Aromaticity Help

[Aletheia]

Hi,

I am having a terrible time understanding aromaticity so if anyone could help me with this concept, that would be awesome.

So, I know that a double bond is made up of one sigma and one pi bond. I don't understand how you can tell, from this, how many pi electrons exist. I think you have to follow Huckel's rule. How do I do this? What is "n" and how do I determine it? One website said that benzene has one electron in its free p orbital thus n=1. But how do you know it has one electron in its p orbital? How do you know this orbital is free?

I also know that each atom is supposed to have a p orbital to satisfy aromaticity. I'm not sure what this means. I went to this site and was confused: http://www2.chem.ucla.edu/~mouser/General/labzone/130BL/problems/hint/aromaticity.html

In one of their examples, they said that the compound is not aromatic because the Carbon is saturated and has no p orbital. I am confused as to how it cannot have a p orbital if 1s2 2s2 2p2 are the electrons in Carbon (and there is clearly a p orbital).

Thanks so much for any help you can give me.

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[Crisoneogenesis]

Hi,

I am having a terrible time understanding aromaticity so if anyone could help me with this concept, that would be awesome.

So, I know that a double bond is made up of one sigma and one pi bond. I don't understand how you can tell, from this, how many pi electrons exist. I think you have to follow Huckel's rule. How do I do this? What is "n" and how do I determine it? One website said that benzene has one electron in its free p orbital thus n=1. But how do you know it has one electron in its p orbital? How do you know this orbital is free?

I also know that each atom is supposed to have a p orbital to satisfy aromaticity. I'm not sure what this means. I went to this site and was confused: http://www2.chem.ucla.edu/~mouser/General/labzone/130BL/problems/hint/aromaticity.html

In one of their examples, they said that the compound is not aromatic because the Carbon is saturated and has no p orbital. I am confused as to how it cannot have a p orbital if 1s2 2s2 2p2 are the electrons in Carbon (and there is clearly a p orbital).

Thanks so much for any help you can give me.

I know a couple of the answers to your questions....but its been awhile since Ochem but here goes.....

Benzene specifcally is aromatic because it does follow Huckels rule 4n+2

Benzene as you see here goes double bond, single bond, double bond all the way around...This is also called "conjugated"...so, where the double bond meet the single (The Carbon where H pokes out) there is a p-orbital, where 1 pi electron exists...so whenever you see a conjugated system like benzene, count 1 pi electron at evert one of the those double bond, single bond interfaces (The Carbon where the H pokes out)...so in benzene its 6 of these guys so 6 pi electrons!!

Huckels rule (4n+2) tells you there is aromaticity...so this rule works by picking a WHOLE NUMER (no fractions) and sticking it into n and seeing if it comes out to the number of pi bonds you found for your structure

For example: Benzene

--6 pi electrons

--Huckels: 4n+2

-- if you put the whole number 1 in for "n" you get 6!! So it is therefore aromatic

--Say you found a structure that had 5 pi electrons...This would NOT be aromatic because there is no whole number you can put in Huckels that would make the equation equal 5.

Hope that helps a little

 

[PiBond]

Hi,

I am having a terrible time understanding aromaticity so if anyone could help me with this concept, that would be awesome.

So, I know that a double bond is made up of one sigma and one pi bond. I don't understand how you can tell, from this, how many pi electrons exist. I think you have to follow Huckel's rule. How do I do this? What is "n" and how do I determine it? One website said that benzene has one electron in its free p orbital thus n=1. But how do you know it has one electron in its p orbital? How do you know this orbital is free?

I also know that each atom is supposed to have a p orbital to satisfy aromaticity. I'm not sure what this means. I went to this site and was confused: http://www2.chem.ucla.edu/~mouser/General/labzone/130BL/problems/hint/aromaticity.html

In one of their examples, they said that the compound is not aromatic because the Carbon is saturated and has no p orbital. I am confused as to how it cannot have a p orbital if 1s2 2s2 2p2 are the electrons in Carbon (and there is clearly a p orbital).

Thanks so much for any help you can give me.

Aromaticity generally equates to stability.

