Lewis acid accept lone pair....


Except C2H6 all have vacant orbital... so a, b have vacant d-orbital where as octet is not complete in BF3... so they can accept lone pair while d can't... so d is correct...

Explanation:

Definition of Lewis acid:
A Lewis acid is a chemical species that can accept a pair of electrons from a Lewis base to form a coordinate bond.

Explanation of options:

a) SiF₄ (Silicon tetrafluoride)
- Silicon tetrafluoride is a compound in which the silicon atom is bonded to four fluorine atoms.
- The silicon atom does not have an empty orbital to accept a pair of electrons, so it cannot act as a Lewis acid.

b) FeCl₃ (Iron(III) chloride)
- Iron(III) chloride is a compound in which the iron atom is bonded to three chlorine atoms.
- The iron atom has an empty d orbital, which can accept a pair of electrons, making it a Lewis acid.

c) BF₃ (Boron trifluoride)
- Boron trifluoride is a compound in which the boron atom is bonded to three fluorine atoms.
- The boron atom has an empty p orbital, which can accept a pair of electrons, making it a Lewis acid.

d) C₂H₄ (Ethylene)
- Ethylene is a hydrocarbon compound in which two carbon atoms are double bonded to each other.
- Ethylene does not have an empty orbital to accept a pair of electrons, so it cannot act as a Lewis acid.

Conclusion:
The compound that is not a Lewis acid among the given options is ethylene (C₂H₄), option (d).