4n+2 tt electron rule

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

Hiickel 1931 Molecular orbital theory, 4n+2 tt-electron rule... 

Three years earher, in 1972, Banister had proposed that planar S-N heterocycles belong to a class of electron-rich aromatics , which conform to the Hiickel (4n 2) TT-electron rule. These two aspects of S N... 

All of the exercises in this problem involve counting the number of tt electrons in the various species derived from cyclooctatetraene and determining whether they satisfy the (4n + 2) tt electron rule. 

The Discovery of Benzene 627 Nomenclature of Benzene Derivatives 628 Reactions of Benzene 630 The Kekule Structure for Benzene 631 The Thermodynamic Stability of Benzene 632 Modern Theories of the Structure of Benzene 634 Huckel s Rule The 4n +2 tt Electron Rule 637 Other Aromatic Compounds 645... 

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. 

Huckel s rule (Section 15.3) A rule stating that monocyclic conjugated molecules having 4n + 2 tt electrons n = an integer) are aromatic. 

The approach of the two orbitals on the same side of the surface is called supra , that on opposite sides is called antara . The above rules, 1 and 2, also apply in the case of sigmatropic rearrangements, since the changes of orbital symmetries are the same on going from a HOMO to a LUMO or from a chain of 4n to a chain of 4n + 2 tt electrons. 

Seven equivalent VB structures can be written for the tropylium cation so only one seventh of the positive charge is expected to be on each carbon. Because the cation has six tt electrons, it is expected from Hiickel s (4n + 2)7r-electron rule to be unusually stable for a carbocation. 

Certain cyclic, completely conjugated, rr systems show unusual stability. These systems are said to be aromatic. Hiickel originally narrowly defined aromatic compounds as those completely conjugated, monocyclic carbon compounds that contain (4n + 2) tt electrons. In this designation, n can be 0, 1, 2, 3,..., so that systems that contain 2, 6, 10, 14, 18,...ir electrons are aromatic. This criterion is known as Hiickel s rule. 

For monocyclic conjugated polyenes, high stabilization is found for systems with (4n + 2) TT electrons but not for systems with (4n) tt electrons. The relationship is formulated as Hiickel s rule, which states that completely conjugated planar hydrocarbons are strongly stabilized (aromatic) when they have 4n -I- 2) tt electrons. Benzene (6 tt electrons) is aromatic but cyclobutadiene (4 tt electrons) and cyclooc-tatetraene (8 tt electrons) are not. 

Solution The criterion for aromaticity in a compound is the satisfaction of Hiickel s rule, which states that "only monocyclic carbocycles containing 4n + 2 tt electrons (where n is an integer) in a closed, planar, conjugated circuit are capable of enhanced thermodynamic stability." Actually, this rule can also be applied to heterocycles and polycyclic carbocycles. For instance, naphthalene,... 

According to Huckel s rule, aromatic compounds are cyclic, fully conjugated, planar, and have (4n+2) tt electrons. Antiaromatic compounds are cyclic, fully conjugated, planar, and have 4n tt electrons,... 

Therefore, photochemical interconversion is allowed in the conrotatory pathway. These generalizations are true for all the systems containing (4n - - 2) TT-electrons, where n = 0, 1, 2, etc. Thus, Woodward—Hoffmann rules for electrocyclic reactions may be summed up as given in Table 2.1. 

Sol 1. (a) In each reaction sequence, there is a hexatriene system bearing (4n + 2) TT-electrons. Therefore, under thermal conditions, this system follows disrotatory ring closure as per selection rules. 

The predictions of the Woodward-Hoffmann rules that thermal concerted cycloadditions are allowed for combinations in which 4n + 2 tt electrons are involved has stimulated the search for combinations with ten and larger numbers of participating electrons. An example of a [6 -I- 4] cycloaddition is the reaction of tropone with 2,5-dimethyl-3,4-diphenylcyclopentadienone ... 

Thus if and have similar signs, the anion is aromatic and the cation antiaromatic, while if and have opposite signs, the cation is aromatic and the anion is antiaromatic. The signs of the coefficients can be seen from equation (3.31). The signs are the same if the odd AH has 4n + 1 atoms and opposite if it has 4n + 3 atoms. Thus anions with 4n + 1 atoms and cations with 4n + 3 atoms are aromatic, while cations with 4n + 1 atoms and anions with 4n + 3 atoms are antiaromatic. Since the anion has one n electron more than the radical and the cation one n electron less, and since the radical has as many n electrons as conjugated carbon atoms, one can see that the ions with (4n + 2) tt electrons are aromatic, while those with 4n n electrons are antiaromatic. Thus Hiickel s rule applies equally well to odd, cyclic conjugated ions. 

Hiickel s rule states that, for a compound to be aromatic, its cloud of electrons must contain (4n + 2)tt electrons, where n is an integer. This is the same as saying that the electron cloud must contain an odd number of pairs of 77 electrons. 

Benzene is described by molecular orbital theory as a planar, cyclic, conjugated molecule with six it electrons. According to the Htickel rule, a molecule must have 4n + 2 77 electrons, where n - 0, 1, 2, 3, and so on, to be aromatic. Pianar, cyclic, conjugated molecules with other numbers of tt electrons are antiaromatic. 

An aromatic compound has a molecular structure containing cyclic clouds of delocalized tt electrons above and below the plane of the molecule, and the TT clouds contain a total of 4n -f 2) tt electrons (where n is a whole number). This is known as Hiickel s rule (introduced first by Erich Hiickel in 1931). Eor example, benzene is an aromatic compound. 

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