To be aromatic, a compound must have either 2, 6, 10, 14 pi electrons (this is also called a huckle number of electrons described by the equation 4n+2)--I got these numbers by plugging in 0, 1, 2, and 3 ("n" values) into the 4n+2. The compound must also be planar and have overlapping p-orbitals. In other words, if an sp3 hybridized atom exists in the compound then then compound is not aromatic.

For every pi bond there exists 2 electrons. The letter "n" is just a number. It could be 0, 1, 2, 3, etc. Benzene has 6 pi electrons and therefore is aromatic. By plugging 1 into to our equation we can prove this: 4(1)+2 = 6.

For heteroatoms in a compound that have lone pairs on them the general rule is that if they can donate their lone pairs to the cyclic pi system then they will to achieve aromaticity--this is a complicated concept to explain without being able to draw it for you. However, google the compound furan and you'll see what I mean.

 

[HippocratesX]

how do you know when those heteroatoms you were talking about, can donate their lone pair (if they happen to have one) to the ring or not? I remember doing some kaplan problem and they were all cyclic/conjugated heteroatoms and one of them had a lone pair and it said something about the lone pair being perpendicular? or maybe it was parallel? and so it WAS NOT aromatic because of this. How can you tell if the lone pair will contribute to conjugation?

 

[HippocratesX]

how do you know when those heteroatoms you were talking about, can donate their lone pair (if they happen to have one) to the ring or not? I remember doing some kaplan problem and they were all cyclic/conjugated heteroatoms and one of them had a lone pair and it said something about the lone pair being perpendicular? or maybe it was parallel? and so it WAS NOT aromatic because of this. How can you tell if the lone pair will contribute to conjugation?

 

[HippocratesX]

how do you know when those heteroatoms you were talking about, can donate their lone pair (if they happen to have one) to the ring or not? I remember doing some kaplan problem and they were all cyclic/conjugated heteroatoms and one of them had a lone pair and it said something about the lone pair being perpendicular? or maybe it was parallel? and so it WAS NOT aromatic because of this. How can you tell if the lone pair will contribute to conjugation?

 

[HippocratesX]

how do you know when those heteroatoms you were talking about, can donate their lone pair (if they happen to have one) to the ring or not? I remember doing some kaplan problem and they were all cyclic/conjugated heteroatoms and one of them had a lone pair and it said something about the lone pair being perpendicular? or maybe it was parallel? and so it WAS NOT aromatic because of this. How can you tell if the lone pair will contribute to conjugation?

 

[PiBond]

how do you know when those heteroatoms you were talking about, can donate their lone pair (if they happen to have one) to the ring or not? I remember doing some kaplan problem and they were all cyclic/conjugated heteroatoms and one of them had a lone pair and it said something about the lone pair being perpendicular? or maybe it was parallel? and so it WAS NOT aromatic because of this. How can you tell if the lone pair will contribute to conjugation?

Look at the molecule and determine if the adjacent atoms are sp2 hybridized--if they are and the donation of the electrons from the heteroatom would yield a huckel number of pi electrons then the heteroatom donates (if there's two lone pairs, then only donates). However, keep in mind some compounds like pyridine and pyrimidine have lone pairs but do NOT donate because the compound is already aromatic (has huckel number of pi electrons) AND there's a double bond to the nitrogen.

It's really difficult to outline this concept without examples. Here's a list of heteroatoms at you might see on the MCAT that fit this description. Practice makes perfect and all of these are aromatic: furan, thiophene, pyrrole, and imidazole.

When you hear about the lone pair existing parallel/perpendicular if refers to the orbital that they reside in. Generally the 2p orbitals are considered parallel to each other (and above the internuclear region on the ring) in a aromatic compound like benzene whereas the sp2 orbital is considered perpendicular to the aforementioned 2p orbitals.

Hope that helps.

 

[Aletheia]

Thank you so much! With your responses I was able to figure it out. I was missing some really basic concepts and I didn't even realize it. This video http://www.youtube.com/watch?v=r9ZV_t16x0k was really helpful. If anyone else is struggling with this concept, take a look. So hopefully this will help you HippocratesX. Specifically, I think video 7 will address exactly what you are looking for (there are examples).