What is Heterolytic Bond Breaking?

Heterolytic bond breaking is also known as heterolysis or heterolytic fission or ionic fission.  It is defined as breaking of a covalent bond between two different atoms in which one atom gains both of the shared pair of electrons. The atom that gains both electrons is more electronegative than the other atom in covalent bond. The energy needed for heterolytic fission is called as heterolytic bond dissociation energy.

A general heterolytic fission is given as:

Covalent Bond

Covalent bond is defined as a chemical bond between two same or different atoms in which two electrons are equally shared by both atoms.  These electrons in covalent bond are called as shared pair electrons.  Sharing of two electrons by atoms in covalent bond helps them to achieve octet number of electrons in outer shell. 

Examples: Covalent bonds between same types of atoms are $C{l}_2,B{r}_2,O_2$

Covalent bonds between different types of atoms are $PC{l}_5,C{H}_4,P{H}_3$

Types of Covalent Bond

• Single covalent bond
• Double covalent bond
• Triple covalent bond
• Polar covalent bond
• Non-polar covalent bond

Single covalent bond

This type of bond formed from two electrons shared by atoms involved in covalent bonding. The electron density in single covalent bond is less than double and triple bonds. Therefore, it is weaker bond. An example is hydrogen bromide, $H-Br$.

Note: Two electrons involved in covalent bond do not belong to one particular atom.   Each atom donates an electron to form covalent bond.

Double covalent bond

Double bond in covalent bond is represented as =. Two electron pairs between participating atoms are involved to form double covalent bond. Double bond means total of two shared pairs of electrons are there in between the atoms. It is stronger than single covalent bond. An example for compound with double bond is $H_{2}C=C{H}_2$.

Triple bond

Three electron pairs between participating atoms are involved to form triple covalent bond.  It is represented by three dashes ≡.  Bond between carbon-carbon in acetylene is a triple bond.

Note:  Triple bond does not mean three electrons in each bond.  It consists of three covalent bonds.

Polar covalent bond

This type of bonds exists when the sharing of electrons is not equal between the atoms involved in covalent bond. It occurs because of presence of higher electronegative atom in the participating atoms. Atom with greater electronegativity has greater domination over shared pair of electrons. In this kind of bond, difference in electronegative between atoms must be between zero to two. Bond between hydrogen fluoride $H-F$ is a polar covalent bond. Atoms involved in polar covalent bond must not have similar electronegativity.

Non-polar covalent bond

Non-polar covalent bonds are formed due to mutual sharing of electrons between the atoms involved in bond. The electronegativity difference between participating atoms should be zero. Atoms with similar electron affinity have tendency to form non-polar covalent bond. Example: hydrogen gas ($H_2$). Atoms involved in non-polar covalent bond must be same type of atoms.

Bond Fission

Bond fission is also known as bond breaking or bond cleavage. Bond fission is defined as breaking of chemical bonds. During a chemical reaction, reactants react to form products in which covalent bonds in reactants breaks to form new bonds in products.  Two types of bond fission is given as,

• Homolytic fission
• Heterolytic fission

Homolytic fission

Homolytic fission involves bond breaking of molecule in such a way that each electron is retained by two atoms equally. When a neutral molecule undergoes homolytic fission, it forms free radicals as products. The energy needed to perform hemolytic fission is called as homolytic bond dissociation energy.    

Outcome of heterolytic fission

During the heterolytic fission of a neutral molecule, an atom that retains both of the shared pair of electrons in covalent bond is called as anion which is negatively charged.  Similarly, the other atom in covalent bond that does not retain electrons is known as a cation, which is positively charged. Between the chemical species in a covalent bond, species with higher electronegativity attracts the shared pair of bonded electrons and gets negative charge on it but electropositive species get positive charge due to loss of shared pair of electrons. An example of heterolytic fission is given by

$$\begin{array}{l}H-F\stackrel{}{\to }{}^{}{H}^{+}+F^{-}\\ NeutralmoleculeCationAnion\end{array}$$

In the above heterolysis, bond fission occurred in hydrogen fluoride molecule. Fluorine is more electronegative compared to hydrogen which results in fluorine atom retaining shared pair of electrons. Thus, fluorine is negatively charged and hydrogen is positively charged. 

Note:  In heterolysis, electrons are not mutually shared between atoms.  Atom with greater electronegative attracts both electrons in bond. 

Ring opening of cyclic compound

In the heterolytic fission of cyclic compound, ring opening occurs in which the broken molecule gives a single unit as product. A single acyclic structure is formed when ring opening of epoxide compound by heterolysis of $C-O$bond.

Note: Ring opening of cyclic compound does not yield two different products. After heterolytic fission, only one compound forms from cyclic compound.

Factors that favor heterolytic fission

• Higher electronegativity difference between chemical species in covalent bond.
• Reaction occurs at lower temperature.
• Polar solvent in the reaction.

Importance

• In biochemistry, the splitting of internal bonds of large molecules by heterolytic fission. This process is called as catabolism.
• In proteomics, small peptide units are formed by cleavage of proteins using cleaving agents. The cleaving agents are trypsin, pepsin, and cyanogen bromide.

Context and Applications

This topic is important in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.

Practice Problem

1. Identify the type of bond fission in the below reaction.

$C{H}_{3}Br\stackrel{}{\to }C{H}_3{}^{+}+B{r}^{-}$

A.  Homolytic fission

B. Heterolytic fission

C.  Free radical formation

D.  All the above.

Solution

In the given reaction, methyl bromide undergoes dissociation to form methyl cation and bromide anion.  The polar covalent bond between carbon and bromine atom breaks and both shared electrons moves toward bromine atom.  Therefore, the bond breaking involved in the given reaction is heterolytic fission.