Question

Which of the following is not a Lewis acid?
A.$Si{F_4}$
B.$FeC{l_3}$
C.$B{F_3}$
D.${C_2}{H_2}$

Answer 1

At first think about the definition of Lewis acid. A Lewis acid is a chemical species that contains an empty orbital that is capable of accepting an electron pair from a Lewis base to form a Lewis adduct.

Complete step by step answer:
Lewis acid is defined as an electron pair acceptor and Lewis base is defined as an electron-pair donor.
Lewis base is any species that has a filled orbital containing an electron pair that is not involved in bonding but may form a dative bond with Lewis acid to form Lewis adduct.
For example, $N{H_3}$ is a Lewis base because it can donate its lone pair of electrons. $M{e_3}B$ is a Lewis acid as it can accept lone pairs of electrons. So these two chemical compounds form a Lewis adduct.
Coming to the given question, In option A there is $Si{F_4}$ molecule. It acts as Lewis acid because $Si$ can expand its octet by accepting a lone pair of electrons.
$FeC{l_3}$ also acts as Lewis acid because it can accept lone pairs of electrons from chlorine atoms.
In $B{F_3}$ molecule, the boron atom is an electron pair and has an empty orbital so it can accept a pair of electrons. So it is also a Lewis acid.
${C_2}{H_2}$ acts as Lewis base because the $\pi$ bond between the carbon atoms is used for donating an electron pair. So it acts as an electron-pair donor in nucleophilic addition reactions.

So correct answer is option D

Note:
Don’t get confused with the terms Lewis acid and Lewis base. Acid is used to accept electron pairs and Base is used to donate electron pairs. So usually a compound with a positive charge is acid and a compound with a negative charge is base.