Electrophiles and Nucleophiles
A carbon supplying reactant molecule (substrate) can have two main regions, the electron rich region and electron deficient region. These two regions together determine the way the substrate will react and the point the substrate will act on.
For example, the electron rich part of the substrate will attack the electron deficient region of the atom, and vice-versa. Thus a substrate is of two types: electron deficient and electron rich.
Electrophiles
Electrophiles
This is the first type of reagent or carbon compound. This type of substrate molecule has an electron deficient region and thus they attack the electron rich region of other substrate molecule.
As they have an electron deficient region, they are positively charged molecules; it seems opposite to the charge decided by the valence shell, but if we look carefully it makes sense. So, they attack the substrate at the point of maximum electron density.
Some examples of electrophiles are \(\ce{H+}, \ce{I+}, \ce{Na+},\) etc.
Which of the following pairs represents a set of electrophiles?
(a) \(\ce{Br+}\) and \( \mathbf{\colon}\ce{CCl2}\)
(b) \(\ce{H+}\) and \(\ce{H2O}\)
(c) \(\ce{BF3}\) and \(\ce{NH3}\)
(d) \(\ce{H-}\) and \(\ce{AlCl3}\)
Nucleophiles
Nucleophiles
This is the second type of reagent or the carbon compound. This type of substrate molecule has an electron-rich region and thus they attack the electron deficient region of another substrate molecule.
Nucleophiles have an electron-rich region and are often negatively charged. Nevertheless, some neutral molecules containing an atom with a lone pair may also behave as (weak) nucleophiles. One important thing to remember is that all alkenes are nucleophiles. Some other examples of nucleophiles are \(\ce{OH-}\), \(\ce{CN-}\), \(\ce{NH2-}\), \(\ce{NH_3}\), and \(\ce{H_2O}.\)
Electrophiles - electron loving, Nucleophiles - nucleus loving.
Reaction Mechanism
A series of steps involved in the transformation of reactants into products is called reaction mechanism. It will give us an outline of what actually occurs during a chemical reaction.
The organic reactions and its mechanism are classified into three groups:
- substitution reaction
- addition reaction
- elimination reaction.
As this wiki doesn't focus much on reaction mechanism, we will limit ourselves to this. Let's move on to the next topic.
Free Radical and Ion Formation
The covalent bonds can break down under the influence of a strong electrophile or nucleophile. They can break down in two ways: homolytic fission and heterolytic fission.
Homolytic fission (or homolysis) occurs when the two atoms forming the covalent bond have the same electronegitivity. So both the atoms split in such a way that they get an equal share of electrons.
In this process each atom takes away one of the two electrons forming a single covalent bond. It will produce two new species having an unpaired electron. These chemical species with only one unpaired electron are called free radicals. These species actively take part in chemical reactions and are highly reactive.
\[\huge\ce{A-B->A^{.} +B^{.} }\]
How does \(\ce{Cl2}\) divide under an influence?
We know that all chlorine atoms have similar electronegitivity. They will undergo homolytic fission as follows: It will produce \(\ce{2Cl-},\) which is highly reactive and will form a bond with an electrophile.
Heterolytic fission (or heterolysis) occurs when the two atoms forming the covalent bond have the different electronegitivity. So both the atoms split in such a way that they get an unequal share of electrons.
In this process the atom with more electronegitivity will attract the bonded pair of electrons towards itself, resulting in two ions. Similarly here, the charged species carrying a positive charge will attack the substrate at the point of maximum electron density and vice-versa.
\[\huge\ce{A-B->A- +B+}\\ \huge\ce{A-B->A+ +B-}\]
In the figure above if A is more electronegative, it will become a negative ion and B will be positive; if B is more electronegative, it will become a negative ion and A will become positive.
How will \(\ce{HCl}\) split, provided \(\ce{Cl}\) is more electronegative than \(\ce H?\)
As the electronegitivity of hydrogen and chlorine are not similar, they undergo heterolytic fission. They split into ions as follows:
\[\huge\ce{H-Cl-> H+ +Cl-}\]