# Why is BCl3 a Lewis acid?

May 26, 2018

Boron trichloride ${\text{BCl}}_{3}$ is capable of accepting pair(s) of electrons from electron-rich species- e.g., ammonia- due to its electron-deficient nature.

#### Explanation:

The Lewis Acid-base theory defines acids as species accepting pairs of electrons.

The central boron atom in boron trichloride ${\text{BCl}}_{3}$ is electron-deficient, enabling the molecule to accept additional pairs of electrons and act as a Lewis Acid.

Each boron atom forms three single bonds with chlorine atoms with all of its valence electrons, such that there are $2 \cdot 3 = 6$ valence electrons available to the boron atom in a ${\text{BCl}}_{3}$ molecule.

As a period $2$ element, boron demands a total of $8$ electrons in its valence shell to achieve an octet; therefore, borons atom in ${\text{BCl}}_{3}$ molecules are electron deficient and would be willing to accept additional electrons to form an octet.

The reaction between boron trichloride ${\text{BCl}}_{3}$ and ammonia ${\text{NH}}_{3}$- in which the central nitrogen atom carries a lone pair of electrons- is an example in which ${\text{BCl}}_{3}$ acts as a Lewis Acid despite possessing no hydrogen atoms. [1]

As seen on the diagram, the ${\text{BCl}}_{3}$ molecule accepts a lone pair of electrons from an ammonia molecule. In this reaction, ammonia donates electrons and is, therefore, a Lewis base whereas ${\text{BCl}}_{3}$ accepts electrons and acts as a Lewis acid.

Reference
[1] "Boron trichloride", the English Wikipedia,
https://en.wikipedia.org/wiki/Boron_trichloride