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The p-Block Elements — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThe p-Block Elements3 Marks
Question
Account for the following observations:
Though fluorine is more electronegative than chlorine yet $\mathrm{BF}_3$ is a weaker Lewis acid than $\mathrm{BCl}_3$
✓

Answer

In boron halides, there exists certain degree of pi bonding involving halogen lone pairs and empty $2 p$ orbital of boron. This bonding is strongest in case of $\mathrm{BF}_3$ and when a boron halide accepts electrons from a donor molecule, this bonding is lost. Due to this, $\mathrm{BF}_3$ resists this change most strongly followed by other halides and thus the acceptor strength increases in the order $\mathrm{BF}_3<\mathrm{BCl}_3<\mathrm{BBr}_3<\mathrm{Bl}_3$.
Due to this, $\mathrm{Bl}_3$ a stronger Lewis acid than $\mathrm{BCl}_3$ and $\mathrm{BF}_3$.

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