Simple Aromaticity Fact that I always get wrong!

[wired202808]

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I always get this wrong! is the + on the outside implying stability through resonance? why is this aromatic and for that matter?

how do i treat all aromatic type questions when I see a + charge?

 

[PooyaH]

The cation is making the carbon sp2. The ring is already satisfying the Huckel rule. Once the cation is made, there is no sp3 interruption. In this case every atom has a p orbital, and it's planar.

 

[flin5845]

I always get this wrong! is the + on the outside implying stability through resonance? why is this aromatic and for that matter?

how do i treat all aromatic type questions when I see a + charge?

1. Cyclic and conjugated
2. can not have any sp3 atoms in the ring
3. planar
4. Meet huckles rule (4N+2e)<-- determines if aromatic, anti-aromatic.

So for the positive charges,
1.Make sure your structure has a continuous pi system
2. Make sure there are no sp3 atoms.
3. Realize that cations are sp2 hybridized
4. make sure it meets huckles rule. an easy way to tell if it meets huckles rule is that if there are an even number of pi bonds, the structure will be anti aromatic. If there are an odd number of pi bonds, the structure will be aromatic. (lone pairs can count as a pi bond)

So for example, if you took the cyclopropane and turned the cation into an anion, it would then have even number of pi bonds, and would be anti aromatic

Hope this helps

 

[wired202808]

The cation is making the carbon sp2. The ring is already satisfying the Huckel rule. Once the cation is made, there is no sp3 interruption. In this case every atom has a p orbital, and it's planar.

so basically if the structure is already aromatic a + does not impact it and therefore its always considered aromatic (as long as all the other rules are obeyed) am i correct?

 

[PooyaH]

so basically if the structure is already aromatic a + does not impact it and therefore its always considered aromatic (as long as all the other rules are obeyed) am i correct?

Well if a structure is already aromatic, it would NOT want to make a cation! That would actually disrupt the aromaticity and ruin the Huckel's rule! What the cation is doing here in this example is that it's making that carbon an sp2 in which it was sp3 before. Think of it this way, if a structure satisfies the Huckel's rule, but has an sp3 interruption, then making a cation on the sp3 carbon would help! Because it would make it aromatic by removing the sp3 interruption. But if a structure already has no sp3 interruption, then making a carbocation would mess up the aromaticity and the Huckel's rule.

 

[wired202808]

Well if a structure is already aromatic, it would NOT want to make a cation! That would actually disrupt the aromaticity and ruin the Huckel's rule! What the cation is doing here in this example is that it's making that carbon an sp2 in which it was sp3 before. Think of it this way, if a structure satisfies the Huckel's rule, but has an sp3 interruption, then making a cation on the sp3 carbon would help! Because it would make it aromatic by removing the sp3 interruption. But if a structure already has no sp3 interruption, then making a carbocation would mess up the aromaticity and the Huckel's rule.

lol u confused me haha so if that structure had no cation. it would still be aromatic no? so cant i just ignore + and says since all other aromaticity rules are obeyed its aromatic?

 

[KyoPhan]

I think I drew it correctly if it was without the cation. Someone correct me if not.

If there is no cation there, the top carbon would be sp3.
From flin's post:
1.Make sure your structure has a continuous pi system
2. Make sure there are no sp3 atoms.
3. Realize that cations are sp2 hybridized

It's sp3, so it's not a continuous pi system.

Also question, how would the cation one be drawn. Is it just 1 hydrogen attached to the top carbon?

 

[wired202808]

asdf

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[PooyaH]

lol u confused me haha so if that structure had no cation. it would still be aromatic no? so cant i just ignore + and says since all other aromaticity rules are obeyed its aromatic?

haha I know it's tough to explain it. It's a lot easier if drawn on paper. In the example you gave, imagine if the cation wasn't there. In that case, the structure would not have been aromatic, because it would have an sp3 interruption, since that carbon without the + would've been sp3. However, it satisfies all other aromaticity rules. When the cation is made, NOW all the carbons are sp2! Therefore the sp3 interruption is removed, and now you have an aromatic structure. Now imagine a benzene ring. It's aromatic, because all carbons have p orbital and are sp2, and it obeys the Huckel's rule: 4N+2 = 6 pi electrons. Now if you introduce a cation to the ring, it would mess up the aromaticity, because it would remove one of the double bonds! Now your Huckel's rule is ruined! There are still no sp3 interruptions BUT now you have 4 pi electrons instead of 6! This would ruin the aromaticity and actually making the structure an anti-aromatic.

I'm not sure what you mean by ignoring the cation, but sometimes the cation helps you with aromaticity and sometimes it can ruin it.

Hope it helps.

 

[flin5845]

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[PooyaH]

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No way!

 

[flin5845]

I do not know if my upload worked but Ill try to explain the whole cation/anion thing.

On the left side..
I. non-aromatic. Has sp3 carbon
II. Anti-aromatic. negative charge (even number of pi electrons)
II. Aromatic. positive charge, turned the sp3 carbon from I into sp2 carbon, and meets huckles rule with odd number of electrons

on the right side...
A. non-aromatic. Has sp3 carbons
B. Aromatic. Lone pair gives odd number of pi electrons to meet huckles rule
C. Anti-aromatic. the sp3 carbon in A is now sp2, but this does not meet huckles rule, even number of pi electrons.


If this is still confusing think about it this way. If you have an odd number of pi electrons with a single sp3 carbon, if that sp3 became a cation the structure would then be aromatic.

 

[wired202808]

I do not know if my upload worked but Ill try to explain the whole cation/anion thing.

On the left side..
I. non-aromatic. Has sp3 carbon
II. Anti-aromatic. negative charge (even number of pi electrons)
II. Aromatic. positive charge, turned the sp3 carbon from I into sp2 carbon, and meets huckles rule with odd number of electrons

on the right side...
A. non-aromatic. Has sp3 carbons
B. Aromatic. Lone pair gives odd number of pi electrons to meet huckles rule
C. Anti-aromatic. the sp3 carbon in A is now sp2, but this does not meet huckles rule, even number of pi electrons.


If this is still confusing think about it this way. If you have an odd number of pi electrons with a single sp3 carbon, if that sp3 became a cation the structure would then be aromatic.

flin u nailed it! good job mate! thanks a bunch this helps a ton! I knew about the sp3 carbons making things non aromatic but visualizing this made it 100% times better than again to everyone.

Cheers for beers that will come after this hell is done.