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Aromatic ions

An aromatic ion has 4 + 2 71 electrons in a ring in which every ring atom is sp hybridized and therefore contains a2p orbital. Lets consider 1,3,5-cydoheptatriene, a conjugated triene. [Pg.402]

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. [Pg.402]

let s consider 1,3-cyclopentadiene. Four of its carbon atoms are sp hybridized. However, one is sp hybridized, and like cycloheptatriene, it is not aromatic. [Pg.403]

Treating 1,3-cyclopentadiene with a base yields the cyclopentadienyl anion. [Pg.403]

3-Cyclopentadiene is surprisingly acidic. It has value of 16, which is comparable to [Pg.403]

To be aromatic, a molecule must have 4u + 2 jt electrons and must have cyclic conjugation. 1,3,5.7,9-Cyclodecapentaene lullills one of these criteria but not the other and has resisted all attempts at synthesis. Hxplain. [Pg.525]

Thomson O tV Click Organic Interactive to learn to recognize and identify aromatic systems. [Pg.525]

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have a p orbital on each carbon) and have 4// + 2 7T electrons. Nothing in this definition says that the number of/ orbitals and the number of tt electrons in those orbitals must be the same. In fact, they can be different. The 4/z + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. I or example, both the cyclopcntadienyl union and the cyclohcptatrienyl cation are aromatic. [Pg.525]

I VV e could remove the hydrogen att)in and both electrons (H ) from the C-H bond, leaving a cyclopentadienyl cation. [Pg.525]

To see why the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic, imagine starting from the related neutral hydrocarbons, 1,3-cyclo-pentadiene and 1,3,5-cycloheptatriene, and removing one hydrogen from the saturated CH2 carbon in each. If that carbon then rehybridizes from sp to sp, the resultant products would be fully conjugated, with a p orbital on every carbon. There are three ways in which the hydrogen might be removed. [Pg.544]

Pattern of molecular orbitals in a cyclic conjugated system. In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has (4iV + 2) pi electrons, it will have a filled shell. If it has 4N electrons, there will be two unpaired electrons in two degenerate orbitals. [Pg.717]

A compound has a filled shell of orbitals if it has two electrons for the lowest-lying orbital, plus 4N electrons, where N is the number of filled pairs of degenerate orbitals. The total number of pi electrons in this case is (4N + 2). If the system has a total of only 4A electrons, it is two electrons short of filling N pairs of degenerate orbitals. There are only two electrons in the A th pair of degenerate orbitals. This is a half-fiUed shell, and Hund s rule predicts these electrons will be unpaired (a diradical). [Pg.717]

We can draw a tive-membered ring of sp -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), Hiickel s rule predicts this system to be antiaromatic. With six pi electrons (an anion), Hiickel s rule predicts aromaticity. [Pg.718]

Because the cyclopentadienyl anion (six pi electrons) is aromatic, it is unusually stable compared with other carbanions. It can be formed by abstracting a proton from cyclopentadiene, which is unusually acidic for an alkene. Cyclopentadiene has a pA a of 16, compared with a pA a of 46 for cyclohexene. In fact, cyclopentadiene is nearly as acidic as water and more acidic than many alcohols. It is entirely ionized by potassium f-butoxide  [Pg.718]

Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion. Cyclopentadiene contains an sp -hybrid (—CH2—) carbon atom without an unhybridized p orbital, so there can be no continuous ring of p orbitals. Deprotonation of the —CH2— group leaves an orbital occupied by a pair of electrons. This orbital can rehybridize to a p orbital, completing a ring of p orbitals containing six pi electrons the two electrons on the deprotonated carbon, plus the four electrons in the original double bonds. [Pg.718]

FIGURE 11.13 The tt molecular orbitals of cycloheptatrienyl (tropylium) cation. [Pg.427]

Cycloheptatrienyl cation (commonly referred to as tropylium cation) [Pg.427]

Tropylium bromide was first prepared, but not recognized as such, in 1891. The work was repeated in 1954, and the ionic properties of tropylium bromide were demonstrated. The ionic properties of tropylium bromide are apparent in its unusually high melting point (203°C), its solubility in water, and its complete lack of solubility in diethyl ether. [Pg.428]

