Boron Halides PYQs MCQs

Boron Halides MCQs
Asked in NEET, JEE, JAM, GATE, CSIR-NET

JEE Advanced 2017 (Paper-1)

Question: Which of the following statements is correct for BF₃ and BCl₃?

1. BF₃ is a stronger Lewis acid than BCl₃
2. BCl₃ is a stronger Lewis acid than BF₃
3. Both have the same Lewis acid strength
4. Neither acts as a Lewis acid

Correct Answer: B

Detailed Explanation:
Although back-bonding (pπ–pπ) is stronger in BF₃ than in BCl₃, the Lewis acidity order is reverse of what one expects from back-bonding alone. In gas phase or non-coordinating solvents, BCl₃ > BBr₃ > BI₃ > BF₃ in Lewis acidity. This is because the B–F bond has significant ionic character and the tendency of fluorine to form stronger σ-bonds reduces the electron deficiency on boron less effectively in larger halides. Experimentally, BCl₃ forms stronger adducts with amines, phosphines, etc., than BF₃. Hence BCl₃ is a stronger Lewis acid than BF₃.

CSIR-NET Chemical Sciences Dec 2019

Question: The order of Lewis acidity of boron trihalides is

1. BF₃ > BCl₃ > BBr₃ > BI₃
2. BI₃ > BBr₃ > BCl₃ > BF₃
3. BCl₃ > BF₃ > BBr₃ > BI₃
4. BF₃ > BBr₃ > BCl₃ > BI₃

Correct Answer: B

Detailed Explanation:
The accepted order of Lewis acidity of boron trihalides is BI₃ > BBr₃ > BCl₃ > BF₃. The major factor is the decreasing pπ–pπ back bonding from F to I due to poor overlap of 2p(B)–3p(Cl), 4p(Br), 5p(I) orbitals. As back bonding decreases, the electron deficiency on boron increases, hence acidity increases from BF₃ to BI₃.

IIT-JAM Chemistry 2021

Question: Boron trifluoride behaves as a Lewis acid with ammonia, but as a Lewis base with

1. BF₃
2. HF
3. F⁻
4. Ni²⁺

Correct Answer: C

Detailed Explanation:
BF₃ behaves as a Lewis base toward F⁻ ion and forms the tetrahedral [BF₄]⁻ anion by accepting a pair of electrons from fluoride. This is a classic example of BF₃ acting as a Lewis acid acceptor (base) in the presence of excess fluoride. Reaction: BF₃ + F⁻ → [BF₄]⁻.

GATE Chemistry 2022

Question: The geometry of BF₃ is planar triangular, but that of [BF₄]⁻ is

1. Planar triangular
2. Tetrahedral
3. Trigonal pyramidal
4. Square planar

Correct Answer: B

Detailed Explanation:
BF₃ has only 6 electrons around B (sp² hybridized, trigonal planar). When it accepts a pair of electrons from F⁻ to form [BF₄]⁻, boron becomes sp³ hybridized with 8 electrons and tetrahedral geometry.

NEET 2020

Question: In which of the following, boron does not act as Lewis acid?

1. BF₃
2. BCl₃
3. B₂H₆
4. [BH₄]⁻

Correct Answer: D

Detailed Explanation:
In [BH₄]⁻, boron already has 8 electrons (4 bonds) and a negative charge, so it cannot accept any more electron pair. Hence it does not behave as a Lewis acid. In BF₃, BCl₃, and B₂H₆ (which has vacant orbitals on boron), boron acts as Lewis acid.

JEE Main Jan 2019

Question: Among the following, the molecule that has back bonding is

1. BCl₃
2. AlCl₃
3. GaCl₃
4. BF₃

Correct Answer: D

Detailed Explanation:
BF₃ shows strong pπ–pπ back bonding between empty 2p orbital of boron and filled 2p orbital of fluorine. In BCl₃ the overlap is poor due to size mismatch (2p–3p), and in AlCl₃ and GaCl₃ the central atom does not have suitable vacant p-orbital of matching energy. Hence only BF₃ shows significant back bonding among the options.

CSIR-NET June 2022

Question: The correct order of boiling points of boron trihalides is

1. BF₃ < BCl₃ < BBr₃ < BI₃
2. BCl₃ < BF₃ < BBr₃ < BI₃
3. BF₃ < BBr₃ < BCl₃ < BI₃
4. BI₃ < BBr₃ < BCl₃ < BF₃

Correct Answer: A

Detailed Explanation:
BF₃ (b.p. −100 °C) is monomeric and has the lowest molecular weight and least polarizability. BCl₃ (12.6 °C), BBr₃ (91.3 °C), BI₃ (210 °C) show increasing molecular mass and polarizability, hence increasing van der Waals forces and boiling points. So the correct order is BF₃ < BCl₃ < BBr₃ < BI₃.

