Huckel’s Rule: What Does 4n+2 Mean?
Last updated: October 27th, 2022 |
Hückel’s Rule: What Does 4n+2 Mean?
“4n+2 is not a formula that you apply to see if your molecule is aromatic. It is a formula that tells you what numbers are in the magic series. If your pi electron value matches any number in this series then you have the capacity for aromaticity.” – a wise student
Table of Contents
- If You’re Looking For “n” In A Molecule, You’re Looking For The Wrong Thing
- “n” Is Not A Characteristic Of The Molecule!
- In Huckel’s Rule, The Formula (4n+2) Is An Algebraic Expression Of The Series 2, 6, 10, 14… Where ‘n’ Is A Natural Number
- Summary: “n” Comes From Algebra, NOT From Chemistry
- (Advanced) References and Further Reading
1. Stop Looking For “n” In A Molecule, Because You’re Looking For The Wrong Thing
The other night a student came to me with a question about aromaticity.
“There’s one thing I don’t get”, she said. “They say a molecule has to have 4n+2 electrons to be aromatic. How do you find ‘n’ ?”
2. In the [4n+2] Rule (“Huckel’s Rule”), “n” Is Not A Characteristic Of The Molecule!
“n” is not a characteristic of the molecule! Let me explain.
In order for a molecule to be aromatic, it has to have the following characteristics:
- It must be cyclic
- It must be conjugated (i.e. all atoms around the ring must be able to participate in π-bonding through resonance)
- It must be flat
And, it must have a certain number of π-electrons. This is known as Huckel’s rule. The number of π electrons must equal one of the numbers in this series:
2, 6, 10, 14, 18….and so on. For example, we can find aromatic molecules with 2 pi electrons, 6 pi electrons, 10 pi electrons, 14 pi electrons, 18 pi electrons, and so on.
But we have never found aromatic molecules with 0, 1, 3, 4, 5, 7, 9, 11, 12, 13, 15, 16, 17 (and so on) pi electrons. Those numbers are not in the series.
To reprise: the number of pi electrons in an aromatic molecule will always be found in the series [2, 6, 10, 14, 18 …and so on]
However, there has to be a better way of expressing it than [2, 6, 10, 14, 18… and so on”]. Right?
3. In Huckel’s Rule, The Formula (4n+2) Is An Algebraic Expression Of The Series 2, 6, 10, 14… Where ‘n’ Is A Natural Number
There is! This is where we use algebra. This is where n is going to come in – we are going to use math (algebra) to replace “2, 6, 10, 14, 18… and so on” with a condensed formula.
2, 6, 10, 14, 18… is an example of an algebraic series.
Algebraically, you can express this with the formula (4n +2), where n is a natural number (0, 1, 2, 3…)
Let’s plug in different values of n (we will put n in bold)
For the formula 4n + 2
- For n = 0, we get (4 × 0 + 2) = 2
- For n = 1, we get (4 × 1 + 2) = 6
- For n = 2, we get (4 × 2 + 2) = 10
- For n = 3, we get (4 × 3 + 2) = 14
- For n = 4, we get (4 × 4 + 2) = 18
We can keep going, but do you get the idea?
By using the formula [4n +2], we are expressing the same idea as [2, 6, 10, 14, 18… and so on] but it is a lot more condensed – and precise.
4. Summary: “n” Comes From Algebra, NOT From Chemistry
Molecules that have the 3 characteristics listed above (cyclic, conjugated, flat) and have this number of π electrons [4n +2] will be aromatic.
The letter “n” is not a characteristic of the molecule!
(Advanced) References and Further Reading
- CYCLOHEPTATRIENYLIUM OXIDE
W. von E. Doering and Francis L. Detert
Journal of the American Chemical Society 1951 73 (2), 876-877
In this paper, esteemed Harvard chemist William Eggers von Doering succinctly summarized the Huckel rule as 4n + 2 pi electrons (although writes it (2n+4 here) in his synthesis of cycloheptatrienylium oxide (“tropone”).
94 thoughts on “Huckel’s Rule: What Does 4n+2 Mean?”
I find some students dissatisfied with the idea that n doesn’t actually refer to anything in the molecule. I don’t know what else to tell them. It seems like they can still apply the rule though.
Well, it’s not so much that it doesn’t refer to anything… the original description is just missing an implied “…where n can be any natural number”
Thanks for mentioning that. I’m going to put that in there explicitly.
I think ,there’s no need of (4n+2),
Learn only this series 2,6,10,14,..etc am i right?
(4n + 2) is shorthand for the series that you mention.
n is a natural number. And how can v find that
When the n will be equal to 0,1,2,3..
