O Chem Destroyer #67

[happyana]

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Ok so I"m not grasping the whole concept on aromatic compounds. Can anyone explain why B is aromatic? Why don't you count both pair of electrons on the oxygen into equation for Huckel's rule?

 

[Orgodox]

Hey happy 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

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 things

 

[happyana]

wow, awesome explanation. thanks!

 

[harrygt]

Nice explanations by orgo. I'm sure everyone who read that a couple of times, will get all the aromaticity questions on the test.
I gotta question for you orgo.
What answer would you pick for 10 annulene? aromatic or non-aromatic?
let me know

 

[Orgodox]

AHHHA Harry now your delving into fun stuff! Annulenes are a whole nother game and dont need to know for the DAT.
But since you asked....
I beleive 14 and 18 all trans are aromatic. Also some of the big ones can kinda adopt planarity and minimize strain and would be considered aropmatic although wouldnt have the same stability as benzene because still some strain.

10 annulene fits the first three rules but is not aromatic because it has too much strain when flat so adopts a different configuration. But again this is too detailed probably even for a college class.

 

[HoangDDS]

jeez, I had to break out my old school ochem textbook to figure out what Annulenes even was. You are guys are too much!

But let me get in this 😀 . Annulene (CnHn) when n =14 or 18 seems to fit the planar and Huckle's rule fine. But Cyclobutadiene (4) - Annulene is anti-aromatic b/c its only 4n pi e- .

I am sure there are many other Annulene that are aromatic and anti-aromatic.

Harry, thanks for filling my head with more over the top info that I will not need for the DAT so I can not remember some of the stuff that I should remember! Thanks man!!!

AHHHA Harry now your delving into fun stuff! Annulenes are a whole nother game and dont need to know for the DAT.
But since you asked....
I beleive 14 and 18 all trans are aromatic. Also some of the big ones can kinda adopt planarity and minimize strain and would be considered aropmatic although wouldnt have the same stability as benzene because still some strain.

10 annulene fits the first three rules but is not aromatic because it has too much strain when flat so adopts a different configuration. But again this is too detailed probably even for a college class.

 

[PooyaH]

AHHHA Harry now your delving into fun stuff! Annulenes are a whole nother game and dont need to know for the DAT.
But since you asked....
I beleive 14 and 18 all trans are aromatic. Also some of the big ones can kinda adopt planarity and minimize strain and would be considered aropmatic although wouldnt have the same stability as benzene because still some strain.

10 annulene fits the first three rules but is not aromatic because it has too much strain when flat so adopts a different configuration. But again this is too detailed probably even for a college class.

Indeed! 14 and 18 trans would be aromatic (since their hydrogens are inside the ring) and they can fullfill planar conformations. 👍