.
Note: The positive electrophile attacks an electron rich substrate and accepts an electron pair for sharing, thereby forming a neutral molecule.
- Neutral electrophiles: The negative electrophiles are those species which are deficient in electron but do not carry negative charge. They include lewis acids and carbenes. Examples: AlCl3, SO3, BF3 etc.
Note: The neutral electrophile attacks on electron rich substrates to produce a highly negatively charged molecules.
2. Nucleophiles: Neucleophiles are those species which can donate electron pair to the electron deficient species. They are also called nucleus loving species. A Nucleophile acts as a lewis base. It is of two types:
- Negative nucleophiles: The negative nucleophiles are those which carry an electron pair and are negatively charged because they contain one extra electrons. They include hydride ions, anions, carboanions etc. Example: H–, Cl–, OH– etc.
Note: A negative nucleophile attacks an electron deficient substrate and becomes a neutral molecule.
- Neutral nucleophiles: A neutral molecule are those which possess an unshared pair of electrons. Neutral electrophiles are not charged. They are electrically neutral. They include lewis base or neutral molecules wih free electrons. Examples: ammonia, water, alcohol, amines etc.
Note: A neutral nucleophile will attack electron rich substrate and gives positively charged products.