let's walk through it step by step |dw:1439682015489:dw|
first we find the number of valence electrons for all the elements in the compound S - > 6 Cl - > 7 O - > 6 6 + 6 + 7 + 7 = 26, giving us 13 pairs. we already used 3 pairs so far (1 pair between S and Cl, 1 pair between S and the other Cl, and 1 pair between S and O so we have 10 pairs left
now, let's complete the octet for chlorine by putting 3 lone pairs around each, like so:|dw:1439682298737:dw|
|dw:1439682445756:dw| I have 13 pairs of electrons though!
well, true, you have the correct number of electrons, but generally speaking, the least electronegative element (sulfur in this case) forms the central atom, with the other atoms branching off of it
(I don't really have a good theoretical explanation as to why your structure is chemically unstable, @aaronq would probably do a better job of explaining > >)
we have 4 pairs left, and we could theoretically put 3 lone pairs on oxygen like we did for chlorine (this isn't the best arrangement because of formal charge, but let's roll with it for now)
and that leaves 1 pair|dw:1439683232757:dw| for sulfur
^ the above lewis structure has the correct number of electron pairs, but it's still not the "best" structure once we take formal charge into account, where formal charge = number of valence electrons - (0.5)number of covalently bonded electrons - number of free electrons
we want each atom to have a formal charge of 0, so we take a look at each atom and calculate the formal charge... you can verify this yourself, but: FC of Cl = 0, so we're good FC of O = -1 FC of S = +1 we can fix this by changing one of the lone pairs on Oxygen to a double-bond on sulfur, like so...
and that completes our lewis structure...
SOCL2 can be used to create an acyl chloride; if I remember correctly it's like this I kind of find this compound fascinating; forgive some of the arrows in those figures. |dw:1439688743268:dw| |dw:1439688835198:dw| |dw:1439688975434:dw|
In O-chem you can see why that compound SOCl2 is useful. BTW i may have drawn some of the arrows wrong.
Yeah, this is a great reaction in practice because it almost always goes to completion due to the fact that \(SO_2\) gas leaves. This is really handy for fun reactions like the Arndt-Eistert Homologation. Only problem is diazomethane is kind of dangerous.