Calculating Pi Electron Count with Huckel Rule

[snshusat161]

I was studying Huckel Rule. And I stuck on one of the point.

(iv) The total number of pi electrons in the molecular species or ion should be 4n + 2 where n = 0, 1, 2, ...... etc.


But I don't know how to calculate the number of pi electron. Do you have any idea about it.

 

[Borek]

You need to know structure of the molecule and types of bonds between atoms. If you know the first, the second should be a breeze.

--
methods

 

[snshusat161]

I know there are three types of bonds i.e., single bond, double bond and triple bond. I'll also have structure. for instance take the structure of benzene. Now how to calculate the number of pi electrons?

 

[Char. Limit]

Well, you know that it's going to equal 4n+2 for some integer n.

Then you can just count them, as long as you know the structure.

Take Benzene... the structure has been ingrained into my head by now, might be the same for you. How many pi electrons does it have?

Spoiler

Hint: How does bonding work between carbons?

 

[Rohit Mallya]

6 pi electron in Benzene..!

 

[Rohit Mallya]

and in Huckle's rule n=1.

 

[snshusat161]

[PLAIN]http://csrchem2005.bootsglobal.com/library/benzene.gif[/CENTER]

Here is the structure for benzene. Rohit, I know that benzene has 6 pi electron but I need to know how to calculate it. Benzene is the simplest structure in our course and I'm going to face lot more complicated structures so I need to know the exact method to calculate not the theoretical data.​

 

[snshusat161]

I can see that there are three double bonds so should I think that each double bond contribute two pi electron to molecule?

 

[Borek]

6 pi electron in Benzene..!

and in Huckle's rule n=1.

Do you think you have helped OP? I doubt.

Do you care to explain how you calculated pi electrons in benzene to give an example?

 

[Rohit Mallya]

Oh Borek Sir I didn't mean to say that...!
Very Sorry Sir :(
One double bond is equivalent to 2 pi electrons so 3[double bonds]*2[electrons] = 6 pi electrons

 

[Rohit Mallya]

Yes you are correct snshusat161..!

 

[Char. Limit]

I can see that there are three double bonds so should I think that each double bond contribute two pi electron to molecule?

Careful, though, the double bonds that are part of the conjugated system are the only ones that count towards Hückel's Rule.

So, if you had an ethylene group on the benzene, that wouldn't count towards Hückel's Rule.

 

[snshusat161]

Careful, though, the double bonds that are part of the conjugated system are the only ones that count towards Hückel's Rule.

So, if you had an ethylene group on the benzene, that wouldn't count towards Hückel's Rule.

Do you mean Huckel's Rule is applicable only to planar molecules. And what bout triple bond. How many pi electrons are there?

 

[snshusat161]

since 2 bonds contribute two pi electron so three bonds must contribute 3 pi electron but single bond doesn't contribute 1 pi electron so I think both the pi electrons are from the second bond in double bond. So triple bond must contribute 4 pi electron. Am I right?

 

[Char. Limit]

since 2 bonds contribute two pi electron so three bonds must contribute 3 pi electron but single bond doesn't contribute 1 pi electron so I think both the pi electrons are from the second bond in double bond. So triple bond must contribute 4 pi electron. Am I right?

Actually, no.

It has to do with the configuration of the third bond in the triple bond. The second bonds all stick "up" from the main molecule. This allows them to lie in the same plane. The third bonds stick "out" from the main molecule, and thus can't lie in the same plane.

So a triple bond contributes 2 pi electrons.

 

[snshusat161]

Thanks Char. Limit!
You mean that the compounds containing triple bonds are not planar? This means that I can't use Huckel's Rule with compounds containing triple bond.

 

[Char. Limit]

Thanks Char. Limit!
You mean that the compounds containing triple bonds are not planar? This means that I can't use Huckel's Rule with compounds containing triple bond.

Oh no, they're still planar. It's just that the third bond doesn't count towards Hückel's Rule.

Look up Benzyne for a cool example of a non-linear triple bond in a molecule that obeys Hückel's Rule.

 

[snshusat161]

Last two questions.

(1) What is the affect of positive and negative charge on number of pi electrons?

(2) One of the compound containing six pi electron is given "Not Aromatic".

Reason: Cycloheptatriene contains 6 pi electrons but the electrons are only delocalized over six carbon atoms and not over all the seven carbon atoms of the ring. Further more it is not planar. Therefore, it is not aromatic and behaves like a typical alkene undergoing electrophilic addition reaction.

Can You explain me this. Everything going above my head without leaving a single trace of meaning behind.

 

[Char. Limit]

All right, I'm pretty sure I can explain this.

One of the requirements for aromaticity is a cyclic, conjugated system of pi electrons. Now, what does this mean?

Conjugated refers to a system of alternating single and double bonds. Beta-Carotene is a good example of a conjugated system. However, for a system of pi electrons to be cyclic conjugated, the alternating single and double bonds must go in a circle without
end, creating a ring of pi electrons above the molecule. A quick look at cycloheptatriene shows this; there are two single bonds in a row, so it's not cyclic conjugated.

Hope this helps.

 

[Rohit Mallya]

Sir may I know the structures of A-Carotene...??

 

[Char. Limit]

Well, I was talking about B-Carotene, not A-Carotene, but here's the structure of A-Carotene.

Note the alternating single and double bonds.

 

[snshusat161]

But when charges are there, then this alternate single and double bond trick doesn't work. What about that case.

 

[Char. Limit]

It depends on the charge...

Let's go back to your cycloheptatriene molecule. Now let's say we take away one of the two H atoms from the molecule. Now, there are three possibilities, and two of them have charges. The third is no different from the first.

One possibility is that an H+ ion is taken away, leaving 8 total pi electrons. Now, carbanions like this one usually change from an sp3 orbital to an sp2 orbital and a p orbital... (carbocations do the same thing) so you have 7 p orbitals, making a system. However, this system has 8 electrons in it, and this actually makes the molecule antiaromatic for having electrons 4n. So it doesn't happen.

The other possibility is that an H- ion is taken away, leaving six total pi electrons. 6 is a Hückel number, so the molecule is stable.

Really, you need to look into sp3 and sp2 bonds to get it...