Htickel’s rule

Based on these results, which ion, if either, appears to be resonance stabilized How does Htickel s rule describe these ions ... 

The investigation of [18]annulene is the oldest of the X-ray annulene studies reported, and it was stated that the hydrogens have not been reliably located. The molecular structure closely resembles that of coronene89. This rules out the possibility of a structure with alternate long and short C—C bonds. The observed spread of CC distances in [14]annulene and in [18]annulene is ca 0.06 A, while that in [16]annulene is twice as large, ca 0.12 A. The annulene molecules therefore have structures that are similar to what is expected on the basis of Htickel s rule. 

Htickel s rule states that planar cyclic % systems involving 4n+2 electrons will be unusually stable ( aromatic ), while cyclic 7t systems with 4n electrons will be unstable ( antiaromatic ). 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

Cyclooctatetraene would be antiaromatic if Htickel s rule applied, so the conjugation of its double bonds is energetically unfavorable. Remember that Htickel s rule applies to a compound only if there is a continuous ring of overlapping p orbitals, usually in a planar system. Cyclooctatetraene is more flexible than cyclobutadiene, and it assumes a nonplanar tub conformation that avoids most of the overlap between adjacent pi bonds. Hiickel s rule simply does not apply. 

We can draw a five-membered ring of sp2 hybrid carbon atoms with all the unhybridized p orbitals lined up to form a continuous ring. With five pi electrons, this system would be neutral, but it would be a radical because an odd number of electrons cannot all be paired. With four pi electrons (a cation), Htickel s rule predicts this system to be antiaromatic. With six pi electrons (an anion), Htickel s rule predicts aromaticity. 

Although the tropylium ion forms easily, the corresponding anion is difficult to form because it is antiaromatic. Cycloheptatriene (pKa = 39) is barely more acidic than propene (piTa = 43), and the anion is very reactive. This result agrees with the prediction of Htickel s rule that the cycloheptatrienyl anion is antiaromatic if it is planar. 

Similar arguments can be used to predict the relative stabilities of the cycloheptatrienyl cation, radical, and anion. Removal of a hydrogen from cycloheptatriene can generate the six-7r-electron cation, the seven-Tr-electron radical, or the eight-w-electron anion (Figure 15.7, p. 572). Once again, all three species have numerous resonance forms, but Htickel s rule predicts that only the six-rr-electron cycloheptatrienyl cation should be aromatic. The seven-7r-electron cycloheptatrienyl radical and the eight-w-electron anion are antiaromatic. 

On the basis of Htickel s rule, label the following molecules as aromatic or antiaromatic, (a) [30]Annulene (b) [20]annulene (c) frans-15,16-dihydropyrene (d) the deep blue (see Figure 14-17) azulene (e) S-indacene. 

In 1931, Erich Htickel postulated that monocyclic (single ring) planar compounds that contained carbon atoms with unhybridized atomic p orbitals would possess a closed bond shell of delocalized n electrons if the number of n electrons in the molecule fit a value of 4 + 2 where n equaled any whole number. Because a closed bond shell of n electrons defines an aromatic system, you can use Hiickel s Rule to predict the aromaticity of a compound. For example, the benzene molecule, which has 3 n bonds or 6 n electrons, is aromatic. 

Furthermore another extremely productive interaction of theoretical and practical organic chemistry, the Woodward-Hoffmann rules, owe at least part of their ancestry to Htickel s proposals. 

According to Htickel s (4n - - 2) electron rule, if a carboca-tion has an aromatic character, it is stabilized by resonance. 

An electrons vis-d-vis Htickel s An Ti-electron rule. Now it has been under-stood that the said system by nature possesses a conflicting aromaticity. 

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a pK of 16.0. The pA s of triphenylcyclopropene and trimethylcyclopropene have been estimated as 50 and 62, respectively, using electrochemical cycles (see Section 6.1). The unsubstimted compound would be expected to fall somewhere between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-311-i-G(2fi(/,2pfi() and B3LYP/6-3ll+G(2df,2pd) calculations indicate a small destabilization of the cyclopropenyl anion, relative to the cyclopropyl anion. Thus the six Tr-electron cyclopentadienide anion is enormously stabilized relative to the four ir-electron cyclopropenide ion, in agreement with the Htickel rule. 

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