Nucleophile vs Electrophile
Consider a reaction between HCl and hydroxide ion. In this reaction, the chloride will be replaced by hydroxide anion. For this reaction to take place, there should be positive and negative charge separation within the HCl molecule. The species arising due to a charge separation like this are known as electrophiles and nucleophiles. What is exactly a nucleophile or an electrophile is discussed in this article. Electrophiles and nucleophiles are important to initiate chemical reactions. Further, they are important to describe how the reactions proceed. In organic chemistry, reaction mechanisms are categorized based on the initial species (either an electrophile or nucleophile) that begins to attack the other species. Nucleophilic substitution, nucleophilic addition, electrophilic substitution and electrophilic addition are the four major types of mechanisms describing organic reactions.
What is Nucleophile?
A nucleophile can be any negative ion or any neutral molecule that has at least one unshared electron pair. Nucleophile is a substance that is very electropositive, therefore, like to interact with positive centers. It can initiate reactions using the lone electron pair. For example, when a nucleophile reacts with an alkyl halide, the lone pair of the nucleophile attacks the carbon atom that bears the halogen. This carbon atom is partially positive charged due to the electronegativity difference between it and the halogen atom. After the nucleophile attaches to the carbon, the halogen leaves. This type of reaction is known as nucleophilic substitution reaction. There is another type of reaction initiated by nucleophiles, called nucleophilic elimination reaction. Nucleophilicity tells about the reaction mechanisms; thus, it is an indication of the reaction rates. For example, if the nucleophilicity is high, then a certain reaction can be fast, and if the nucleophilicity is low, reaction rate is slow. Since nucleophiles donate electrons, according to the Lewis definition, they are bases.
What is Electrophile?
“Electrophiles are reagents, which in their reactions seek the extra electrons that will give them a stable valence shell of electrons.” Carbocations are electrophiles. They are electron deficient and have only six electrons in their valence shell. Because of this, carbocations can act as Lewis acids. They are accepting an electron pair from a nucleophile and fill the valence shell. Electrophiles may have a formal-positive charge, partial-positive charge, or a valance shell with incomplete octet. Electrophilic substitution and electrophilic addition reactions are initiated by electrophiles. In electrophilic substitution reaction, an electrophile displaces an atom or group in a compound. This is most seen in aromatic compounds. For example, this is the mechanisms by with a nitro group is attached to the benzene ring by displacing a hydrogen. In electrophilic addition reaction, a pi bond in a molecule is broken and a new sigma bond is formed between the molecule and the electrophile.
What is the difference between Nucleophile and Electrophile?
• Nucleophile is a substance that seeks a positive center. Electrophiles seek negative centers which have extra electrons.
• A nucleophile can be any negative ion or any neutral molecule that has at least one unshared electron pair. Carbocations and other, completely or partially, positive charged species can act as electrophiles.
• Nucleophile is a Lewis base, and an electrophile is a Lewis acid.
• In a reaction mechanism, arrows are always drawn to show the electron flow from a nucleophile to the electrophile.