You can work out the number of pi electrons from the molecule (x)
Then working backwards using Huckel’s Rule –> (4n+2) = x
(x-2)/4 = n
If n is a natural number (0,1,2,3…) and the structure is a planar monocycle with an uninterupted cyclic pi-cloud then the molecule is aromatic.
n is just any natural number which is used to satisfy the 4n 2 rule.
1. Count the number of pi electrons.
2. If that number becomes equal 4n 2 for any value of n then that compound is aromatic(or in other words if the number of pi electrons come in the series – 2, 6, 10, 14, 18….. then that compound will be aromatic)..
Hope all understand what I mean.
I find this a little bit confusing. How do i get that the molecule is flat? I always thought that the fact that the aromatic molecule is planar is caused by aromaticity itself (or even distribution of electrons in the rings above and below aromatic core). From this article it seems to me like the planarity is something like prerequisite (i think of prerequisite like that it must be cyclic and conjugated). I’m not sure if i explained my problem properly but if you got some of my idea, can you please explain it?
Good question. It’s generally a good assumption that if the molecule fits the first three criteria, the molecule is flat, unless there is some steric impediment to it doing so. One notable exception is one of the isomers of -annulene (see figure 3 in this page) which cannot lie flat due to steric clash between the hydrogens.
Thanx for this post; there is a lot of confusion about the Hueckel rule. I personally like azulene more for the 10-electron example, as each individual ring does not conform the Hueckel rule, while your example still has to separate rings with each 6 π electrons. Another aspect of this is that ‘aromatic’ bonds, those in the delocalized bond system, have a bond length somewhere in between that of a single and of a double bond.
Azulene is a much better example. Good idea Egon.
Huckel model is obsolete. See
ARKIVOC, 2008, Part xi, p. 24 – 45
According to above comments the very aromatic hydrocarbon pyrene is anti-aromatic because it includes 16 pi electrons?
According to above James’ amusing comments regarding ‘algebraic sens of n’ the aromatic hydrocarbon pyrene is anti-aromatic because it includes 16 pi electrons! Also, the non-existent hexazine should be quite stable because his Huckel aromaticity!
Huckel model became obsolete many years ago, see, for instance, ARKIVOC, 2008, Part xi, p. 24-45
Yes, it’s worth noting that the Huckel model starts to fall apart for even moderately complex polyaromatic hydrocarbons, so thank you for pointing that out. However, as far as the most common examples students will encounter in an introductory class (i.e. the intended audience of this site), the Huckel model is fine. By all means learn the exceptions once you’ve mastered the basics.
I have confusion that Benzenoids obey Huckel’s rule where non benzenoids not but we call them aromatic compounds?
Dear Adil! The term aromatic is an old name used for those compounds having pleasant odor, although they are not in the category what we call as “Aromaticity”.
It should read:
For n = 0, we get (4 × 0 + 2) = 2
For n = 1, we get (4 × 1 + 2) = 6
For n = 2, we get (4 × 2 + 2) = 10
For n = 3, we get (4 × 3 + 2) = 14
Thanks, Jamey, that confused me, too.
In 4n+2rule where n is????????plz.sir n is pi electron or rings or pi bonds
thanks i had the same problem
Very helpful. Thanks
I get it!!!! You have no idea what this means… Thank you!!!
I have little confusion … n is not the characteristic of a molecule so what is it???
all have same value of n’ or every molecule has different n’ value how we can find the value of ‘n???
n, is the number of ring or rings
NO it absolutely is not!!
No, its the number of electrons in the pi-bonds. Take benzene, for example, we construct this as alternating pi bonds in a six membered ring. If the pi, bonds are alternating, then we know that there must be half the number of double bonds as there are single (3 in this case). So, we find that that the number of electrons in the pi bonds are 3×2 (because there are two electrons in every double bond), so we see that the 6 electrons in the pi bonds, satisfies the 4n+2 rule. Another way to put the 4n+2 rule is that if you set 4n+2 equal to the number of electrons in the pi bond and solve for n, you will find that n will be a whole number. Therefore n must be a whole number that satisfies this equation 4n+2=x, where x = the number of electrons in the pi bonds. The reason for this is probably related to quantum mechanics, since n must be a whole number, it must be a quantized value of some kind…that’s just a guess though.
Hi I’m confused on the last structure: I don’t see how the last dicyclic structure has 10 pi electrons. At the tertiary carbons that the cycloheptane and cyclopentane are connected to: aren’t there no space for electrons? Or they don’t have Hydrogens, which I’ve been counting as electrons for the previous examples.
