Canonical forms stability

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

When the ortho-para directing group is one with an unshared pair (this of course applies to most of them), there is another effect that increases the amount of para product at the expense of the ortho. A comparison of the intermediates involved (p. 683) shows that C is a canonical form with an ortho-quinoid structure, while D has a para-quinoid structure. Since we know that para-quinones are more stable than the ortho isomers, it seems reasonable to assume that D is more stable than C, and therefore contributes more to the hybrid and increases its stability compared to the ortho intermediate. 

The problem statement. We pursue the stability analysis of two-layer schemes by having recourse to their canonical form... 

The problem statement. In this section we establish sufficient stability conditions and a priori estimates for three-layer schemes on the basis of their canonical form... 

Many modes of constructing schemes of a particular form can be treated as simplest regularization modes. The canonical form of a scheme is convenient not only for practical tests of stability, but also for proper... 

Resonance theory [15] contains essentially three assumptions beyond those of the valence bond method. Perhaps the most serious assumption is the contention that only unexcited canonical forms, non-polar valence bond structures or classical structures need be considered. Less serious, but no more than intuitive, is the proposition that the molecular geometry will take on that expected for the average of the classical structures. This is extended to the measurement of stability being greater the greater the number of classical structures. These concepts are still widely used in chemistry in very qualitative ways. 

This is because the greater the number of canonical forms, the greater is the stability. Let us see the canonical forms from a tertiary and a primary carbocation. 

Canonical forms of benzene that are calculated to contribute about 22% to the resonance stabilization of benzene. Such resonance structures have no separate physical reality or independent existence. For the case of benzene, the two Kekule structures with alternating double bonds i.e., cyclohexatriene structures) contribute equally and predominantly to the resonance hybrid structure. A dotted circle is often used to indicate the resonance-stabilized bonding of benzene. Nonetheless, the most frequently appearing structures of benzene are the two Kekule structures. See Kekule Structures... 

The energy released by a molecular entity as a consequence of resonance, the magnitude of which equals the difference between the actual molecule s energy and the energy of the least energetic canonical form. Because the isolated canonical form does not exist factually, the resonance stabilization energy can only be estimated. 

The effect of the ring heteroatom can be seen in the reactivity of alkylpyridines. Deprotonation from the a-carbon of a substituent at position 4 or 2 is much easier than from a similar substituent at position 3. For example, mesomeric stabilization of the negative charge in (4a) and (5a) will involve contributions from canonical forms (4b) and (5b) in which the charge is carried on ring nitrogen, but a similar contributor cannot be drawn for (6a Scheme 2). 

Simple acyl cations RCO+ have been prepared45 in solution and the solid state.40 The acetyl cation CH3CO is about as stable as the f-butyl cation (see, for example, Table 5.1). The 2,4,6-trimethylbenzoyl and 2,3,4,5,6-pentamethylbenzoyl cations are especially stable (for steric reasons) and are easily formed in 96% H2S04.47 These ions are stabilized by a canonical form containing a triple bond (G), though the positive charge is principally located on the carbon,48 so that F contributes more than G. 

However, in a nitrogen ylide R3 —C 2 (see p. 39), where a positive nitrogen is adjacent to the negatively charged carbon, only the field effect operates. Ylides are more stable than the corresponding simple carbanions. Carbanions are stabilized by a field effect if there is any hetero atom (O, N, or S) connected to the carbanionic carbon, provided that the hetero atom bears a positive charge in at least one important canonical form,90 e g.,... 

With resonance possibilities, the stability of free radicals increases 149 some can be kept indefinitely.150 Benzylic and allylic151 radicals for which canonical forms can be drawn similar to those shown for the corresponding cations (pp. 168, 169) and anions (p. 177) are more stable than simple alkyl radicals but still have only a transient existence under ordinary conditions. However, the triphenylmethyl and similar radicals152 are stable enough to exist in solution at room temperature, though in equilibrium with a dimeric form. The concen-... 

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