JEE Advanced Chemistry question

Consider the following boron halides I. BF3 II. BCl3 III. BBr3 IV. BI3 The Lewis acid characters of these halides are such that

(A)I < II < III < IVCorrect
(B)I < III < II < IV
(C)IV < III < II < I
(D)IV < II < III < I

Answer: (A)

Solution

Lewis acidity in boron trihalides is reduced by pπ–pπ back-donation from a filled halogen p-orbital into boron's empty p-orbital, which partially satisfies boron's electron deficiency. This back-bonding is strongest for fluorine (best orbital-size match with boron's small 2p

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Question ID VK-159359