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Huckel's rule for Aromaticitywhat is n?by brady.fosse
Tags: organic aromatic 
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#1
Mar1109, 02:33 AM

P: 3

1. The problem statement, all variables and given/known data
Probably a really stupid question. When using Huckel's rule to figure out if a cyclic compound is aromatic or not, and considering that it must have 4n + 2 pi electrons, how do I figure out what n is? 2. Relevant equations 4n + 2 3. The attempt at a solution I'll just use Benzene as an example, but I have tried on many other compounds. I have tried using the number of carbons in the ring (n is one for benzene but there are six carbons, so that doesn't work), the degree of unsaturation (which is 4 for benzene, not one), the number of hetero/electronegative atoms (obviously this doesn't work), and some other less impressive ways of thinking about it. I can't find an explanation anywhere in the textbook and am a bit confused about why they changed n from signifying 'number of carbons' to something else without telling anyone. Someone told me n is the number of cyclic rings within a compound, which makes sense for benzene (1) and napthalene (2) but if that's the case then I don't know what to do with bigger aromatics such as octo/nano/decatrieneI'm not sure if those in particular are even aromatic due to my lack of understanding of this simple rule, but I did see some examples of bigger rings that had n values of two. If anyone can shed any light on this at all I would appreciate it. I really can't advance with this material unless I grasp this. Thanks. 


#2
Mar1109, 02:38 AM

P: 3

oh...does the rule just mean that if the total number of pi electrons are a multiple of four plus two (n is whatever that multiplier is) then it qualifies as aromatic?



#4
Mar1109, 03:50 PM

P: 3

Huckel's rule for Aromaticitywhat is n?
thank you very much



#5
Jan3110, 09:02 AM

P: 10

True, n is just any integer, but it has to signify sumthing! i mean, come on, we're talking science, you can't just use any 'n' straightway without challenging its credibility.
Yes, it's true that there are hardly few books which mention what 'n' is. I don't think it's there even in Morrison and Boyd. To answer the question, it's fundamentally based on the MOT: aromatic systems have 4n+2 electrons, where n is the number of pairs of degenarate bonding orbitals. Consider Benzene as an example. we concern ourselves only with the piorbital system. Benzene has six atomic porbitals, which give six pi molecular orbitals (MO's): three bonding orbitals,say [tex]\psi[/tex]_{1}, [tex]\psi[/tex]_{2}, [tex]\psi[/tex]_{3}, and three antibonding, say, [tex]\psi[/tex]_{4}, [tex]\psi[/tex]_{5}, [tex]\psi[/tex]_{6}. the 6 pelectrons arrange themselves in the 3 bonding orbitals. [tex]\psi[/tex]_{1} has no node, while [tex]\psi[/tex]_{2} & [tex]\psi[/tex]_{3} have one node each. Furthermore, The energy level of the orbitals increases with increasing number of nodes. Thus, [tex]\psi[/tex]_{1} is at a lower energy level than [tex]\psi[/tex]_{2} and [tex]\psi[/tex]_{3}, which share the same energy level, having one node each. [tex]\psi[/tex]_{2} and [tex]\psi[/tex]_{3} are said to be degenerate. Benzene, thus has one pair of degenerate bonding orbitals (i.e, n=1). For higher aromatic systems, the number of pairs of degenerate bonding orbitals increases. Napthlene has 10 atomic porbitals, thus, 10 MO's. The 5 bonding orbitals contain 2 pairs of degenerate orbitals and along with [tex]\psi[/tex]_{1}. [tex]\psi[/tex]_{1} can contain 2 electrons, while each degenerate pair has a capacity of 4 electrons. Thus, the rule: 4n+2, which is the configuration having all pibonding orbitals completely filled, associated with extra stability. 


#6
Apr1411, 11:11 PM

P: 1

There are some conditions for application of Huckel's rule.One of the most important among them is that the molecule must have a planar structure. It must not contain any sp3 or dsp2 & other hybidised C atom which correspond to a three dimensinal structure. If the molecules do not obey this condition, then even if they have 4n+2 pi electrons, they will not looked upon as an aromatic compound.
the next speculum is that they must have a cyclic structure. If the concerned molecule satisfies the above said conditions, we may apply the Huckel's rule that is it must contain 4n+2 pi electrons.(n=an integer, ie 0,1,2,3.....) .Each single bond correspond to 0 pi electrons ,each double bond correspond to 2 pi electrons,& each triple bond corresponds to 2*2=4 pi electrons. If the concerned compound follows all these said conditions, it may be looked upon as an aromatic organic compound. eg Benzene(C6H6) 1)All carbon atoms in Benzeze is sp2 hybridised. So it has a planar structure.( 1st condition fulfilled) 2) it has a cyclis ring structure,(2nd condition fulfilled) 3) A benzene molecule contains 3 single & 3 double bond. Hence the total no. of pi electrons= 3*0+3*2=6 putting n=1, hence 4n+2=6 Hence benzene follows 4n+2 rule.(3rd condition fulfilled) all the conditions being fulfilled Benzene may be looked upon as an aromatic compound. 


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