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Aromaticity and the Huckel 4n 2 Rule

Let s list what we ve said thus far about benzene and, by extension, about other benzene-like aromatic molecules. [Pg.541]

Cyclobutadiene has four tt electrons and is antiaromatic. The tt electrons are localized in two double bonds rather than delocalized around the ring, as indicated by an electrostatic potential map. [Pg.542]

Cyclobutadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at 78 C, however, cyclobutadlene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. [Pg.542]

Chemists in the early 1900s believed that the only requirement for aromaticity was the presence of a cyclic conjugated system. It was therefore expected that cyclooctatetraene, as a close analog of benzene, would also prove to be unusually stable. The facts, however, proved otherwise. When cyclooctatetraene was first prepared in 1911 by the German chemist Richard Willstatter, it was found not to be particularly stable but to resemble an open-chain polyene in its reactivity. [Pg.542]

Test your knowledge of Key Ideas by answering end-of-chapter exercises marked with.  [Pg.543]


Cycloheptatrienyl radical has seven tt electrons Therefore it does not satisfy the Huckel 4n + 2 rule and is not aromatic... [Pg.1219]

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. [Pg.17]

Tropylium cation and the substituted cyclopropenyl cations cited in Table 5.2 are of interest in that their ease of formation provides experimental support for the Huckel 4n + 2 rule of aromatic stability. Both types contain planar monocyclic systems of 5p -hybridized atoms with two tt electrons (n = 0) in the cyclopropenyl cations and six tt electrons (n = 1) in tropylium cation. [Pg.256]

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... [Pg.530]

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. [Pg.577]

The inscribed polygon method is consistent with Huckel s 4n + 2 rule that is, there is always one lowest energy bonding MO that can hold two Jt electrons and the other bonding MOs come in degenerate pairs that can hold a total of four Jt electrons. For the compound to be aromatic, these MOs must be completely filled with electrons, so the magic numbers for aromaticity fit Huckel s 4n + 2 rule (Figure 17.11). [Pg.629]

It is important to examine aromaticity in its wider concept at this point. There are many compounds and systems besides benzene that are aromatic. They possess common features in addition to planarity and aromatic stability. MO calculations carried out by Hiickel in the 1930s showed that aromatic character is associated with planar cyclic molecules that contained 2, 6, 10, 14 (and so on) n-electrons. This series of numbers is represented by the term 4n + 2, where n is an integer, and gave rise to Huckel s An + 2 rule that refers to the number of n-electrons in the p-orbital system. In the case of benzene, n— 1, and thus the system contains six n-electrons that are distributed in MOs as shown above. [Pg.5]

Aromatic systems that obey Huckel s 4n + 2 rule where n = 0 and so possess two n-electrons do exist and are indeed stable. The smallest possible ring is three membered and the derived unsaturated structure is cyclopropene. The theoretical loss of a hydride ion from this molecule leads to the cyclopropenyl cation, which contains two 71-electrons distributed over the three carbon atoms of the planar cyclic system (Figure 1.8). [Pg.7]

This last requirement is an important characteristic of all aromatic systems. It s known as Huckel s rule, or the 4n + 2 rule. To apply this rule, begin by assigning 4n + 2 = number of -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 Hiickel number), so the last requirement to be classified as an aromatic is satisfied. Figure 6-7 contains several aromatic species with n = 1. [Pg.86]

Using theoretical data, Huckel formulated the 4n + 2 rule which states that exceptional resonance stability is to be expected with ir electron systems containing 4n + 2 electrons in planar, cyclic structures, where n is an integer. This applies to a continuous series of p orbitals which are capable of effective overlapping. For example, benzene has 6, or 4(1) +2 (with n=l), if electrons. Therefore, according to Huckel s rule, benzene is aromatic and is stabilized by resonance. [Pg.302]

An aromatic compound is a cyclic structure that contains continuous tt electron clouds above and below the plane of the molecule. The ir clouds must also contain a total of kn+2 tt electrons, where n can be any integer. This requirement is called the 4n+2 rule or Huckel s rule and is derived from quantum mechanics. Thus, to determine whether any given compound is aromatic, first inspect the structure to see if it is cyclic and planar. Then detennine the number ofTf electrons and see if all these satisfy Kuckel s rule. [Pg.304]

Benzene is aromatic because its tt electrons are delocalized in a cyclic cloud above and below the plane of the molecule, and its tt cloud contains 4n+2 it electrons (where n = 1 for benzene). This stipulation, known as Huckel s 4n+2 Rule, is the defining condition for aromaticity. This special stability, due to the property of aromaticity, is based on quantum mechanics and has to do with the filling up of the bonding orbitals of the tt electrons. [Pg.309]

According to the 4n - -2 Huckel s rule for aromaticity and the 4n Huckel s rule for antiaromaticity, triatomic systems with the electronic configuration are... [Pg.289]

Therefore the X3 (X = Sc, Y, La) clusters satisfy the 4n -f 2 counting rule for a-aromaticity and the 4n - - 2 Huckel rule for Tt-aromaticity in cyclic systems with odd numbers of atoms (see Table 1). Thus, all three anions are d-orbital based doubly (a- and rr-) aromatic systems. The obtained via the AdNDP method 3c-2e d-AO based Or-bond and 3c-2e d-AO based rtf-bond are similar to the 3c-2e la i and la model molecular orbitals presented in Figs. 5 and 11, respectively. [Pg.299]

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... [Pg.509]


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