Cyclopentadienide anion is an aromatic anion. It has six ir electrons delocalized over a completely conjugated planar monocyclic array of five 5p -hybridized carbon atoms. [Pg.428]

A convincing demonstration of the stability of cyclopentadienide anion can be found in the acidity of cyclopentadiene. [Pg.428]

FIGURE 11.13 The TT molecular orbitals of cydoheptatrlenyl (tropyllum) cation. [Pg.427]

Hiickel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons. He pointed out that cycloheptatrienyl cation, also called tropylium ion, contained a completely conjugated closed-shell six- r electron system analogous to that of benzene. [Pg.460]

It is important to recognize the difference between the hydrocarbon cycloheptatriene and cycloheptatrienyl cation. [Pg.460]

Cycloheptatriene lacks cyclic conjugation, interrupted by CH2 group [Pg.460]

The carbocation is aromatic the hydrocarbon is not. Although cycloheptatriene has six 7T electrons in a conjugated system, the ends of the triene system are separated by an 5p -hybridized carbon, which prevents continuous cyclic tt electron delocalization. [Pg.460]

Cycloheptatrienyl radical (CyHy ) contains a cyclic, completely conjugated system of tt electrons. Is it aromatic Is it antiaromatic Explain. [Pg.461]

Cycloheptatriene lacks cyclic conjngation, interrnpted by CH2 gronp [Pg.437]

Cycloheptatrienyl cation completely conjugated, six TT electrons delocalized over seven carbons [Pg.437]


Other aromatic ions include cyclopropenyl cation (two rr electrons) and cycloocta tetraene dianion (ten tt electrons)... [Pg.459]

Figure 11.14 shows a molecular orbital diagrfflTt for cycloheptatrienyl cation. There are seven tt MOs, three of which are bonding and contain the six tt electrons of the cation. Cycloheptatrienyl cation is a Hiickel (4n + 2) system and is an aromatic ion. [Pg.456]

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. [Pg.539]

Recently, Weissman and his colleagues52 showed that the product is paramagnetic indicating that it results from an electron transfer process giving one unpaired electron to the hydrocarbon ion. Furthermore, they demonstrated30 that electron transfer reactions easily proceed in systems containing aromatic" ions and neutral aromatic hydrocarbon molecules, e.g., naphthalene" + phenathrene - naphthalene -j- phenanthrene". [Pg.154]

The various aromatic ions which are summarized in Fig. 10 can he determined relatively easily on the basis of these considerations. The discussion of this field supports the theoretically required isomeric proton addition complexes in which proton addition can take place both in the o-position and in the jj-position relative to the methyl group. Thus a reaction in the m-position or at the carbon atom carrying the methyl group is less favoured compared to the other possibilities. This accords... [Pg.220]

Kilpatrick and Luborsky (1953) used a single value of Ag for all aromatic ions in the evaluation of the measurements summarized in Table 11. A direct test of this assumption is not possible for most methyl-9... [Pg.249]

In contrast to the HMO method, the repulsion forces of the electrons are taken into account when carrying out the SCF method, so that it may be expected that the results of these calculations will give better results for the positive aromatic ions than do the HMO calculations. If the pif -values are plotted against the localization energies obtained by this method, one obtains excellent conformity with the requisite linearity, as shown in Fig. 27. The figures for peri-condensed aromatic... [Pg.288]

In order to explore the significance of the term in determining Aa, Musher (1962) calculated its effect for the C7H7 and CsHs ions and found that the correlated points provided a near perfect fit with a correlation line of slope 11-2. Fraenkel et al. (1960) introduced a correction for the variation of ring current with ring size and found it to be small. In Table 1, the raw data for the pnmr shifts for these ions as well as some other monocyclic aromatic ions are given, along... [Pg.137]

Chemical Shifts and Corrections for Monocyclic Aromatic Ions... [Pg.137]

