Problem 213 - A Complete Resource Book in Chemistry for JEE Main

Question

The bond dissociation energy of \(\mathrm{B}-\mathrm{F}\) in \(\mathrm{BF}_{3}\) is 646 \(\mathrm{kJ} \mathrm{mol}^{-1}\) whereas that of \(\mathrm{C}-\mathrm{F}\) in \(\mathrm{CF}_{4}\) is \(515 \mathrm{~kJ} \mathrm{~mol}^{-1}\). The correct reason for higher B - F bond dissociation energy as compared to that of \(\mathrm{C}-\mathrm{F}\) is: (a) stronger bond between \(\mathrm{B}\) and \(\mathrm{F}\) in \(\mathrm{BF}_{3}\) as compared to that between \(\mathrm{C}\) and \(\mathrm{F}\) is \(\mathrm{CF}_{4}\) (b) significant \(\mathrm{p}-\mathrm{p}\) interaction between \(\mathrm{B}\) and \(\mathrm{F}\) in \(\mathrm{BF}_{3}\) whereas there is no possibility of such interaction between \(\mathrm{C}\) and \(\mathrm{F}\) in \(\mathrm{CF}_{4}\) (c) lower degree of \(\mathrm{p}-\mathrm{p}\) interaction between \(\mathrm{B}\) and \(\mathrm{F}\) \(\mathrm{BF}_{3}\) than that between \(\mathrm{C}\) and \(\mathrm{F}\) in \(\mathrm{CF}_{4}\) (d) smaller size of \(\mathrm{B}\) - atom as compared to that of C- atom

Step-by-Step Solution

Answer

Option (b): significant \( p-p \) interaction between \( \text{B} \) and \( \text{F} \) in \( \text{BF}_3 \).

Step 1: Analyze Given Bond Dissociation Energies

We are provided with the bond dissociation energies for two different bonds. In \( \text{BF}_3 \), the \( \text{B}-\text{F} \) bond has a dissociation energy of 646 \( \text{kJ mol}^{-1} \), whereas in \( \text{CF}_4 \), the \( \text{C}-\text{F} \) bond has a dissociation energy of 515 \( \text{kJ mol}^{-1} \). This indicates that the \( \text{B}-\text{F} \) bond is stronger than the \( \text{C}-\text{F} \) bond.

Step 2: Analyze Options for Underlying Causes

We have four options explaining why the \( \text{B}-\text{F} \) bond is stronger. Option (a) suggests a stronger bond, option (b) suggests \( p-p \) interaction in \( \text{BF}_3 \), option (c) suggests lower \( p-p \) interaction in \( \text{BF}_3 \), and option (d) suggests the smaller size of the \( \text{B} \) atom.

Step 3: Consider p-p Interaction Hypothesis

In \( \text{BF}_3 \), the \( \text{B} \) atom has empty \( p \) orbitals that can participate in \( p-p \) back bonding with \( F \)'s filled \( p \) orbitals. This can enhance the bond strength through resonance, unlike in \( \text{CF}_4 \), where carbon cannot participate in \( p\)-\( p \) back bonding with fluorine.

Step 4: Evaluate Explanation for Answer Selection

Option (b) correctly explains the higher bond dissociation energy for \( \text{B}-\text{F} \) in \( \text{BF}_3 \) as compared to \( \text{C}-\text{F} \) in \( \text{CF}_4 \). The significant \( p-p \) interaction between \( \text{B} \) and \( \text{F} \) in \( \text{BF}_3 \) greatly strengthens the bond making it harder to break.

