4n + 2 rule

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

Cycloheptatrienyl radical has seven tt electrons Therefore it does not satisfy the Huckel 4n + 2 rule and is not aromatic... 

While HiickeTs 4n + 2 rule applies only to monocyclic systems, HMO flieory is applicable to many other systems. HMO calculations of fused-ring systems are carried out in much the same way as for monocyclic species and provide energy levels and atomic coefficients for the systems. The incorporation of heteroatoms is also possible. Because of the underlying assumption of orthogonality of the a and n systems of electrons, HMO dieory is restricted to planar molecules. 

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have ap orbital on each carbon) and have 4n + 2 tt electrons. Nothing in this definition says that the number of p orbitals and the number of nr elections in those orbitals must be the same. In fact, they can he different. The 4n + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. For example, both the cydopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

A7-Phenylsydnone, so-named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain, using the Hiickel 4n + 2 rule. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

According to Hiiekel s rule, turcasarin should not be aromatic, but even if the macrocycle should fulfill the (4n +2) rule for aromatic systems the lack of planarity due to the loop conformation would prevent aromatic stabilization. In fact, the existence of the loop conformation in which the whole macrocycle is twisted was demonstrated by X-ray structure analysis and NMR investigations. 

The electrochemistry of nonbenzenoid hydrocarbons has attracted much interest because their structures offer unusual insights into r-electron systems, which undergo geometric changes upon reduction and obey both the Hiickel An and 4n + 2 rules (Table 2). 

This last requirement is an important characteristic of all aromatic systems. It s known as Hilckel s rule, or the 4n + 2 rule. To apply this rule, begin by assigning 4n + 2 = number of 7 electrons in a cyclic system. Next, solve for n, and if n is an integer (a whole number), the system is aromatic. In the case of benzene, 4n + 2 = 6, so n = 1. One is an integer (a Hilckel number), so the last requirement to be classified as an aromatic is satisfied. Figure 6-7 contains several aromatic species with n = 1. 

It is evident from the structure of the oxazol-5(4//)-one, also sometimes called an azlactone or oxazolinone, why it is so susceptible to epimerization. Removal of the a-proton generates a five-membered ring with six n-electrons, an aromatic system according to the Hiickel 4n + 2 rule (Scheme 5). 

See Fig. 20-1. The four C s and the heteroatom Z use ip -hybridized atomic orbitals to form the a bonds. When Z is O or S, one of the unshared pairs of e s is in an sp HO. Each C has a p orbital with one electron and the heteroatom Z has a p orbital with two electrons. These five p orbitals are parallel to each other and overlap side-by-side to give a cyclic rr system with six p electrons. These compounds are aromatic because six electrons fit Hiickel s 4n + 2 rule, which is extended to include heteroatoms. 

A special case of carbocation stability arises where the cation complies with the Hiiekel (4n+2) rule governing aromatic structures. Of these, the best known and most useful is the cycloheptatrienyl cation, more frequently referred to as the tropylium ion. For an informative and wide ranging account of structures, stabilities, properties and reactions of almost every type of carbocation, reference should be made to the series of monographs edited by Olah and Schleyer (18). 

These predictions indeed are borne out by many experiments, some of which we will discuss later. That the 4n + 2 rule does not apply to noncyclic systems in the same way will be seen by working Exercise 21-18d and 21-18e. 

On the basis of this result and the 4n + 2 rule, work out a mechanism for the reaction and then use this mechanism to predict what product will be formed from the Cope rearrangement of 3,4-dimethyl-1,5-hexadiene. Show your reasoning. 

We have mentioned already (Section 22-3C) the large differences in nmr chemical shifts between the inside and outside hydrogens of [18]annulene —a substance which with 18 tt electrons should be aromatic by the 4n + 2 rule. These differences are observed only at low temperatures. The proton nmr spectrum of [18]annulene at room temperature is a single resonance, which indicates that the inside (Ha) and outside (Hb) hydrogens are equilibrating rapidly. This can take place only if cis-trans interconversion occurs about the double bonds (marked c and t) ... 

The perturbation and symmetry theories can be used to find molecular orbitals of conjugated chains and to explain the aromaticity 4n + 2 rule. 

C=C)—n or cumulene =( C )=K type. Larger clusters, C12-C20, were also found in which C, Cj5 and Cj9 were prominent this phenomenon was ascribed to the fact that these species might be cyclic as the electron counts in the ions conform to the Hiickel 4n + 2 rule. 

Consider now phthalic anhydride, (X = Z = CO, Y = O) (XVIII). This species has eight 7t electrons. Is it antiaromatic HiickeTs 4n + 2 rule would suggest this although we acknowledge that this rule strictly applies only to monocyclic species. The relevant reaction is ... 

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