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: [2009](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 \(\mathrm{p}\) - 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

(b) Significant p-p interaction between B and F in \(\mathrm{BF}_3\).

Step 1: Understand the Problem

We are given bond dissociation energies for B-F in \(\mathrm{BF}_3\) and C-F in \(\mathrm{CF}_4\) and need to identify why B-F has a higher dissociation energy. The options suggest different reasons related to bond strength and atomic interactions.

Step 2: Analyze Bond Interactions

Option (b) suggests that the B-F bond in \(\mathrm{BF}_3\) benefits from significant p-p interactions, which increases bond strength. This interaction is described as absent or less significant in C-F bonds within \(\mathrm{CF}_4\).

Step 3: Compare Bond Energies

The given bond dissociation energies show that B-F has a higher energy (646 \(\mathrm{kJ} \mathrm{mol}^{-1}\)) than C-F (515 \(\mathrm{kJ} \mathrm{mol}^{-1}\)), implying that B-F bonds are generally stronger.

Step 4: Evaluate Atomic Properties

Both (b) and (d) relate to atomic properties: (b) with orbital interactions and (d) with atomic size. Smaller atoms like B could form stronger bonds due to closer proximity, but the significant \(\mathrm{p}-\mathrm{p}\) interactions proposed in (b) provide a more definitive reason for increased bond strength.

Step 5: Select the Best Explanation

Given the role of orbital interactions in bond strength, option (b), which highlights the significant p-p interactions contributing to a stronger B-F bond in \(\mathrm{BF}_3\), is the most comprehensive reason for the observed higher bond dissociation energy compared to \(\mathrm{CF}_4\).

Key Concepts

p-p Interaction

When we talk about the bond strength in chemistry, one essential aspect to consider is orbital overlap. Orbital interactions significantly influence the energy and stability of bonds. The term 'p-p interaction' refers to the overlap between p orbitals of two atoms, which is a crucial factor in bond formation. In

• p-p interactions, both participating atoms must have available and compatible p orbitals.
• This overlap increases the electron density between the atoms, strengthening the bond.
• In \( ext{BF}_3\), significant p-p orbital overlap occurs between boron and fluorine, resulting in stronger bonds.

However, this sort of interaction is less pronounced in \( ext{CF}_4\).

Thus, the presence and efficiency of p-p interactions can determine why certain bonds, like B-F, might have higher bond dissociation energies compared to others.

B-F Bond

The bond between boron and fluorine in

• \(\text{BF}_3\) is highly polar, with fluorine being more electronegative than boron.
• This polarity results in a bond with partial ionic character.
• Additionally, the significant p-p orbital overlap, as explained before, contributes to an exceptionally stable and strong bond configuration.

In essence, the blend of bond polarity and orbital interaction makes B-F bonds particularly robust, leading to a higher dissociation energy than one might find in less polar covalent bonds.

C-F Bond

In contrast to the B-F bond, the carbon-fluorine bond in

• \(\text{CF}_4\) also features a considerable electronegativity difference.
• This bond also leans towards being polar, yet it lacks the p-p interactions found in B-F.
• In carbon tetrafluoride, the structure forms through sp3 hybridization, which does not favor the same level of p orbital overlap.

The absence of such orbital interactions means the C-F bond has a lower dissociation energy compared to B-F, even though it still remains relatively strong due to its polarity.

Atomic Size

The atomic size plays a pivotal role in bond strength.

• Boron, being a smaller atom than carbon, can form tighter and more effective bonds due to greater orbital proximity.
• Fluorine, being the same size in both B-F and C-F, adds to this interaction by bringing its high electronegativity to play.
• Smaller atoms allow for closer interaction and overlap of orbitals, which can account for increased bond stability and energy in boron-fluorine bonds.

Therefore, while atomic size might not seem crucial initially, it contributes significantly to the strength variations seen in B-F and C-F bonds.

Orbital Interactions

Orbital interactions define how atoms approach and affect each other during bond formation.

• In molecules like \(\text{BF}_3\), the interaction between atomic orbitals contributes to creating strong bonds.
• These interactions depend on how well the orbitals can overlap, their shapes, and energy similarities.
• Efficient orbital overlap in B-F stems from available and compatible orbitals, creating stronger bonds due to increased electron sharing.

Such orbital overlaps in \(\text{BF}_3\) showcase how essential these interactions are in defining molecular stability, directly impacting bond dissociation energies.