My chemistry book says, an electrophile is a reagent which is atrracted to electrons, therefore positive ions and reagents with incomplete octets act as electrophiles, Sulfur trioxide belongs to neither category, so how does it act as an electrophile?

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    $\begingroup$ An electrophile is an electron pair acceptor. $\endgroup$ – Ali Caglayan Jun 10 '15 at 18:28
  • $\begingroup$ In organic chemistry almost all electrophiles are like that - only have partial positive charge. "positive ions and reagents with incomplete octets" you're rather talking about Lewis acids it seems - why SO3 is lewis acid is more interesting. $\endgroup$ – Mithoron Jun 10 '15 at 19:01

Sulfur is bonded to three oxygens, twice each. The sulfur in the middle technically has a formal charge of zero, but the oxygens it is bonded to are extremely electronegative (they're electron hogs), and so the sulfur atom in the middle has a partial plus charge on it. Ergo, it will be willing to accept electrons to try to compensate.

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    $\begingroup$ Not really, structures with formal charge describe it better. $\endgroup$ – Mithoron Dec 26 '17 at 23:43

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