[skyfire095]

I'm a little confused about how aromaticity is affected when drawing resonance structures of aromatic compounds with substituents.  A particular example is the benzylic cation, which we know to be an incredibly stable compound.  I know that in benzylic resonance structures, aromaticity is lost.  Can antiaromaticity be gained? (For example, drawing a resonance structure placing a double bond from the benzene ring to the carbon substituent and leaving a positive charge in the ring--does that make it a two pi bond system with an empty p orbital and therefore antiaromatic?)

And going off of this, if anyone could offer any insight into this problem I'd very much appreciate it:
When comparing an ionized cyclobutadiene methyl chloride (so essentially, a cyclobutadiene with a methyl cation substituent), why is that species much more stable (about 25 kcal/mol) than the benzylic cation?

Thanks a lot!

[FeLiXe]

aromaticity is something that exists essentially only for neutral monocyclic systems. so you can't really use that term here. you have to think about all the resonance structures as a whole. there cannot be one antiaromatic resonance structure. In a way you may still be able to use Hückel's rule: you have 6 electrons

when you say 25kcal/mol more stable, how do you compare it?

[azmanam]

aromaticity is something that exists essentially only for neutral monocyclic systems.

technically not true.

napthalene and cyclopenadienyl anion are both aromatic.

(updated to fix formatting)

[azmanam]

When considering whether a compound is aromatic or not, you need to consider the total number of electrons over the total number of atoms in the extended pi system.

For example, cyclopentadiene is not aromatic.  The 5th carbon atom is sp3, not part of the pi system, so there are four electrons in a four atom system - which isn't even cyclic.  not aromatic.

The cyclopentadienly cation and cyclopentadienyl anion are a bit different.  the hybridization of the 5th carbon atom changes to sp2, and now we have a cyclic pi system.  but are they aromatic?  the cation has 4 electrons over the 5 atom pi system - antiaromatic.  the anion has 6 electrons over the 5 atom pi system - aromatic (think ferrocene)

Similarly, pyrrole can be analyzed for aromaticity.  At first glance, it may appear similar to the neutral cyclopentadiene and might not be aromatic.  But the nitrogen atom has a lone pair of electrons which contributes to the extended pi system.  6 electrons over 5 atoms - aromatic.

Thus, when dealing with the benzyllic cation we should keep 2 principles in mind.  One is this concept of aromaticity - total number of electrons over the total number of atoms in the extended pi system.  And the other is that when dealing with resonance, no single line structure accurately represents what the molecule looks like.  The actual molecule is a weighted average of ALL possible resonance structures.

Benzyl chloride can be ionized to the benzyl cation.  You are correct that several resonance structures can be drawn which move one double bond to the exocyclic position, leaving a positive charge at either the ortho or para positions.  Let's consider aromaticity for this compound (not necessarily a single resonance structure, as it won't exist like that - rather as that weighted average).  Over the entire cation, there are still 6 electrons, but unlike benzene the electrons are now extended over a 7 atom pi system.  As far as aromaticity is concerned, the only part that is affected is the length of the pi system. 

Thus, my analysis says the benzyllic cation is still aromatic.  6 electrons, 7 atom pi system = aromatic.  Aromaticity, then, is not lost in the cation.  The benzyllic carbon atom, the 2 ortho carbon atoms, and the para carbon atom all accept a partial positive charge, but there is still 6 electrons.

I don't have an answer for you about the 2nd part.  Nothing immediately jumps out at me.

[FeLiXe]

thanks, the one with charge I take back

but not the monocyclic. consider biphenyle and benzopyrene, both have an even number of electron pairs but are aromatic

[azmanam]

Perhaps I'm misunderstanding what you mean by 'monocyclic' then.  Can you elaborate?  Benzopyrene is an interesting example (along with pyrene or coronene), as they are aromatic compounds which fail Huckel's rule.

http://en.wikipedia.org/wiki/H%C3%BCckel%27s_rule

[FeLiXe]

by monocyclic I mean just one ring: cyclopropenium cation, ..., benzene, ..., octadienyl-dication; no substituents
from a theorist's point of view I think that's all that Hückel's rule really talks about. because these systems' Hückel matrices are simple band matrices that can be easily diagonalised and you notice that they have to be filled with an odd number of electron pairs.
there are some generalisations for linear condensed systems: naphthalene, anthracene, tetracene, ... (all with 4n+2 electrons)

but I just would not say that there is a magic 4n+2 rule

in my eyes you have to get back to hückel theory or some higher level computation to know anything about any other system

[azmanam]

Ah, I see.

indeed, on the huckel's rule wikipedia page, it notes that for more complicated systems quantum calculations are required (wikipedia suggests pariser-parr-pople method - http://en.wikipedia.org/wiki/Pariser-Parr-Pople_method)

In your opinion, then, would you conclude that the benyl cation is no longer aromatic?

[FeLiXe]

between Hückel's rule and Pariser-Parr-Pople there is still the Hückel's method http://en.wikipedia.org/wiki/H%C3%BCckel_method (I'll add that to the article). if you apply that to the benzylic cation you will notice that there is a rather large energy gap. so I guess you can call it aromatic
I am not a specialist on the topic. i am just trying to point out that theorists and organic chemists have a different view about aromaticity

[azmanam]

Very interesting.  I am no expert myself, so thanks for the discussion.

I hope we haven't completely confused skyfire...

[skyfire095]

Haha no, I'm still with you guys.  Thanks for the input.  To clarify on the second part of my question:
Benzyl chloride and methyl chloride cyclobutadiene were both ionized to their cations and Cl-.  The methyl cyclobutadiene cation is 25 kcal/mol less endothermic than the benzyl cation, implying that the cyclobutadiene ring stabilized its carbocation substituent by 25 kcal/mol more than the benzene ring.
Any thoughts on how this is occurring?

[AWK]

methyl chloride cyclobutadiene

These are two compounds.

[FeLiXe]

you are probably talking about (chloro-methyl)-cyclobutadiene. I guess it has to do with the fact that the cation is  not antiaromatic anymore, or at least not as antiaromatic as the cyclobutadiene

[FeLiXe]

I looked at the hueckel energy schemes with this little package I wrote: http://stud4.tuwien.ac.at/~e0425252/chemprogs/python.htm
the MO schemes are attached. c-butadiene (if it is a square) has degenerate singly occupied orbitals which makes it very unstable
the cation still has a small HOMO-LUMO gap but at least it has a HOMO-LUMO gap

with the benzyl-chloride it's the other way around: you have the perfect aromatic system to start with and only some resonance in the cation