Question

Why is $S{O_3}$ molecule an electrophile?

Answer 1

An atom or molecule that seeks an atom or molecule containing an electron pair available for bonding is an electrophile. In simple words species that are electron deficient and can accept an electron pair electron-rich species are electrophile. Nucleophiles have the tendency to donate the pair of electrons to the electrophiles therefore, they behave as Lewis bases.

Complete answer:
According to Bronsted-Lewis concept, an acid is a substance that requires electron as they accept lone pair and behaves as an electrophile.
Electrophiles are electron loving species, they accept the lone pair of the other elements as they are electron deficient whereas nucleophiles are electron-rich species having at least one pair of electrons present in them, they are nucleus loving chemical species as the nucleus of an atom is positively charged.
$S{O_3}$ is sulphur trioxide and it is an electrophile because, in sulphur trioxide, sulphur is in the middle and is bonded to three oxygen, although the oxygen bonded to it are extremely electronegative thereby making the sulphur electron deficient.
Due to the resonance sulphur atom acquires a partial plus charge on it and will accept electrons so $S{O_3}$ is an electrophile as well as a Lewis acid.

Note:
Lone pairs are present on the oxygen atom in the $S{O_3}$ molecule but not on the sulphur atom. But if it gets bonded to another compound through an oxygen atom it will behave as a nucleophile.
$S{O_3}$ is prepared industrially by contact process. Sulphur trioxide is a strong oxidizing agent and is an important reagent in sulfonation reactions. The reaction of benzene with sulphur trioxide and sulphuric acid mixture is known as sulphonation of benzene. Sulphonic acid is formed on the reaction of benzene with the mixture of sulphur trioxide acid.