The chemical entity that can accept or donate electron pair(s) is known as an electrophile or nucleophile respectively.
Electrophiles are those reactants that are either positively charged or neutral with no lone pair of electrons. These positively charged or electron-deficient chemical species can accept electron pairs from other molecules or atoms. Nucleophiles, on the other hand, are those reactants that are either negatively charged or that is neutral with one or more lone electron pairs. The nucleophilicity or electrophilicity of an element is the extent to which it is capable of accepting or donating a pair of electrons. In acid-base reactions, nucleophiles (base) attack electrophiles (acid).
Nucleophiles in Details
A nucleophile is that chemical species that has negative charge or that has lone pairs of electrons. Lone pair of electrons is those electrons that do not get used in the bond. They remain unused in the molecule. As a result, this type of chemical species can get attracted to the positive area of another compound or molecule. This attraction results in chemical reaction and bonds.
Chlorine (Cl) has 3 lone pairs of electrons in just its atomic form. So it can donate them to other electron-deficient atoms or molecules by getting attached to them.
OH- can act as a great nucleophile because of its electronegativity.
NH3 has a lone pair of electrons. Hence it is a nucleophile.
Features of Nucleophiles
Nucleophiles can donate one or more pair(s) of electrons in a chemical reaction.
Since nucleophiles get attached to protons - they are called nucleophiles. Why? Protons and neutrons reside in the same region. This region is called the nucleus. So, when nucleophiles get attracted to protons, they actually get attracted to the nucleus. Phile means lover. Hence nucleophiles are named that way.
Nucleophiles can donate electrons because they have either extra electrons or lone pairs of electrons. In other words, they are rich in electrons.
Nucleophiles are anions - negatively charged ions. In other cases, they are neutral chemical species with free electrons or lone pairs of electrons.
Electrophiles in Detail
Electrophiles are just the opposite of nucleophiles. They are either positively charged or they have still some space left in their valence shells to accept electron pairs. It is this positive charge or vacant valence shell that makes it get attracted to nucleophiles - the electron-rich species. Since they tend to accept electrons, they are known as electron acceptors.
The ionic form Chlorine - Chlorine ion or Cl+ is an electrophile.
A hydrogen ion is also an electrophile.
Borane or Boron Trihydride ( BH₃) has its p orbital vacant. Hence it can attract electrons. So it is an electrophile.
AlCl₃ is an interesting compound. The Cl atoms in the compound have full octets. But the Al does not have 8 electrons in its valence shell. Hence it is interested in attracting electron-rich compounds.
Represented by - NU-
Features of Electrophiles
Electrophiles are ready to accept electrons from other atoms or molecules.
Electrophilic species either have cations or they are neutral with a vacancy in valence shells. In other words, their valence shells do not have 8 electrons.
Since they have lesser electrons in their valence shells and since the cations have a positive charge, they are always on the lookout for electron-rich compounds or atoms.
Represented by E+
Now that you know what electrophiles and nucleophiles actually are, let us find out how the two differ.
Nucleophilic & Electrophilic Substitution Reaction
Sometimes there is already a compound attached to the positive area of another compound. A third negatively charged compound or compound that has lone pairs of electrons attacks the main compound and replaces the already existing negatively charged compound. This is called Nucleophilic substitution reaction.
For example - CH₃BR has BR already attached to the positive area of CH₃. When this compound reacts with CN-, the BR atom gets replaced by the CN- atom. So now, the compound changes to CH₃CN.
In the electrophilic substitution reaction, the electrophile replaces another electrophile already attached to a compound. For example, an electrophile can replace the H+ atom attached to a Benzene ring.
Therefore, to recap
Nucleophiles are electron-rich chemical species.
Nucleophiles can be negatively charged ions or anions. They can even be any neutral compound having one or more lone pair(s) of electrons.
Because nucleophiles are electron-rich, they can donate electrons.
Because of the power of nucleophiles to donate electrons, they can get attached to the positive area or the electron-deficient area of another compound.
In a nucleophilic substitution reaction, an electron-rich compound attacks another compound and replaces the already existing electron compound attached to it.
Nucleophiles are named that way because they are attracted to the positive area of a compound or atom. In other words, they are attracted to the protons that reside in the nucleus.
Nucleophiles are symbolised by NU-.
Nucleophiles belong to the Lewis base category - meaning they can attract the proton of the Hydrogen.
Here is a summary of things that we know about electrophiles -
Electrophiles have a positive charge
They have positive charges because they are cations. In other cases, they might be atoms having a vacancy in their valence shells.
Because electrophiles have a positive charge, they attract electrons.
They are represented by E+ symbol.
Electrophiles belong to the Lewis acid category.
In the electrophilic substitution reaction, the cation or compound having vacant valence shell replaces another electrophile already attached to a compound. For example, an electrophile can replace the Hydrogen cation attached to a Benzene ring.
Electrophiles and nucleophiles play an important role in the chemical reaction between different atoms or chemical species. As we all know opposites attract. Nucleophiles and electrophiles are opposites. It is this attraction that results in so many chemical reactions and formation of compounds.
1. Why is Nucleophile a Base?
Ans. Nucleophile can be a base when it attracts the proton of the Hydrogen. This is the basic function of base - attracting the Hydrogen atom. But you must remember that bases get attracted to Hydrogen ions only and form bonds with them. But nucleophiles can attack any electron-deficient species. So all nucleophiles are bases but not all bases are nucleophiles.
2. Why is Nucleophile Neutral?
Ans. A nucleophile is an electron-rich entity. Yet it can be neutral because the compound as a whole can be neutral but the individual atoms can have lone pairs of electrons. So they can act as nucleophiles despite being neutral. This lone pair of electrons makes the whole compound get attracted to the positive area of another compound.
3. Are Nucleophiles Electron-donating Groups?
Ans. Yes. Since nucleophiles have lone pairs of electrons, they can donate that free electronic pair. They are known as electron-rich.
4. What is an Electrophile?
Ans. An electrophile is that species that are positively charged. It can also be a neutral compound with a vacancy in its valence shell. This is why it can accept electrons and form bonds. Electrophiles are electron deficient.
5. What are the Main Differences Between Electrophile and Nucleophiles?
Ans. Electrophiles are electron acceptors while nucleophiles are electron donors. Electrophiles accept electrons because they are either positively charged or they have empty valence shells. Nucleophiles donate electrons because they either have a negative charge or they have lone pair(s) of electrons in their valence shells. Electrophiles are of the Lewis acid category and nucleophiles are of the Lewis base category. Lastly, electrophiles are symbolized as E+ and nucleophiles are symbolized as Nu-.
6. Are CO2 an Electrophile?
Ans. Yes CO2 is an electrophile. The two strong electronegative Oxygen atoms pull the electrons to their side. The central carbon atom thus gains a partial + charge.
7. Why is BF4 not a Nucleophile?
Ans. BF4 is not a nucleophile because the formation of the atoms makes it difficult for the nucleophile to attack. This effect is called steric hindrance.
8. Is Electrophile Electron-rich?
Ans. No, electrophiles are not electron-rich. They are electron deficient. They attract electrons.