KeY Aromatic Question that Always shows up?

[urDDS]

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the three ring cyclopropane, in kaplan it had a + charge on top and said that its aromatic, in destroyer #27 with a lone pair said not arom. and in
# 67, just a plain ring said also not aromat. how does this work please?

 

[eilrahcmi]

yea that question was on my diagnostic last night

 

[yankees27th]

It's only aromatic with a positive charge.
First of all, with no charge you have an sp3 carbon. If you ever see an sp3 carbon the molecule is not aromatic. Ever.
I'm pretty sure with a negative charge you have an extra lone pair. That makes for 4 electrons...which is bad. (someone confirm this?)
With a positive charge the molecule meets all requirements for aromaticity. 2 pi electrons, planar, etc.

 

[drillbit]

Remember Huckels rule for aromaticity: 4n + 2 pi electrons (so for a compound to be aromatic this will equal 2, 6, 10, 14, etc.) If n = 0, then the answer is 2. If n = 1, then the answer is 6. and so on.....

Destroyer #27: The cyclopropene anion has 4 pi electrons (2 in the double bond and 2 lone pairs) - so 4n + 2 pi electrons equals 4. This indicates it is not aromatic (as shown above - to be considered aromatic it must equal 2, 6, 10, etc.)

Kaplan: The cyclopropene cation has 2 pi electrons (2 in the double bond) - so 4n +2 pi electrons equals 2. This indicates it is aromatic - as shown above.

Of course the compound must be planar too like yankees27th said! If the compound was neutral cyclopropene - it would not be aromatic even though it has 2 pi electrons because it is not planar.