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In BF₃, the B–F bond dissociation energy is 646 kJ mol⁻¹, while in CF₄ the C–F bond dissociation energy is 515 kJ mol⁻¹. What best explains why the B–F bond in BF₃ has the higher bond dissociation energy?

1. The σ-bond between B and F in BF₃ is stronger than the σ-bond between C and F in CF₄.
2. There is appreciable pπ–pπ overlap between B and F in BF₃, whereas such overlap is not possible between C and F in CF₄.
3. The extent of pπ–pπ overlap between B and F in BF₃ is less than that between C and F in CF₄.
4. Boron has a smaller atomic size than carbon, which makes the B–F bond stronger.

Correct answer: There is appreciable pπ–pπ overlap between B and F in BF₃, whereas such overlap is not possible between C and F in CF₄.

Solution

The presence of significant pπ–pπ overlap between boron and fluorine in BF₃ enhances the bond strength, resulting in a higher bond dissociation energy compared to CF₄, where such overlap is not feasible due to the larger size of carbon.

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