Resonance descriptions

A (iipoie moment results wtier molecule has a permanent urc-i . en elect ton density, Individua) Gttarge separations in bonds cannot be measured, only the sum of ati individual bond moments. A dipole moment is eKpresscJ in debye units (D). 

Of ily completely symmetrical nnoleoules fail to have a dipde rrionitint. but lew have dipoie moments greater tlwi 7 D. 

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Twentieth century theories of bonding m benzene gave us a clearer picture of aromatic ity We 11 start with a resonance description of benzene... 

Because the carbons that are singly bonded m one resonance form are doubly bonded m the other the resonance description is consistent with the observed carbon-carbon bond distances m benzene These distances not only are all identical but also are intermediate between typical single bond and double bond lengths... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

Resonance description of electron delocalization in carboxylate anion... 

The corresponding resonance description shows the delocalization of the nitrogen lone pair electrons m terms of contributions from dipolar structures... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

This IS the same resonance description shown in Section 24 4... 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

The regioselectivity of substitution in furan is explained using a resonance description. When the electrophile attacks C-2, the positive charge is shared by three atoms C-3, C-5, and O. 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

Evidently the positive charge is more readily accepted by the dimethyl-amino group in the excited state, or the resonance description of the excited state of the molecule weights the charge-separated form LXXXIV more heavily. 

The mathematical criterion for resonance description of delocalization effects... 

Fig. 3 Resonance description of the r-electron structure in (benzo)phospholides (R=H, PH3 +>)...

From the resonance description you might conclude that although the primary site for electrophilic attack is at N-1, reactions at carbon C-3(5) might be possible, even if not as likely. However, an important point to remember is that the N atom of pyridine carries a lone pair of electrons these electrons are NOT part of the jc-system. As a result, pyridine is a base 5.2), reacting with acids, Lewis acids and other electrophiles (E ) to form stable pyridinium salts (Scheme 2.2), in which the heterocycle retains aromatic character. 

Another feature that is clear from the resonance description of the pyri-dinium cation is that attack by nucleophiles is favoured at C-2(6) and C-4. This has importance in some reactions where at first sight it may appear that electrophilic reagents combine quite easily with pyridine. These reactions are more subtle in nature ... 

The bond lengths of quinoline, which are irregular, support the resonance description thus, the 1,2-, 5,6- and 7,8-linkages are shorter than that of the carbon-carbon bond in benzene (more double bond character ). There is also a dipole of 2.19 D directed towards the nitrogen atom. 

Like pyrrole, its resonance description requires the delocalization of one of the lone pair electrons of the oxygen atom with the cyclopentadiene unit (Scheme 6.21). 

Molecular orbital descriptions offer a number of significant advantages over conventional resonance structures. For one, they often provide more compact descriptions, e.g., the LUMO in planar benzyl cation conveys the same information as four resonance structures. Second, orbital descriptions are quantitative, compared to resonance structures which are strictly qualitative. Finally, molecular orbital descriptions may be applied much more widely than resonance descriptions. Of course, molecular orbital descriptions cannot be generated using a pencil as can resonance structures, but rather require a computer. It can be argued that this does not constitute a disadvantage, but rather merely reflects a natural evolution of the tools available to chemists. 

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