JEE Advanced 2021 (Paper-2)

Question: Hydrolysis of which boron halide produces hypoborous acid (H₃BO₃)?

1. BF₃
2. BCl₃
3. BBr₃
4. All of these

Correct Answer: D

Detailed Explanation:
All boron trihalides undergo hydrolysis in water to ultimately give boric acid (H₃BO₃) and corresponding hydrohalic acid.
BX₃ + 3H₂O → H₃BO₃ + 3HX (X = F, Cl, Br, I)
However, BF₃ forms a stable hydrate BF₃·2H₂O first, but on heating or in excess water also gives H₃BO₃.

JEE Advanced 2023 (Paper-1)

Question: The correct statement regarding boron trihalides is

1. BF₃ is more volatile than BCl₃ because of stronger back bonding
2. BCl₃ has higher boiling point than BF₃ mainly due to dimerisation
3. BF₃ is monomeric while BCl₃, BBr₃ and BI₃ form dimers in solid state
4. All boron trihalides are monomeric in gaseous state

Correct Answer: D

Detailed Explanation:
All boron trihalides (BF₃, BCl₃, BBr₃, BI₃) exist as monomeric molecules in the gaseous state. Unlike AlCl₃, which dimerises to Al₂Cl₆ even in vapour phase at moderate temperature, boron trihalides do not dimerise because boron is too small to accommodate four large halogen atoms around it. BF₃ has very low boiling point (−100 °C) due to low molecular mass and weak intermolecular forces despite back bonding. BCl₃, BBr₃ and BI₃ have progressively higher boiling points due to increasing molecular mass and polarisability.

CSIR-NET Chemical Sciences Dec 2021

Question: The compound that does NOT form a stable adduct with BF₃ is

1. NH₃
2. PH₃
3. EtvN
4. H₂O

Correct Answer: B

Detailed Explanation:
BF₃ forms stable adducts with hard nitrogen and oxygen bases (NH₃, Et₃N, ethers, H₂O). However, with soft phosphine (PH₃), the adduct BF₃·PH₃ is very weak and dissociates easily because of poor soft–soft interaction and steric reasons. In contrast, BCl₃ and BBr₃ form stronger adducts with PH₃ than BF₃ does. Hence BF₃ does not form a stable adduct with PH₃.

GATE Chemistry 2023

Question: The dipole moment of boron trihalides follows the order

1. BF₃ > BCl₃ > BBr₃ > BI₃
2. BF₃ = BCl₃ = BBr₃ = BI₃ = 0
3. BI₃ > BBr₃ > BCl₃ > BF₃
4. BF₃ < BCl₃ < BBr₃ < BI₃

Correct Answer: B

Detailed Explanation:
All boron trihalides (BF₃, BCl₃, BBr₃, BI₃) have perfectly trigonal planar geometry (D₃h symmetry) with 120° bond angles. The individual B–X bond dipoles cancel out completely due to symmetry, resulting in zero dipole moment for all boron trihalides.

IIT-JAM Chemistry 2022

Question: When BF₃ is treated with LiH in diethyl ether, the main product formed is

1. Li[BF₄]
2. B₂H₆
3. LiBH₄
4. BH₃

Correct Answer: C

Detailed Explanation:
BF₃ is reduced by LiH in ether to give lithium borohydride:
BF₃ + 4LiH → LiBH₄ + 3LiF
This is one of the standard laboratory methods for preparing LiBH₄, a powerful reducing agent.

JEE Main April 2024 (Shift-1)

Question: Which of the following boron compounds has the highest bond angle?

1. BCl₃
2. BF₃
3. BBr₃
4. All have same bond angle

Correct Answer: D

Detailed Explanation:
All neutral boron trihalides BF₃, BCl₃, BBr₃ and BI₃ are trigonal planar (sp² hybridised) with ideal bond angle of 120°. Although back bonding is strongest in BF₃, it does not significantly distort the bond angle from 120°. Experimental bond angles are very close to 120° for all (BF₃: 120°, BCl₃: 120°, BBr₃: 120°). Hence all have the same bond angle.

Read also: Boron Halides: Preparation, Properties, Structure, Bonding and Uses