Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Val on September 09, 2019, 04:16:52 PM
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I am trying to learn how to determine if a molecule is aromatic, antiaromatic, or neither, by counting the number of electrons in the pi system. When oxygen has two lone pairs, how do I determine how many lone pairs participate in the aromaticity. In a problem set, one examples says one lone pair participates, but another example says no lone pairs participate. Both had oxygens with two lone pairs and two sigma bonds, I do not understand why it's different?
Should one exclude counting oxygen's lone pairs in the pi system if it will make the compound aromatic? Or if it will prevent it from being antiaromatic? This is the only reasoning I can think of.
Also I am so not sure if the second example would be considered antiaromatic or neither?
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In example 1, including one lone pair gives 6 pi electrons and makes the system aromatic. In example 2, you can't make an aromatic structure. Including a lone pair from each oxygen gives 8 electrons - antiaromatic. Including a lone pair from just one O would give 6 pi electrons, but not in a ring.
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As mjc123 points out, in number 2 if both Oxygens contribute a lone pair, it is 8 electrons and antiaromatic. If only one O contributes a lone pair, the system is not conjugated all the way around the ring.
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In checking for aromaticity we count only those lone pairs which can undergo delocalization with rest of delocalized system.