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aromatic

Discussion in 'DAT Discussions' started by arpitpatel86, Jun 4, 2008.

  1. arpitpatel86

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    ahhhhhhhhhhhhhhhhh i hate this part....how can u tell if a compound is aromatic.....i knwo the huckel rule 4n + 2....but doesnt really work for me like when u have lone pairs of electrons someitmes u count them sometimes u do..WHATS THE SECRET LOL
     
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  3. ak47

    ak47 flossy flossy

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    huckels rule works but you have to remember to include lone pairs ONLY when the adjacent bonds on both sides of that thing with the long pairs are single bonds

    ex: http://en.wikipedia.org/wiki/Furan
    Furan has 4 pi electrons in the ring + 2 from the O = 6= 4n + 2
    n=1

    goodluck champ
     
  4. Orgodox

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    Hey arpit I hope I can help you out on the aromatic issue. I posted this a while ago so here it is again. Many people find this confusing but its so overrated and not that bad...
    Lets start with the 4 rules. Im sure you have heard the 4 rules many times but bear with me I think this will help.

    Cyclic
    Fully conjugated
    Huckles rule (4n+2)
    Planar

    So right away when you see something and want to know if it might be aromatic check to see if its a ring. If it is not then you know it 100% is not aromatic if it is then continue on to number 2

    2. Fully conjugated-- If you see that one of the carbons in the ring is sp3 hybridized then you know its not aromatic. For example you have a carbon with no double bonds and 2 Hydrogens it cant be aromatic because the electrons cant delocalize ( they cant move around because that sp3 carbon cant help spread around the charge. If however there is a positive charge at a carbon then that means there is a vacant p orbital that can help with delocalization, for example the cycloheptatrienyl cation, and you should continue to 3 to see if it can be aromatic...)

    3. Huckles rule: So the number of PI electrons needs to satisfy the 4n+2 rule where n = an integer. n can be 0,1,2,3... so for example in benzene there are 6 pi electrons so 4n+2=6 and n = 1 so that would pass the test. (As an aside note if there are 4 or 8 or 12 pi electrons IE 4N pi electrons then this is destabilizing and called anti-aromatic)

    So here comes the part where I think many people are confused. What if there is a heteroatom (non carbon or hydrogen atrom) in the ring and it has lone pairs, do the lone pairs contribute tot he pi electrons or not? (We can also ask this by a carbon with a negative charge for that matter as in cyclopentadienyl anion)
    You have to realize that the atom with the lone pair in question can only have ONE pair of electrons in the Pi system and would be counted towards the pi electrons to apply the 4n+2 rule. So if the atom is already doubly bonded to a neighboring atom then you know its lone pair cant be in the PI system. Additionally if the atom has two lone pairs only one of them will be in the pi system. For example Furan. Only of the lone pairs should be counted but the other CANT because its electrons are not in the same plane.

    Just to be 100% clear lets take a hard example Imidazole
    [​IMG]
    Looking at the form all the way on the left. The nitrogen with the hydrogen has a lone pair. It does not have any double bonds associated with it in this resonance form and the lone pair can be included in the PI system. The other Nitrogen though already has a double bond. Its lone pair must lie in a different plane and is not part of the pi system and should not be counted. Since we have 6 Pi electrons this is aromatic


    4. About planarity You don't really need to know much if it passes these 3 criteria I think its safe to assume for the DAT that its aromatic (even though theres more to it)

    I hope this helps clarify thing. Good luck on your test!
     
  5. MoooShuuu

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    In destroyer number 27 has a propene ring with a lone pair... (sorry I can't name it)...can someone explain to me why this is anti aromatic?

    Also, if a lone pair lies outside of the ring then it is not counted, right?

    One more thing...when there is a negative or positive charge on an atom on the ring...what does this do?
     
  6. MoooShuuu

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  7. Contach

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    yeah, lone pairs outside the ring aren't counted... it sort of defeats the purpose of it being connected in a circle.. unless of course you can draw a resonance structure that brings that lone pair into the ring.

    a positive charge on a carbocation usually orients the Carbon in sp2 fashion. Thus you have a 4th orbital that is directly vertical (aka an empty orbital) where electrons from adjacent bonds can resonate to fill.

    and please stop bumping your own posts - it's really annoying
     
  8. MoooShuuu

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    Yes, sorry I'm sure it is. I was just looking for some last minute answers and by bumping them some people who haven't been on the website may see it and be able to answer it.

    You're in luck because I am not asking anymore questions after tonight.

    Thanks for your help in answering my questions.
     

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