Question

why does bcl3 act as lewis acid

Answer 1

The central atom in BCl3 (that is Boron, in trivalent state) has only six electrons around it — it is electron deficient and accepts electrons to complete it's octet. Therefore it acts as a Lewis acid by accepting electrons.

Answer 2

Answer: Because it accept electrons.

Explanation: According to the Lewis concept, an acid is defined as a substance that accepts electron pairs and base is defined as a substance which donates electron pairs.

As $BCl_3$ is a covalent compound which contains 6 electron pairs and thus is short of two electrons to complete its octet, thus it can easily accept an electron pair from a lewis base.

The electronic configuration of boron is $1s^22s^22p^1$

Number of valence electrons of boron are 3.

The electronic configuration of chlorine is $1s^22s^22p^63s^23p^5$

Number of valence electrons of chlorine is 1.

Thus one electron each from three chlorine atoms share electrons with boron and form three single bonds.