Boron Halides: Preparation, Properties, Structure, Bonding & Uses

Boron Halides
(BX₃ where X = F, Cl, Br, I)

Boron halides are covalent compounds of boron with halogens. The most important and stable are the trihalides BF₃, BCl₃, BBr₃, and BI₃. They are strong Lewis acids due to the electron-deficient nature of boron (only 6 electrons in valence shell).

1. Methods of Preparation

Compound	Common Preparation Methods
BF3 (Boron Trifluoride)	Reaction of B2O3 or borax with HF and H2SO4: Na2B4O7 + 12HF + 6H2SO4 → 4BF3 + 6NaHSO4 + 7H2O CaF2 + B2O3 + 3H2SO4 → 2BF3 + 3CaSO4 + 3H2O Direct combination at high temperature (industrial): B2O3 + 6NaBF4 → 8BF3 + 3Na2O + ... (complex)
BCl3 (Boron Trichloride)	Direct combination of elements at 500–700°C: 2B + 3Cl2 → 2BCl3 Passing Cl2 over heated boron in presence of carbon: 2B + 3Cl2 + C → 2BCl3 + C (catalyst) From B2O3: B2O3 + 3C + 3Cl2 → 2BCl3 + 3CO (high temp)
BBr3 (Boron Tribromide)	Direct reaction: 2B + 3Br2 → 2BBr3 (red heat) Exchange reaction: BCl3 + BBr3 ⇌ BBr3 + BCl3 (using excess Br2) AlBr3 + B → BBr3 + Al (high temp)
BI3 (Boron Triiodide)	Reaction of B with I2 in CS2 solvent From BCl3 or BBr3 by halogen exchange with KI

2. Physical Properties

Property	BF3	BCl3	BBr3	BI3
Appearance	Colorless gas	Colorless fuming liquid	Colorless fuming liquid	White crystalline solid
Melting Point (°C)	−126.8	−107	−46	49.9
Boiling Point (°C)	−100.3	12.6	91.3	210 (sublimes)
Density (g/cm³, liquid)	—	1.35	2.65	3.35 (solid)
Thermal stability	Very stable	Stable	Less stable	Least stable

Trend: As the size of halogen increases from F → I, boiling point increases (except BF₃ due to low molecular weight and strong intermolecular forces in heavier halides). Hydrolysis becomes more vigorous from BF₃ to BI₃.

3. Chemical Properties (Important Reactions)

1. Lewis Acid Behavior
   BX₃ + :L → BX₃·L (adducts with NH₃, ethers, amines, phosphines)
   Example: BF₃ + NH₃ → H₃N·BF₃
2. Hydrolysis
   BCl₃ + 3H₂O → H₃BO₃ + 3HCl (vigorous)
   BF₃ hydrolyzes slowly because of strong B–F bond and the formation of tetrafluoroboric acid (H[BF₄]).
   4BF₃ + 3H₂O → H₃BO₃ + 3H[BF₄]
3. Reaction with alcohols → alkyl borates (used in borate ester synthesis)
4. Formation of tetrahaloborates
   BX₃ + X⁻ → [BX₄]⁻ (tetrahedral)
5. As Friedel-Crafts catalyst (especially BF₃ and BCl₃)

4. Structure and Bonding

• All boron trihalides are monomeric in gas and liquid phase.
• Boron uses sp² hybridization → trigonal planar geometry (bond angle 120°).

• Boron has only 6 electrons in valence shell → strong Lewis acid.
• B–X bonds have partial π-character due to back-donation from filled p-orbitals of halogen into empty p-orbital of boron (most significant in BF₃ → shortest B–F bond).

• B–X bond length (1.30 Å) is shorter than expected due to strong pπ–pπ back bonding.
• Back bonding decreases from F > Cl > Br > I.
  Order of back Bonding: BF₃ > BCl₃ > BBr₃ > BI₃
• B–X bond length increases so Lewis acidity increases from BF₃ to BI₃.
  Order of Lewis acid strength: BF₃ < BCl₃ < BBr₃ < BI₃
• Nucleophilicity order is inversely proportional to the Lewis acid character.
  Nucleophilicity order is: (reaction with nucleophile/water): BI₃ > BBr₃ > BCl₃ > BF₃

Effects of Back Bonding:

1. It always leads to an increase in bond order between the participating atoms.
2. It always leads to an increase in bond strength between participating atoms.
3. It always leads to a decrease in bond length between participating atoms.

5. Uses of Boron Halides

Compound	Major Uses
BF3	Strong Lewis acid catalyst in organic reactions (polymerization, isomerization, alkylation) Production of diborane (B2H6) Flux in soldering and brazing In neutron detectors (¹⁰BF3 proportional counters)
BCl3	Refining of Al, Mg, Zn, Cu alloys Plasma etching in semiconductor industry Production of boron fibers and boron nitride Catalyst in organic synthesis
BBr3	Selective cleavage of ethers (especially aryl-alkyl ethers) in organic synthesis Demethylation reagent (converts Ar–OCH3 → Ar–OH) Source of boron in chemical vapor deposition (CVD)
BI3	Less common; used in preparation of other boron compounds and some organic transformations

Key Takeaway: Boron trihalides are electron-deficient, trigonal planar, strong Lewis acids. Their reactivity and applications increase with the size of the halogen (except BF₃, which is most stable thermally).

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

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