Order of stability

The order of stability of carbocations, carbanions and radicals bearing electron-donating (+1) alkyl groups is as follows. 

Carbanions can be stabilised by electron-withdrawing groups (-1, -M), whereas carbocations can be stabilised by electron-donating groups (+1, +M). 

For the formation and reaction of ester enolate ions see Section 9.11 

PhjC is commonly called the trityl cation, whereas PhCH2 is the benzyl cation (Section 5.3.1.2) 

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Tautomerism has been discussed in Section 4.04.1.5.2. It concerns prototropic tautomerism and the decreasing order of stability is (hydrazone) >A (azo)> A (enehydrazine). The isomerization A -> A occurs via a A -pyrazoline (65BSF769). Pyrazolidones and amino-A -pyrazolines exist as such. The only example of non-prototropic tautomerism deals with the isomerization (403) —> (404) (74CJC3474). This intramolecular process is another example (Section 4.04.1.5) of the thermodynamic analogy between prototropy and metallotropy. 

In the case of esters, carboxylate anions, amides, and acid chlorides, the tetrahedral adduct may undergo elimination. The elimination forms a ketone, permitting a second addition step to occur. The rate at which breakdown of the tetrahedral adduct occurs is a function of the reactivity of the heteroatom substituent as a leaving group. The order of stability of the... 

Class-b acceptors on the other hand are less electropositive, have relatively full d orbitals, and form their most stable complexes with ligands which, in addition to possessing lone-pairs of electrons, have empty n orbitals available to accommodate some charge from the d orbitals of the metal. The order of stability will now be the reverse of that for class-a acceptors, the increasing accessibility of empty d orbitals in the heavier halide ions for instance, favouring an increase in stability of the complexes in the sequence... 

SnI reactivity follows the order 3°>2°>1°. Is this the ordering of stabilities of the carbocation intermediates ... 

Combination of the hydroxyl ion with the mesomeric cation involves the removal of a double bond. For the quaternary pyridinium compounds this causes the total loss of the aromaticity. For quaternary quinolinium and isoquinolinium compounds, the aromatic character of one of the two rings is lost, and for the quaternary acridinium compounds that of one out of three. Hence., the order of stabilities of these compounds (determined by Hantzsch ) is explained. - Comparison of quaternary 3,4-dihydroisoquinolinium compounds and their isoquinolinium analogs with respect to the equilibrium (5) (4) shows that a much higher hydroxyl ion concentration is necessary for the isoquinolinium ions to form the carbinolamine. This is because the transition from the quaternary 3,4-dihydroisoquinolinium ions into the undissociated carbinolamine involves significantly smaller loss of mesomeric energy than that for the quaternary isoquinolinium hydroxides. ... 

If the formation of a 2,5-dihydro isomer 98 is not allowed, other dihydro-1,2,4-triazines are formed aeeording to their order of stability ealeulated by various theoretieal methods (see Table III) (98JOC5824). 

Notes. In the case of a single oxide (or hydroxide) existing in different allotropic states, indicated by the letters a, b, c, d, etc. the oxides are arranged in descending order of stability, i.e. in ascending order of standard chemical potentials (expressed for an identical chemical formula). 

Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability. 

Solid PtH2X2(PEt3)2 (X = Cl, Br) is isolated by removal of solvent at -20°C the order of stability is Cl > Br > I and solutions decompose at room temperature, eliminating H2. Monohydrides PtHX3(PR3)2 are less stable [178],... 

As judged by vacuum stability test data (see below), Explosive D is of a very high order of stability. The material has been found to withstand storage at ordinary temps for a period of twenty years with no evidence of deterioration, and at 50° for more than five years without marked deterioration... 

Table 11 summarizes the relative conformation stabilities of various sulfmyl carbanions, based on the H/D exchange rates of the corresponding sulfmyl compounds 36-39. The results are in good agreement with the order of stabilities obtained from the MO calculations using the 3-21G basis set. This is remarkable, since the calculation did not take into consideration the solvent effect, despite the strong unsymmetrical solvation on the a-sulfmyl carbanion. 

When considering the solvent CH2C12, by employing the model of Huron and Cla-verie, the order of stability of the hydrocarbon cations remains unchanged (Table 14), but the oxygen-containing cations are clearly preferred by the solvent ... 

In our group we have also studied some diquat systems of the type (propyl)3N(CH2)xN(propyl)3. Molecular mechanics calculations (ref. 4) showed the following order of stability of the zeolite including the template x = 5[Pg.205]

Carbanions Increase in Stability With an Increase in the Amount of s Character at the Carbanionic Carbon. Thus the order of stability is... 

A number of alkylated and ring-annelated derivatives of cyclopentadienyl-thallium(I) have been reported. All were prepared by the same procedure used for the parent compound, and relevant experimental data are listed in Table I. None of these compounds is as stable as cyclopentadienylthallium-(I) the methyl-substituted derivative, for example, undergoes essentially spontaneous oxidation on exposure to the atmosphere (25), and, qualitatively, the order of stability has been assessed (105) as... 

A// 2 is significantly higher for Cr than for either Mo or W, and this is in agreement with the generally observed order of stability for those group VI compounds which are seven-coordinated. 

In the case of functionalized olefins, heterogeneous catalysts have usually not performed well [84,85]. Methyl oleate is the typical test substrate the following order of stability towards the ester functional group for the different SOM catalysts has been observed (based on the number of turnovers) Re W > Mo, which is similar to what has been described for related homogeneous systems. In the specific case of Re, 900 TON can be reached, while TON for other Re-based heterogeneous catalysts do not exceed 200 [79,84]. 

This is a reasonable inference, because site binding is significant only with multivalent cations and strong electrostatic interactions. Under these conditions ion polarization occurs and bonds have some covalent character (Cotton Wilkinson, 1966). This is illustrated by the data of Gregor, Luttinger Loebl (1955a,b). They measured the complexation constants of poly(acrylic acid), 0 06 n in aqueous solution, with various divalent metals, which, as it so happens, are of interest to AB cements (Table 4.1). The order of stability was found to be... 

Picric acid, in common with several other polynitrophenols, is an explosive material in its own right and is usually stored as a water-wet paste. Several dust explosions of dry material have been reported [1]. It forms salts with many metals, some of which (lead, mercury, copper or zinc) are rather sensitive to heat, friction or impact. The salts with ammonia and amines, and the molecular complexes with aromatic hydrocarbons, etc. are, in general, not so sensitive [2], Contact of picric acid with concrete floors may form the friction-sensitive calcium salt [3], Contact of molten picric acid with metallic zinc or lead forms the metal picrates which can detonate the acid. Picrates of lead, iron, zinc, nickel, copper, etc. should be considered dangerously sensitive. Dry picric acid has little effect on these metals at ambient temperature. Picric acid of sufficient purity is of the same order of stability as TNT, and is not considered unduly hazardous in regard to sensitivity [4], Details of handling and disposal procedures have been collected and summarised [5],... 

The delocalization of charge and the order of stability of carbocations parallel the number of attached methyl groups. 

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