JEE Main · 2023 · 31 Jan 2023 · Shift 2 · Chemical Bonding and Molecular Structure

The Lewis acid character of boron tri halides follows the order :

(A) $BBr_{3} > BI_{3} > BCI_{3} > BF_{3}$
(B) $BCl_{3} > BF_{3} > BBr_{3} > BI_{3}$
(C) $BF_{3} > BCl_{3} > BBr_{3} > BI_{3}$
(D) $BI_{3} > BBr_{3} > BCl_{3} > BF_{3}$ ✓ Correct

Answer: (D)

Solution

The Lewis acidity of a boron trihalide is governed by the back-donation of a halogen lone pair into the empty 2p orbital of boron. This overlap is most effective for the small fluorine atom, so boron in

B F_{3}

is least electron deficient, and it weakens as the halogen grows larger through chlorine, bromine and iodine. Since weaker back-bonding leaves boron more electron deficient and hence a stronger acceptor, the Lewis acid strength increases in the order

B I_{3} > BB r_{3} > BC l_{3} > B F_{3}

JEE Main · 2023 · 31 Jan 2023 · Shift 2 · Single correct option

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JEE Main · 2023 · 31 Jan 2023 · Shift 2 · Chemical Bonding and Molecular Structure

Solution

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