The NMR spectra of all the ions exhibit a significant downfield shift in comparison to the neutral precursors, and the spectra are fully consistent with delocalized aromatic ions. The C2,5-H coupling constant in 282 increases from 175 to 195 Hz upon ionization, and C3,4-H from 161 to 170 Hz. The resonance signals of C2,5 are relatively little affected by the ionization process, and are shifted only by about 1-3 ppm, while the shift at C3,4 amounts to 20-30 ppm, and that of Cla,5a to about 45 ppm. That this is not due to an effect of the fluoro substituent is apparent from the data of 290, which may be obtained by protonation of240. The resonance lines of 290 are in the same range as those of the fluoro-substituted 288. Analogous protonation of237 afforded a very short-lived cation which decomposed before its C spectrum could be measured. The C chemical shifts of benzocyclo-... [Pg.80]

Carbonyl substituted 5/7-1,2,3-dithiazoles (45) lose the carbonyl substituent to give a stable aromatic ion (46) as a main peak in their mass spectra (Equation (1)) <8lBCJ354i>. The ring carbonyl... [Pg.415]

The other structures all represent cases in which the Group IV element is interacting with 3 lithium atoms, and in each case a three-dimensional lithium aggregate is formed. The lithium-lithium and lithium-carbon distances are summarized in Table VIII for those structures that have been determined. In addition, lithium-carbon distances in several lithium-aromatic ion pair systems are included in Table VIII for comparison (18, 19), as well as the observed distances in the hexamer of trimethylsilyllithium. In the dimeric molecule, the Li—Li distance of... [Pg.259]

Simple 4//-pyrazoles break down in mass spectroscopy by loss of one 4-substituent to give an aromatic ion, followed by elimination of nitrogen and fragmentation of the remaining carbon skeleton.15,66,67 For the fused-ring systems 29, nitrogen loss is not apparent,49,65 and in the case of structures 71 and 72, accurate mass measurements have shown that initial loss of 28 units is due to C2H4 and not Nj.46... [Pg.68]

Fig. 3.2 reflects the dependence of benzenoid aromatic ions correlate linearly with the it electron density relative to benzene (Qn = 1)... [Pg.110]

Fig. 3.2. 71-Electron density versus 13C chemical shift of ring carbons in benzene and non-benzenoid monocyclic aromatic ions [76],... Fig. 3.2. 71-Electron density versus 13C chemical shift of ring carbons in benzene and non-benzenoid monocyclic aromatic ions [76],...
A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. [Pg.254]


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1.2- Dithiolylium ions aromaticity

Aromatic Diazonium Ions as Synthetic Intermediates

Aromatic Substitution by Diazonium Ions

Aromatic and Antiaromatic Ions

Aromatic hydrocarbon ions

Aromatic hydrocarbon ions mass spectra

Aromatic ion pairs

Aromatic ion-radical

Aromatic ions cyclooctatetraene dianion

Aromatic substitution diazonium ions

Aromatic substitution via diazonium ions

Aromaticity aromatic hydrocarbon ions

Aromaticity, cycloheptatrienyl cation ions and

Aromatics acylium ions

Aromatics carbonium ions

Aromatics negative ions

Aromatics positive ions

Bromonium ions aromatic bromination

Carbonium ions aromatic

Carbonium ions electrophilic aromatic substitution

Carbonium ions electrophilic aromatic substitution reactions

Diazonium ions, aromatic

Diazonium ions, aromatic azides

Diazonium ions, aromatic fluorides

Diazonium ions, aromatic halides

Diazonium ions, aromatic iodides

Diazonium ions, aromatic phenols

Diazonium ions, aromatic reductive

Diazonium ions, aromatic substitution reactions

Electrophilic aromatic bromonium ions

Electrophilic aromatic substitution nitration with nitronium ions

Huckel rule aromatic ions

Nonbenzenoid aromatic ions

Nucleophilic Aromatic Substitution Diazonium Ions

Nucleophilic aromatic substitution reactions diazonium ions

Other Reactions Involving Formation of Aromatic Diazonium Ions

Pyrylium cations/ions/salts aromaticity

Single crystals of alkali aromatic ion pairs

The Aromatic Diazonium Ion as a Dibasic Acid

Trialkylsilylium Ions in Aromatic Solvents

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