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What resonance structure is the most abundant in sulfur trioxide: the one with one double and two single, the one with two double and one single or the one with all three double bonds? Is it actually correct to write an expanded octet when it is not necessary?

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    $\begingroup$ Have a look at this excellent answer. It contains some interesting information about the bonding in SO3. $\endgroup$ – Philipp Nov 11 '14 at 9:41
  • $\begingroup$ So it is just one double bond. I don't get the sulfite ion explanation. There are only single bonds in it. Is it correct to write expanded octets by Lewis when they aren't necessary, or it is octet over expanded octet? $\endgroup$ – RBW Nov 11 '14 at 9:49
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    $\begingroup$ The problem is that Lewis structures very rarely give a realistic account of the actual bonding situation in a molecule. It is rather good for organic molecules but for molecules like SO3 it is not suitable. Fact is that all S$-$O-bonds in SO3 are the same and they are neither typical single nor double bonds. So maybe it would be best to use some delocalised double bonds like you would with benzene. $\endgroup$ – Philipp Nov 11 '14 at 11:38
  • $\begingroup$ I know that they are delocalised, but what do you suggest me to draw, normal octet or extended octet? What is more useful from these two in organic chemistry? $\endgroup$ – RBW Nov 11 '14 at 12:37
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    $\begingroup$ It does not really matter which one you choose, because they don't carry that much meaning anyway. The most common respresentation I know is the one with three double bonds - it reflects that all bonds are equal. If you choose a Lewis formula with one or two double bonds, you must in principle include the other mesomeric structures as well. That's why I suggested using delocalised double bonds, i.e. dashed ones, they convey the more involved bonding situation better. $\endgroup$ – Philipp Nov 11 '14 at 15:40
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The answer depends on which teaching you follow.

Traditional teaching, dating back to Linus Pauling, would prefer three double bonds to minimise charge separation, requiring an expanded octet.

Even older traditional teaching would favour the validity of the octet rule and take charge separation as a tolerable by-product.

Both sides have their advantage and disadvantage:

  1. The Pauling method

    • correctly shows that all three bonds are equal without resorting to mesomery
    • incorrectly assumes a bond order of $2$ rather than the actual $1.4$
  2. The charge-separation method

    • predicts a bond order of $1.33$ which is much closer to $1.4$
    • requires mesomery of three structures to explain the three equal bond lengths.

I for my part prefer the mesomeric charge-separation description, because we have no trouble drawing benzene in the same way and because I like the octet rule. Others will disagree and prefer the three double bonds.

I wouldn’t know anybody who prefers a description with two double bonds and one single bond. It is just between the two models making it all the less correct.

Please also see this excellent answer to a related question by Martin which includes molecular orbitals for $\ce{SO3}$

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