Question

BF3 acts as lewis acid while CCl4 can not do so. Why?

Answer 1

Lewis acids are those molecules that can accept electron pairs, and those that can donate electron pairs are called Lewis bases.

Since $B{F}_3$ can accept a pair of electrons it is a lewis acid but $CC{l}_4$ molecule does not have a vacant d-orbital where it can not accept pair of electrons so $CC{l}_4$ cannot act as Lewis acid.