Hiickel criteria for aromaticity

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. 

The carbon atoms in benzene are sp2 hybridised, and the hydrogen atoms are in the same plane as the carbon atoms. The remaining six p orbitals are at right angles to the plane of the ring and contain six jc electrons. Benzene fulfils the Hiickel criteria for aromaticity as applied to cyclic polyenes containing 4n + 2 electrons (where n-1 in this case) in filled p orbitals capable of overlap. 

A difficulty arises with five-membered heterocycles such as pyrrole, which at first sight would appear to have only four jc electrons, two short of the An + 2 Hiickel criteria for aromaticity. The nitrogen atom is sp2 hybridised and formally contains a lone pair of electrons in the remaining p orbital at right angles to the ring. However, the system is delocalised, as shown below. 

Each molecule meets the Hiickel criteria for aromaticity Each is cyclic and planar, has one 2p orbital on each atom of the ring, and has six electrons in the pi system. In pyridine, nitrogen is sp hybridized, and its unshared pair of electrons occupies an sp orbital perpendicular to the 2p orbitals of the pi system and thus is not a part of the pi system. In pyrimidine, neither unshared pair of electrons of nitrogen is part of the pi system. The resonance energy of pyridine is 134 kj/mol (32.0 kcal/mol), slightly less than that of benzene. The resonance energy of pyrimidine is 109 kJ/mol (26.0 kcal/mol). 

More accurately, it is used to classify any compound that meets the Hiickel criteria for aromaticity (Section 21.2A). 

Benzene is an aromatic compoimd its six equivalent hydrogens appear as a sharp singlet at 8 7.27. Cyclooctatetraene does not meet the Hiickel criteria for aromaticity because it has 4n -ir electrons and is nonplanar. Therefore, the eight equivalent hydrogens of the cyclooctatetraene ring appear as a singlet at 8 5.8 in the region of... 

It is worthwhile to recap how to recognize aromaticity. After all, it has been described in the context of hydrocarbons, heterocycles, cyclic cations, and cyclic anions. The Hiickel criteria for aromaticity can be summarized as follows Look for 4 + 2 electrons where those electrons are in a cyclic array of parallel p orbitals that is, the molecule is planar or nearly planar. Benzene (CgHg) is the paradigmatic hydrocarbon example, but other planar hydrocarbons that simply increase the number of electrons inp orbitals by a factor of 4n are also aromatic i.e., bridged-[10]-annulene (C,qH,q, page 881) and [14]-annulene page 880). ... 

Because charged ring systems can satisfy the Hiickel criteria for aromaticity, they are highly stabilized compared to other nonaromatic cations or anions. Examples include the cyclopropenyl cation, cycloheptatrienyl cation, and cyclopenta-dienyl anion. 

Hiickel criteria for aromaticity (Secfion 21.2A) To be aromatic, a monocyclic compound must have one 2p orbital on each atom of the ring, be planar or nearly so, and have (4n + 2)... 

Every carbon atom in the cycloheptatrienyl cation is sp hybridized, each carbon has a 2p orbital, and the ring contains 4t7+2 = 6 ti electrons. Therefore, it is aromatic Because the cycloheptatrienyl cation, or tropylium ion, has six 71 electrons, it meets the Hiickel criteria for aromaticity. Although we won t discuss the chemistry of the tropylium ion, it is more stable than a simple secondary car-bocation. Moreover, all its carbon atoms are structurally equivalent, and all its carbon-carbon bonds are of equal length. Lewis structures for cycloheptatrienyl cation are shown below. 

From among the molecules and ions shown, all of which are based on cycloundecapentaene, identify those which satisfy the criteria for aromaticity as prescribed by Hiickel s rule. 

Aromatic ions (Section 14.7D) Cations and anions that fulfill the criteria for aromaticity (planarity, electron delocalization, and a Hiickel number of TT-electrons) and thus have additional (aromatic) stability. 

Show how the outer perimeter of benzo[a]pyrene satisfies Hiickel s criteria for aromaticity. Is the outer... 

Beginning in the 1960s, Franz Sondheimer and his colleagues, first at the Weizmann Institute in Israel and later at the University of London, synthesized a number of larger annulenes, primarily to test the validity of HiickeTs criteria for aromaticity. They found, for example, that both [14] annulene and [18] annulene are aromatic, as predicted by Hiickel. [18]Annulene has a resonance energy of approximately 418 kJ (100 kcal)/mol. Notice that for these annulenes to achieve planarity, several of the carbon-carbon double bonds in each must have the trans configuration. 

More recently Corminboeuf et al. [124] based on structural and NICS analysis provided striking insights into the magnetic criteria for aromaticity in the group 8 metal carbonyl [M3(CO)i2](M = Fe, Ru, Os) clusters. Typically [Fe3(CO)i2] (C2v) exhibits the same type of diatropic behavior as Dsh cyclopropane arising from one 3c-2e radial Hiickel orbital and the 3c-4e peripheral Mobius orbitals. The radial orbitals remain also diatropic in both [Ru3(CO)i2] (i>3h) and [Os3(CO)i2] ( 3h),... 

Erich Hiickel developed a shortcut for predicting which of the annulenes and related compounds are aromatic and which are antiaromatic. In using Hiickel s rule, we must be certain that the compound under consideration meets the criteria for an aromatic or antiaromatic system. 

The double C=C bond isi to some extent susceptible to quantum mechanical treatment Pauling,Slater, E. Hiickel, and more recently R. S. Mulliken in particular have tried, by employing different methods, to derive criteria for the form of the charge distribution corresponding to the aliphatic and the aromatic double bond. It has been proved repeatedly that this chemical primary valence is produced by 4 electrons, two of... 

Construct MO energy diagrams for the cyclopropenyl cation, radical, and anion. Which of these species is aromatic according to the Hiickel criteria ... 

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