Understanding more on heterolytic fission

A chemical bond that exhibits the sharing of electron pairs between atoms is known as a covalent bond. It accomplishes this by breaking existing bonds and forming new ones. Bond fission is the broad phrase used to describe the breaking of bonds. Homolytic Fission and Heterolytic Fission are the two types of bond fission. 

Heterolytic fission: 

When a covalent connection is broken in heterolytic fission, one of the atoms takes the shared pair of electrons. The two bound electrons are not equally divided in this form of fission. The prefix ‘hetero-,’ which means ‘different,’ denotes the fact that the two atoms are now distinct since one contains two electrons while the other does not.

Because electrons are negatively charged, this leads to the production of a negatively charged atom. The atom that receives the electrons will be negatively charged (an anion). The other will be charged positively (cation). 

History: 

The discovery and classification of heterolytic bond fission were clearly linked to the discovery and classification of chemical bonds.

In 1916, chemist Gilbert N. Lewis proposed the electron-pair bond, in which two atoms share one to six electrons, resulting in the single electron bond, single bond, double bond, or triple bond. A covalent bond was modelled after this.

Linus Pauling first presented the concept of electronegativity in 1932, along with the idea that electrons in a covalent bond may not be uniformly divided between the bound atoms. 

Solvation effects: 

The pace of reaction for many unimolecular heterolysis processes is greatly influenced by the rate of ionisation of the covalent bond. The production of ion pairs is usually the limiting reaction step. The significance of nucleophilic solvation and its effect on the mechanism of bond heterolysis were investigated in depth by a group in Ukraine. They discovered that the rate of heterolysis is highly influenced by the solvent type. 

However, there is some disagreement about the effects of the solvent’s nucleophilicity; some papers claim that it has no effect, while others claim that more nucleophilic solvents slow down the reaction rate. 

Conditions behind heterolytic fission: 

Assume WY is a covalently bound molecule. If Y is the more electronegative atom, it will take both electrons during heterolytic fission. Both electrons will migrate onto W if it is more electronegative. 

The two-electron displacement in heterolytic fission is indicated by the curved arrow.

When a positive charge is present on the carbon in an organic compound, the cation is called a carbocation. The anion is called carbanion when the carbon has a negative charge. The reaction has two intermediates: a carbocation and a carbanion. Between the reactants and products, an intermediate is created, which is a short-lived unstable species.

There are some circumstances that favour heterolytic cleavages, such as: 

• Differences in electronegativity between the bonding atoms. 
• The presence of a cold environment. 
• A polar solvent is present. 

Heterolysis of bromoalkane: 

Consider the following example of bromoalkane heterolysis: 

CH3CH2Br + H2O → CH3CH2OH + HBr 

The carbon-halogen bond in bromoalkane becomes weaker than the carbon-carbon and carbon-hydrogen bonds due to the electron-withdrawing effect of halogens. 

The carbon-halogen bond is broken in this process. The creation of an ethyl carbocation intermediate and a free bromide ion was discovered to be a type of heterolytic fission during this bond breaking. Ethanol and hydrogen bromide are formed as a result of the process. 

Cations and Anions: 

Cations: 

Cations are ions with a positive charge. When a metal loses its electrons, they form. They lose one or more electrons but none of their protons. As a result, they have a net positive charge. Calcium (Ca2+), potassium (K+), and hydrogen (H+) are examples of cations. 

Anions: 

Anions are ions with a negative charge. When a nonmetal gains electrons, they form. They receive one or more electrons while retaining all of their protons. As a result, they have a net negative charge. Iodide (I–), chlorine (Cl–), and hydroxide (OH–) are examples of anions. 

Heterolytic bond dissociation energy: 

The heterolytic bond dissociation energy is the amount of energy necessary to cleave a covalent bond by heterolytic cleavage (not to be confused with homolytic bond dissociation energy). This figure is occasionally used to represent a covalent bond’s bond energy. A good example of homolytic fission can be seen in the hydrogen chloride molecule, as seen in the chemical reaction below. 

H-Cl → H+ + Cl– 

Because its electronegativity is stronger than that of hydrogen, the chlorine atom preserves the bond pair of electrons. As a result, the chloride anion and hydrogen cation are generated as products. 

Carbocation: 

It is defined as a collection of atoms containing a positively charged carbon atom in its valence shell with just six electrons. Carbocation was previously known as a carbonium ion. The heterolytic fission of a covalent link produces them. Carbocations are classed as primary, secondary, or tertiary depending on whether one, two, or three carbon atoms are connected to the positive-charged carbon atom, as illustrated below: 

The relative stability of carbocation: 

We know that the methyl group has a positive inductive effect, which means it donates electrons. The positively charged carbon atom’s alkyl group tends to release electrons towards carbon. The +ve charge on the carbon atom decreases as a result of this positive inductive action, but the carbon atom itself becomes relatively positive. As a result, the carbon atom’s positive charge is spread. The dispersal of positive charge causes stability, therefore the more alkyl groups connected to the positively charged carbon atom, the more stable the carbocation becomes. 

As a result, relative stability exists as: 

3° > 2° > 1° > CH3 

Carbanion: 

A carbanion is a species that has a negatively charged carbon atom with eight electrons in its valence shell. These are produced by the heterolytic fission of a covalent bond involving a carbon atom, in which the atom connected to the carbon loses its bonding electrons. Carbon gains a negative charge as a result of this. Methyl carbanion is generated when a group linked to a carbon atom loses its electron pair.

Carbocations are classed as primary, secondary, and tertiary, just like carbanions.

The stability order of carbanion is the inverse of that of carbocation, as seen below: 

CH3– > 1° > 2° > 3°  

The +I inductive effect can explain this. Because the alky group has a positive I-effect, it releases electrons and increases the density of negatively charged carbon, making it more unstable. As a result, the carbanion becomes increasingly unstable as the alkyl group linked to the negatively charged carbon atom increases. 

Conclusion: 

When a covalent link breaks unevenly, heterolytic fission occurs, and one of the connected atoms absorbs both electrons from the bond. A negative ion is formed when an atom accepts both electrons (anion). A positive ion is formed when an atom does not accept electrons (cation).