Sophia !the last structre has 10 pi bond because it contain 5 double bond and each double bond contain 2 pair of electron so they become 10 pi electron.
You would think it would contain 4 electrons (since a double bond implies that there are double the amount of the number of electrons in a single bond), but its important to note that that the 4n+2 requirement isn’t dependent on the electrons of the double bond, but of the pi bond…there is a difference. A double bond = 1 pi bond + 1 sigma bond. The pi bond refers to sideways orbital overlap of one of the 3 p orbitals, resulting in a shorter bond.
We are taught that a p orbitals house 6 electrons, but this is wrong. the p sublevel houses 6 electrons. It also has 3 individual p orbitals in it, each containing 2 electrons. If you’re curious as to why, look up the Zeeman effect. So basically because its only one p orbital for each carbon in the bond, there are 2 electrons total in the pi bond, but four in the double.
Thank you sir,,,,it helped me to understand the meaning of ( 4n+2)
Sir ,, i am still confused why annulene is not aromatic,,,,how steric hinderence affect it????
Is a compound having more than one ring aromatic even if a single ring is aromatic? For eg. if benzene is linked to another five-membered ring (with no pi-bonds) to form a bicyclic compound, can it be called aromatic?
Yes, what you’re describing is simply a substituted benzene which happens to have substituents which form a ring. Doesn’t change aromaticity.
What if you have a molecule that is made from taking 1 benzene and putting 6 benzene rings around it?
You get a cyclohexane ring in the center. Does this benzene bonded to 6 other benzene rings change the aromaticity or is the molecule itself still aromatic even though the ring on the inside of the molecule(cyclohexane) is not?
and why can’t 2 aromatic rings bonded to each other keep their pi bond and form an alkyne? I mean surely there are cases of cycloalkynes.
Thank you for that concise and simplified explanation. Perhaps you could give some tips to the people at Khan academy so they can improve their explanation!
Glad you found it helpful Charlotte!
Hey James, I have 2 questions about ortho meta para directors and i was wondering if you would be able to answer them.
1. If you have HSO3 on a ring and add NaOH I believe the OH replaces the HSO3. Would you need to add NaOH and heat, and then H3O and H2O as the total step to accomplish this? or just NaOH and heat?
2. If you have two ortho/para directors on a ring and one is stronger than the other, say NH2 and CH3, and all other spots are open and you are adding Br2 in say acetic acid, would the Br2 only add to the NH2 directed postions? Or would it go to every spot on the ring?
Actually sorry to say all of you are worng. n in 4n+2pi electron comes from molecular orbital theory where n stand for number of degenerate orbitals. in benzene only one degenerate orbital so n =1. so benzene satisfy huckle rule. same applies to ther members as well.
No, it does not come from molecular orbital theory.
It’s just an algebraic expression of the series: 2, 6, 10, 14….
Thank you ! it was very helpful , however I wanted to know while counting the pi bonds , do the lone pairs of electron play any role ?
It depends whether or not they are aligned with the other electrons of the pi system.
How do you know whether or not the lone pair electrons are aligned with the other electrons of the pie system? Thanks a bunch I really enjoy your site!
I thinks you can assume the pair aligns with the pi system if the molecule where the pair resides, has a sp 2 hybridisation.
Yes, but only one orbital per atom may align with the pi system.
Thank you for this great explanation! I was also confused.
So the pi electrons are those which make a pi bond?
Like in benzene it has 3 alternate pi bonds which keep moving so at a time there are only 3 pi bonds…so the electrons which form these pi bonds are 6.
Am I right?
Can you also tell me why a three carbon ring with a pi bond and alternate “+” charge is aromatic but a three carbon ring with a pi bond and alternate “-” charge is not aromatic?
Count the pi electrons. The first case has two pi electrons (2 in the pi bond, zero in the p orbital of the carbocation), which is a Huckel number. The second case has 4 pi electrons (2 in the pi bond, 2 in the p-orbital of the carbon which bears the negative formal charge). 4 is not a Huckel number. It is not aromatic.
Thank you for this, life saver!
l understand that for a compound to be aromatic using Huckel’s rule,first you should know the number of pi electrons in the compound and if you substitute any integer value into the formula and the result amounts the number of pi electrons in the compound then that integer substituted becomes the value of n for that compound.
This was so helpful, so clear and consice and really helped. Every time I’m stuck on a concept in organic chem, I come to this website, it’s been an absolute life saver. Thank you!
Thanks for the help.
How do you tell if the mlc is planar or not?
You talk about Non-Aromatic and Aromatic compounds, what about Anti-Aromatic compounds?
Antiaromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n electrons in it. Unlike aromatic compounds, which follow Hückel’s rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive.
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Ok, after looking a million places, this site explains it best, but I would like to propose some wording that made it make sense for me.
4n+2 is not a formula that you apply to see if your molecule is aromatic. It is a formula that tells you what numbers are in the magic series. If your pi electron value matches any number in this series then you have the capacity for aromatic. To find how many pi electrons you have, multiply the number of pi bonds by 2 and then compare that number to this formula. It it works out with n as a whole integer, then you have aromaticity. Yes?
How does huckel rule apply on furan?
Two pi bonds on furan = 4 electrons, plus one lone pair on the oxygen = 6 electrons.
n=0 1 2 3 .., what does it mean
i cant understand
while in case of benzene n=1 and has 6 pi electrons.
then from where u bring the value of n
any rule for the value of n ., how we can understand that n = ? in any molecule ?
can w right 2 in case of benzene ?
i have alot of confusion in this plz rply me fast
7. Review. Theory of three-electron bond in the four works with brief comments (full version, 93 p., content). http://vixra.org/pdf/1612.0018v4.pdf
Review (127 pages). Benzene on the Basis of the Three-Electron Bond. (The Pauli exclusion principle, Heisenberg’s uncertainty principle and chemical bond). http://vixra.org/pdf/1710.0326v1.pdf
Flat is important Azonine is an unsaturated heterocycle of nine atoms, with a nitrogen replacing a carbon at one position is aromatic. Oxonine an unsaturated heterocycle of nine atoms, with an oxygen replacing a carbon at one position is not aromatic.
To What Extent Can Nine-Membered Monocycles Be Aromatic?
Paul von Rague´ Schleyer,*[a] La´szlo´ Nyula´szi,*[b] and Tama´s Ka´rpa´ti[b]
i have a question please, what does it mean a molecule being flat to be an aromatic ?
p orbitals must align themselves in the same plane in order to establish conjugation. If the molecule is not “flat”, i.e. if the orbitals are not aligned in the same plane, then aromaticity is not possible.
Hello, found an error on the page (I believe). In your 2pi 4pi 6pi 8pi small picture examples, it states that the cyclo-octane with the 8pi is Not Aromatic but in fact its Antiaromatic. Not Aromatic would be incorrect!
Hi Ryan – haven’t got to antiaromaticity yet, so I didn’t include that designation. However cyclooctatetraene can wiggle out of being antiaromatic because it is sufficiently flexible to avoid being flat. It adopts a tub shape.
Thanks for the comment! James
Quantum-mechanical aspects of the L. Pauling’s resonance theory.
Algebraically, you can express this with the formula (4n +2), where n is a natural number (0, 1, 2, 3…)
It should be n is a *whole* number.
Natural numbers are counting numbers and does not include 0.
Whole numbers are counting numbers plus 0.
Gone through many other sites, but not one has been able to explain it this well! Thank you for helping me understand as well as for the abundance of other valuable information on your page! Much thanks!
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If we put n as a number of pi bonds present in the chemical structure then also we get the one of the number belongs to series
2,6,10,14,18…… Then also we conclude it is aromatic…. Am i right?
No. You’re missing the point of the article.
this helped a lot as a student. it should also kindly suggest mentioning that counting Pi bonds is just counting 2 Pi electrons per every double bond. ignore the sigma bonds. i was confused for a bit
Sure. That works.
Thanks so much foe your useful information but still I have one problem with n
Can we say the meaning of n is the number of ring or not???
How can we apply Huckel’s Rule when we have a hetroatom with lone pairs in the ring? How should we be counting the number of electrons then?
In any aromatic compound, on which basis we take ‘n’.. I’m confused..
I don’t know how to make it any simpler…
What happen if n=1/2
It has to be an integer. N=1/2 is meaningless.
I mean if n is a fraction then compound is said to be nonaromatic?
If you’re using n as a fraction it means you’re not understanding the purpose of 4n + 2
You are correct to say that n isn’t in the molecule, but yet you fail to show your students how to find the n. Not once did you even teach anything over as to where the n actually comes from. I just started studying this in organic chemistry 2 and I’m still researching this myself. I came to this article thinking I got it down, but yet it still fails to teach students how to find n.
This is a completely valid criticism. I should be answering the “question behind the question” here. The specific details are on this post:
but including a discussion on this post would also be useful. Thanks for the feedback.
Is 1,3,5 triphenylbenzene aromatic? As number of electron become 4x(4+2) = 24 which follow antiaromatic rule.
Treat each aromatic ring individually.
Thank you for the simple explanation .
Thank you for your information. Found it helpful, be blessed.