Why BF3 acts as Lewis Acid | Boron trifluoride

Boron trifluoride

Boron trifluoride is a colourless, pungent-smelling gas. It is harmful when inhaled. It is soluble in water and is slowly hydrolyzed by cold water to produce corrosive hydrofluoric acid. It has heavier vapours than air. Exposure of the containers to fire or heat for an extended period of time may result in their violent rupture and propulsion.

Boron trifluoride BF3 Structure

boron trifluoride

Boron trifluoride formula

Chemical formula of Boron trifluoride is BF3

Why BF3 acts as Lewis Acid?

According to Lewis Theory of Acids and Bases

Lewis Acid

A Lewis acid is a substance capable of accepting a pair of electrons to form a coordinate covalent bond.

Lewis Base

A Lewis base is a substance capable of donating a pair of electrons to create a coordinate covalent bond.

Lewis Theory of Base Example

BF3 is accepting a lone pair of electrons from ammonia (NH3)

Boron trifluoride BF 3  is an electron-deficient species that can accept an electron pair, it behaves as a Lewis acid. Here is the arrangement of electrons in Boron is 1s2,2s2 , 2p1 and a vacant p-orbital exists, indicating that the mean BF3 has a propensity to accept lone pair, hence BF3 is a Lewis acid.


The reaction can be summarized as;

  • The electron-deficient core of BF3 is accepting a lone pair of electrons from ammonia (NH3) in this reaction.
  • Thus, lone pair-donating species (NH3) behave like a base whereas electron-accepting species (BF3) behave like acid in accordance with the Lewis concept.
  • BF3’s electron-deficient core allows it to take lone pairs of electrons. Boron’s octet is lacking one element.
  • Therefore, Boron will receive a lone pair of electrons to complete its octet.
  • In addition, NH3 and BF3 form a coordinate covalent bond, which is symbolized by a single-headed arrow. This bond is created by sharing a full lone pair of electrons with an electron-deficient specie.

Is BF3 a strong or weak Lewis acid?

BF3 is a weaker Lewis acid than BCl3 due to its increased back-bonding. In BCl3, the 3p orbitals on Cl are larger than the 2p orbitals on B, making orbital overlap less efficient and back bonding less crucial.

Arrange the following boron halides according to their acidity.


BF3 < BCl3 <BBr3 <BI3 .

BF3 has the strongest back bonding and therefore the weakest acid.

What is boron trifluoride used for?

Boron Trifluoride is an odourless, colourless gas that produces dense, white vapours in humid air. It is utilised as a catalyst in polymerization operations, in soldering fluxes and fibre optics, and as a magnesium fire extinguisher.

Is boron trifluoride ionic or covalent?

The molecule boron trifluoride (BF3) is covalent.

Is BF3 polar or nonpolar?

Boron trifluoride (BF3) is a nonpolar compound,

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