Cleavage of Bond

When a chemical reaction initiates, all the bonds of reactants get broken and new bonds are formed during and after the reaction. This process of breaking down the bond during chemical reaction is called cleavage of bonds. There are two types of cleavage in general.
1. Homolytic cleavage: It involves the fission of covalent bond in such a way that each of the bonded atom takes away one electron out of their shared pair. It is shown by half headed curved arrows. This type of fission generally occurs if the bonded atoms have similar electronegativities. It is generally form in the presence of light, heat, peroxide etc.

Homolytic cleavage

Homolytic cleavage produces:

• Free radical species.
• Paramagnetic species.

2. Heterolytic cleavage: In heterolytic cleavage, the shared pair is taken by one of the most electronegative atoms. It is shown by bend arrow. This type of fission generally occurs if the bonded atoms have different electronegativities.

Heterolytic cleavage

Heterolytic fission produces:

• Ionic species called carbocation or carboanion.
• Diamagnetic species.

Electronic Effects in Covalent Bonds

There are few electronic effect in covalent bonding. These effect arises due to the nature of reacting species, condition of reactions and other different factors. Some of the common electronic effects are:
1. Inductive effect:
2. Electromeric effect
3. Mesomeric effect
1. Inductive effect: When there is a formation of covalent between two atoms having different electronegativities, the bonding electron pair shift towards the more electronegative atom resulting in a certain degree of polarity produced in this bond. This effect is called inductive effect. In other words, the polarity developed in a covalent compound due to the difference in electronegativities of a bonding atom is called inductive effect. It is also called as I effect.
In C-Cl, the bond is displaced slightly towards the chlorine atom as it has higher electronegativity. So, a slightly negative character is developed in chlorine atom while a positive character is developed in carbon atom. This illustrates the I-effect.  In case of C-C, the bond is exactly in between the carbon atoms as they have equal electronegativities. So this bonding does not have I-effect.

Note: It is a permanent effect and this can be forwarded from one carbon to another carbon.

There are two types of Inductive effect:
a. – I effect
b. + I effect:
a. – I effect: The atom or group of atom is said to have – I effect if it attacks the bonded electrons away from the carbon. Examples: -Cl, -Br, -NO₂, -CN, -OH etc.
The decreasing order of I effect is given below:
-CN > NO₂ > COOH > CHO > F > Cl > Br > I
b. + I effect: The atom or group of atom is said to have +I effect if it releases electron towards carbon. Examples: Alkyl group such as CH₃, C₂H₅ etc.
Electromeric effect: The effect in which involves the complete transfer of bonded electrons to one of the atoms joined together by a multiple bond in presence of attacking agent is called electromeric effect. No electrometic effect takes place without the presence of attacking agent. This effect is observed in multiple bonded compounds. This is a temporary effect.
Mesomeric effect: The mesomeric effect is observed in conjugated compounds. The effect which refers to the polarity produced in the molecule as a result of interaction between two pi bonds or one pi bonds and a lone pair of electrons is called mesomeric effect. It is also a permanent effect.

Electrophiles and Nucleophiles

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
2. Nucleophiles
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.
   

  Resonance and Resonance Energy

  Some of the compounds can be represented by two or more than two structures called resonating structure. This phenomenon of representing the molecules by more than one structure is called resonance.
  Resonating structure differs from each other only in the arrangement of electrons.
  

  

  Resonating structure of benzene

  The actual structure of the compounds cannot be represented by any of the resonating structures. Actual structure of the compound is hybrid of all the resonating structure called hybrid structure. In the figure below, first two are the resonating structures and third is the resonating hybrid of benzene.
  The energy of hybrid structure is lower than any of the resonating structure. This dfference in energy between any of the resonating structure and hybrid structure is known as resonance energy.
  

  Note: More the number of resonating structures, the more stable will be compound.