JEE Main · 2021 · Shift-ImediumPB11-030

In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?

p-Block Elements (Class 11) · Class 11 · JEE Main Previous Year Question

Question

In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?

Options
  1. a

    BI3\mathrm{BI_3}

  2. b

    BBr3\mathrm{BBr_3}

  3. c

    BCl3\mathrm{BCl_3}

  4. d

    BF3\mathrm{BF_3}

Correct Answerd

BF3\mathrm{BF_3}

Detailed Solution

Strategy: Relate 2p2p2p-2p orbital overlap to the extent of back-donation in boron halides.

Step 1: Mechanism Back-donation involves the overlap of a filled pp-orbital on the halogen with an empty 2p2p-orbital on Boron. Step 2: Comparison

  • In \ceBF3\ce{BF3}, overlap is 2p(B)2p(F)2p(\text{B})-2p(\text{F}), which is highly effective due to similar size and energy.
  • In \ceBCl3,\ceBBr3\ce{BCl3}, \ce{BBr3}, the overlap is 2p3p,2p4p,2p-3p, 2p-4p, etc., which progressively weakens.
  • Therefore, the strongest back-donation occurs in \ceBF3\ce{BF3}.

Answer: (Option) [d] \ceBF3\boxed{\text{Answer: (Option) [d] $\ce{BF3}$}}

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