Key Concepts

BF3

Boron trifluoride (\(\text{BF}_3\)) is an interesting molecule due to its unique bonding characteristics. Within \(\text{BF}_3\), there are three boron-fluorine bonds. Each of these bonds contributes to the molecule's overall stability. Bond dissociation energy is a key term here. It refers to the energy required to break a bond in a molecule. For the \(\text{B}-\text{F}\) bond in \(\text{BF}_3\), this energy is 646 \(\text{kJ mol}^{-1}\). This high value suggests that the \(\text{B}-\text{F}\) bond is particularly strong compared to other similar bonds.
The reason for this strength is attributed to the possibility of \(p-p\) back bonding. The boron atom in \(\text{BF}_3\) has empty \(p\) orbitals. These orbitals are capable of forming back bonds with the filled \(p\) orbitals of fluorine. This back bonding creates a resonance effect. This resonance increases the stability and strength of the \(\text{B}-\text{F}\) bond.

• \(\text{BF}_3\) has strong \(\text{B}-\text{F}\) bonds.
• High bond dissociation energy of 646 \(\text{kJ mol}^{-1}\).
• \(p-p\) back bonding enhances bond strength.

CF4

Carbon tetrafluoride (\(\text{CF}_4\)) showcases a different bonding scenario compared to \(\text{BF}_3\). In \(\text{CF}_4\), carbon forms four bonds with fluorine atoms. The bond dissociation energy for each \(\text{C}-\text{F}\) bond is 515 \(\text{kJ mol}^{-1}\). While this is a significant amount of energy, it's less than that of the \(\text{B}-\text{F}\) bond in \(\text{BF}_3\).
The difference can be attributed to the lack of \(p-p\) back bonding in \(\text{CF}_4\). Carbon atoms, unlike boron, do not have the capacity for \(p-p\) back bonding with fluorine. This absence means that the \(\text{C}-\text{F}\) bonds do not benefit from the extra stabilization that \(p-p\) back bonding can offer.

• \(\text{CF}_4\) does not form \(p-p\) back bonds.
• \(\text{C}-\text{F}\) bond dissociation energy is 515 \(\text{kJ mol}^{-1}\).
• Lack of \(p-p\) interaction reduces bond strength.

p-p interaction

The concept of \(p-p\) interaction is key to understanding the difference in bond strengths between \(\text{BF}_3\) and \(\text{CF}_4\). \(p-p\) interaction, specifically \(p-p\) back bonding, occurs when there is a resonance between two elements through their \(p\) orbitals.
In \(\text{BF}_3\), boron has empty \(p\) orbitals. These orbitals can overlap with the filled \(p\) orbitals of fluorine, enabling \(p-p\) back bonding. This interaction strengthens the \(\text{B}-\text{F}\) bond, making it more difficult to break. Conversely, in \(\text{CF}_4\), carbon cannot participate in similar interactions because it lacks the required empty \(p\) orbitals interacting with fluorine.

• \(p-p\) back bonding involves \(p\) orbital overlap.
• Occurs in \(\text{BF}_3\), not in \(\text{CF}_4\).
• Strengthens \(\text{B}-\text{F}\) bonds due to resonance.

Bond strength comparison

When comparing the bond strengths of \(\text{BF}_3\) and \(\text{CF}_4\), it becomes evident that the \(\text{B}-\text{F}\) bonds in \(\text{BF}_3\) are stronger than the \(\text{C}-\text{F}\) bonds in \(\text{CF}_4\). This is clear from their bond dissociation energies: 646 \(\text{kJ mol}^{-1}\) for \(\text{B}-\text{F}\) versus 515 \(\text{kJ mol}^{-1}\) for \(\text{C}-\text{F}\).
The key factor influencing this difference is the presence of \(p-p\) back bonding in \(\text{BF}_3\). This back bonding allows for greater bond stabilization through additional bonding interactions. In \(\text{CF}_4\), however, such interactions do not occur. Hence, the bonds are less strong.

• \(\text{B}-\text{F}\) bonds are stronger than \(\text{C}-\text{F}\) bonds.
• Presence of \(p-p\) interaction in \(\text{BF}_3\) enhances strength.
• No \(p-p\) back bonding in \(\text{CF}_4\) leads to lower bond strength.