Q.Why does SO3 act as an electrophile?

SO₃ acts as an electrophile because the sulphur atom is electron-deficient and strongly polarised.

Reasoning:

1. In SO₃ the three electronegative oxygen atoms pull electron density strongly away from sulphur, leaving the sulphur centre electron-deficient with a large partial positive charge.
2. The oxygen atoms pull electron density away from sulphur via resonance, leaving the sulphur centre with a large partial positive charge ($\delta^+$). …

Why $SO_3$ is an Electrophile — The Concept

An electrophile (from Greek philos = loving) is literally an "electron lover" — a species that seeks out electron-rich centres to form a new bond. For a molecule to be a good electrophile, it must have either:

• A positive charge (like $H^+$ or $NO_2^+$), or
• An atom with an incomplete octet (like $BF_3$), or
• An atom that can expand its octet and is electron-deficient due to resonance or high oxidation state.

$SO_3$ falls into the third category. Let's see why.

1. Structure of $SO_3$ Sulphur trioxide has a trigonal planar geometry with $S$ at the centre and three $O$ atoms at the vertices. The sulphur atom is $sp^2$ hybridized. But here's the crucial part: $SO_3$ is a resonance hybrid. One of its major contributing structures shows a double bond between $S$ and each $O$, but another important structure has a formal positive charge on sulphur and a negative charge on one oxygen:

In the resonance form with a positive charge on sulphur, the sulphur atom has only 6 electrons in its valence shell (incomplete octet), making it highly electron-deficient. Even in the hybrid, the sulphur carries a partial positive charge ($\delta^+$).

2. High oxidation state of sulphur

   In $SO_3$, sulphur is in its +6 oxidation state — the highest possible for sulphur. This means sulphur has lost almost all its valence electron density to the highly electronegative oxygen atoms. The sulphur atom is therefore strongly electron-poor and desperately wants to accept a pair of electrons to stabilise itself.

3. The electrophilic attack

   When $SO_3$ encounters a nucleophile (like the $\pi$ electrons of a benzene ring in sulfonation), the electron-deficient sulphur atom accepts a lone pair from the nucleophile. This forms a new $\sigma$ bond, and the sulphur expands its octet to 10 electrons (using its available $3d$ orbitals). The reaction is:

   $\text{SO}_3 + \text{Nu}^- \longrightarrow \text{Nu-SO}_3^-$ …