Orgo Aromatic question

[jjmack]

Hey all,

I'm a recent grad and studying for the april mcat and haven't taken orgo since soph year and have a question about armoatics. I understand the 4n+2 rule, but I'm confused about when the lone electron pair on N can be counted. It seems like sometimes it is and others it is not. Thanks in advance.

Jude

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[UCLAstudent]

The lone pairs are counted when they contribute to the molecule's conjugation (for example, if the N is in a ring structure with double bonds, then the lone pair will be bumped up to a P orbital and be counted).

 

[Sonya]

more important question... didn't they pull aromatics off MCAT (per AAMC webpage, changes to MCAT). please don't hold me to this.

 

[hmgcoa]

yea. none of that stuff should be on the new format..(or at least i hope)...it would be pretty mean if they gave us questions on that stuff.

anyways, yea. back to the original question. it has to do with whether or not the molecule would gain any extra stability by being aromatic. if it could, then yes, the lone pair would line up such that it, too, could get a piece of the pi electron-conjugation "action". so to speak. =)
you determine whether or not the molecule is aromatic by the 4n+2 rule, and if it fails this aromaticity "test", then see if throwing the two electrons from the lone pair into the mix would help satisfy the rule. if it does, throw them in.

consider the case with the imidazole ring in histidine. you'll notice that one N looks like it should be reactive and therefore have its own pKa value. but it doesnt. only one (not two) has a pKa value associated with it. it has to do with the above explanation.

hope this helps.

peace.

 

[deleted9493]

If there is ring with a nitrogen in it that has two lone pairs...only one pair can be in a p orbital that is in the same orientation as the electrons of any double bonds. That being true, you only count one pair of electrons on nitrogen.

 

[hmgcoa]

out of curiosity, hudson,

what school do you go (or went) to?