COMEDK · Chemistry · 12. p Block Elements (Group 13 & 14)

The order of acidic strength of boron trihalides is

A \(\mathrm{BI}_3 < \mathrm{BBr}_3 < \mathrm{BCl}_3 < \mathrm{BF}_3\)
B \(\mathrm{BF}_3 < \mathrm{BCl}_3 < \mathrm{BBr}_3 < \mathrm{BI}_3\)
C \(\mathrm{BBr}_3 < \mathrm{BCl}_3 < \mathrm{BF}_3 < \mathrm{BI}_3\)
D \(\mathrm{BCl}_3 < \mathrm{BBr}_3 < \mathrm{BI}_3 < \mathrm{BF}_3\)

Verified Solution

Answer & Solution

Correct Answer

(B) \(\mathrm{BF}_3 < \mathrm{BCl}_3 < \mathrm{BBr}_3 < \mathrm{BI}_3\)

Step-by-step Solution

Detailed explanation

Boron trihalides are Lewis acids. The back bonding determines its acidic strength, more is the \(\pi\)-back bonding, less is the acidity. Thus, due to better overlapping of orbitals it promotes better \(\pi\)-back bonding, hence \(\mathrm{BF}_3\) is least acidic in nature. Thus,…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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