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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3pb051

Select the correct statement(s) about boron trihalides.

A $BF X_{3}$ exists as a dimer $B X_{2} F X_{6}$ like $AlCl X_{3}$ .
B $BCl X_{3}$ is hydrolysed by water to $H X_{3} BO X_{3}$ and $HCl$ .
C $BF X_{3}$ is a stronger Lewis acid than $BCl X_{3}$ because fluorine is more electronegative.
D $BF X_{3}$ forms an adduct with $NH X_{3}$ in which boron becomes $s p^{3}$ hybridised.

Answer: B, D

BCl X_{3} + 3 H X_{2} O H X_{3} BO X_{3} + 3 HCl

, so A is true.

F X_{3} B \leftarrow NH X_{3}

has four bonds on B, hence

s p^{3}

, so B is true.

BF X_{3}

is weaker because of the strong

2 p π

-

2 p π

back-bonding, not stronger, so C is false. Small boron cannot accommodate bridging halogens and

BF X_{3}

is monomeric, so D is false.

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