What is Bond Dissociation Energy?

Bond dissociation energy, which is also abbreviated as BDE, simply put, is the energy required to dissociate or break the bond. An endothermic process is involved to break a bond that gives two fragments, atomic or molecular, possessing one electron from the initial molecule. The energy required to carry out this process is known as bond dissociation energy. For bond dissociation, the standard enthalpy change is given by the symbol, $\Delta {\text{H}}_{\text{0}}$

Effects on Bond Dissociation Energy

Electronegativity

The tendency of an atom to attract an electron is known as its electronegativity.

Consider a carbon halogen bond $\left(\text{C-X}\right)$when the $\text{C-X}$bond undergoes fission, the carbon becomes electropositive in nature and its tendency to attract the electron from the electronegative element increases. The atom that gained the electron tends to have increased electronegativity. And as the electronegativity of an atom increases the bond energy increases.

The increasing order for bond dissociation energies for the $\text{C-X}$bond is as follows-

$$\text{C-I>C-Br>C-Cl>C-F}$$

This order matches with that of the polarities of the $\text{C-X}$bonds.

Hybridization

The $\text{C-H}$bond energy trend for different hybridization is as follows -

$${\text{sp}}^{\text{3}}{\text{<sp}}^{\text{2}}\text{<sp}$$

Depending on the percentage of the s and p contribution in a molecule the average distance between the electron and the nucleus can be calculated. For instance, in a ${\text{sp}}^3$orbital the $2\text{s}$ orbital contributes $25\%$ whereas it contributes $50\%$ in a sp Hybrid orbital. The smallest s character of the ${\text{sp}}^3$ hybrid orbital causes the distance of the electrons and nucleus to increase and therefore, the weakest bond is formed with this orbital.

The increasing order of electronegativity of the carbon from the hybridization is as follows-

$${\text{sp}}^{\text{3}}{\text{<sp}}^{\text{2}}\text{<sp}$$

Therefore, the bond strength increases with increase in electronegativity and thus the bond dissociation energy also increases.

Based on the trend with hybridization, the triple bond has more bond energy than the double bond which is followed by the single bond. This is also related to the increase in the s character of the hybrid orbital effects. However, the increased number of bonds joining the carbon atoms is largely responsible for the significant increase in carbon-carbon bond strength. As the strength of the bond increases the bond dissociation energy also increases.

How do you Calculate Bond Dissociation Energy?

For the homolysis of a $\text{X-Y}$molecule, the energy of bond dissociation is calculated as difference in the total enthalpy of formation for the reactants and products.

$$\text{BDE = }\Delta {\text{ H}}_{\text{f}}\left(\text{X}\right)+\Delta {\text{ H}}_{\text{f}}\left(\text{Y}\right)–\Delta {\text{ H}}_{\text{f}}\left(\text{X-Y}\right)$$

Where, $\Delta {\text{H}}_{\text{f}}$is the heat of formation. Mostly the bond dissociation energy is calculated for a particular bond and therefore it consists of fragments such as radicals since they undergo hemolytic fission.

How to Compare Bond Dissociation Energy?

The bond dissociation energy between two molecules can be compared by measuring the enthalpy of the reactants and products, the difference between the two would give us a quantitative measure. Another way would be by deducing the electronegativity between the bonds being dissociated and concluding the more electronegative bond breaking to hold the highest value for bond dissociation energy.

This helps in studying the energy requirements for the reactions with specific reactants involved and the thermochemistry of a reaction is well understood by the study of their bond dissociation energy. The periodic table trend is also involved to compare bond dissociation energy. As we go from left to right in the periodic table, the electronegativity increases and therefore the bond dissociation energy also increases. However, this can be influenced by neighboring atoms, if present. The polarity of the neighboring atom in a molecule can alter the bond dissociation energy of the bond under study since the electronegativity of the atom is indirectly influenced.

Bond Dissociation Energy of Halogens

Halogens have a very intriguing part when it comes to bond dissociation energy. As mentioned earlier electronegativity of an element determines its energy of bond dissociation. Therefore, fluorine being the most electronegative in the halogen series is known to have the highest energy for bond dissociation as compared to chlorine and bromine and iodine. Due to its small size the interatomic distance is less and this cause it to have more bond strength. One of such increased bond strengths is seen in the case of $\text{Si-F}$bonds. Both the atoms possess small size and high electronegativity. The bond strength is increased due to the ionic and covalent contributions. Therefore, the $\text{Si-F}$ bonds are known to be the strongest single bonds according to the data of bond dissociation energies. This information is in use when a fluorine or silicon atom is required to be abstracted from a molecule in a reaction. Fluorine atoms are introduced to abstract a silicon atom and vice versa.

Bond Dissociation of Ionic and Covalent Bond

Covalent bond involves sharing of electrons whereas ionic bond involves transfer of electrons in a molecule. Due to the electrostatic forces involved in the ionic bond formation, they are stronger than the bond formed using covalency. The strong attraction of the opposite charges in an ionic bond is difficult to be separated. Therefore, it can be said that the bond dissociation energy for ionic bonds is higher than that of covalent bonds.

Are Bond Energy and Bond Dissociation Energy the Same?

The answer would be no, they are not the same. Bond energy tells the strength of chemical bonds in a molecule collectively with reference to the bond dissociation energy between individual atoms. Therefore, with the information about the bond energy, one can simply put the amount of energy required to completely dissociate a molecule; whereas with the information of the bond dissociation energy one can only dissociate the particular bond between the two atoms in consideration. Bond dissociation energy is required to calculate the bond energy; this further helps in knowing the reaction condition required for a particular reaction in regards to the energy to be applied for bond fission or molecule dissociation. This information are collectively studied under the thermodynamics of the reaction.

The difference between bond energy and bond dissociation is useful when a particular substituent of the molecule needs to be eliminated without disturbing the structure or orientation of the molecule. When protecting groups are involved in the reaction the separation of the element and the group is enhanced with the information of its bond dissociation energy. Auxiliary groups that are used to introduce chirality in a molecule are also released in the similar way. The knowledge of specific reagents and supplying the right amount of energy are the essential parts of a reaction to be carried out in the desired way.

Context and Applications

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