Resonance stabilisation

Acylium ion (Section 16.3) A resonance-stabilised carbo-cation in which the positive charge is located at a carbonyl-... 

Band 3, 3-93yi 2548 cm. ). This absorption is characteristic of carboxylic acids and is due to the 0—H stretching absorption in the resonance-stabilised dimer. (Carboxylic acids generally exist as dimers in the solid state and in all but very dilute solutions.)... 

The peptide bond is characterized by a fixed planar structure, as was discovered by X-ray crystallography of peptides more than 60 years ago. The arrangement of the atoms in the peptide bond is due to resonance stabilisation the lowest-energy state of the system is that in which the four atoms forming the peptide linkage lie in a plane, while the C-N bond has partial double bond character. 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

From appraisal of their respective resonance stabilisation, arylnitrenium ions and alkoxynitrenium ions should form with similar facility. On account of the fact that A-acetoxy-A-acetyl arylamines 7 are penultimate carcinogens in the metabolism of aromatic amines, A-acyloxy-A-alkoxyamides 3 were designed to test their potential as DNA-damaging agents. 

Population of the <7 n-oac orbital weakens the bond rendering it unstable, resulting in formation of a resonance-stabilised nitrenium ion (Fig. 14c). 

Mechanistically it is likely that the resonance-stabilised anion resulting from deprotonation at the 4-position of benzanthrone couples with non-ionised benzanthrone. Linkage across the 3- and 3 -positions under more strongly alkaline conditions may also involve loss of a proton, coupling and oxidation (Scheme 6.15). Chlorination of benzanthrone followed by an Ullmann reaction produces 3,3 -bibenzanthronyl (6.78), which readily undergoes 4,4,-coupling under mild conditions. 3,3 -Bibenzanthronyl is available from benzanthrone directly by treatment with manganese dioxide in sulphuric acid. 

In this case the inhibitor is nitrobenzene which adds on the growing chain P yielding a Polymer chain having a nitrobenzene endgroup carrying a radical site. The nitrocompound end of the chain is, resonance stabilised and the resonance-stabilised free-radical end is not active enough to attract a fresh monomer molecule and further propagate the chain. 

High pressure free radical process is not suitable for propylene due to extensive hydrogen transfer to free radical which results in resonance stabilised alkyl radicals with reduced tendency to... 

It is quite brittle and is insoluble, because of the right nature of the Polymer chain that is caused by the presence of aromatic rings linked together through parapositions. It can withstand temperatures even up to 560°C. This property is attributed to the presence of resonance-stabilised conjugated double bonds in the aromatic rings. 

The carbonyl oxygen of an ester group, (e.g., in acrylates or vinyl esters), is more basic than a vinyl group and it captures protons (or other cations) from the catalytic system to give a resonance-stabilised cation which does not involve the reaction site, namely the olefinic double bond. Hence, acrylates and vinyl esters do not polymerise cationically. 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stability grounds. Notable exceptions are benzene and toluene (AH°f +82.2, 50.0 kJ/mol 1.04, 0.54 kJ/g, respectively), where there is the resonance stabilising effect of aromaticity. Values of thermodynamic constants for elements and compounds are tabulated conveniently [1], but it should be noted that endothermicity may change to exothermicity with increase in temperature [2], There is a more extended account of the implications of endothermic compounds and energy release in the context of fire and explosion hazards [3], Many examples of endothermic compounds will be found in the groups ... 

The formation of the C-X bond in hetero-cross coupling reactions is thought to proceed via a migration of the hetero atom to the aryl group, which develops a negative charge, which is Jt-stabilised by mesomeric interaction with acceptor substituents. Both for this reductive elimination and its reverse (oxidative addition) resonance-stabilised Meisenheimer complexes have been proposed [42,49,50,51], This stabilised structure is depicted in Figure 12.18. 

The acidity of a-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. 

Cracking reactions produced only 3.5% of the product and most of the cracked product was substituted hydrophenanthrenes. One product positively identified was s-HgP resulting from the initial hydrocracking of one or both of the forms of decahydrophenanthrene which have low resonance stabilisation eneigies. This type of cracking reaction would not be unfavourable and could produce a better hydrogen donor compound. 

Hydroquinone oxidation to quinone occurs via two linked one-electron transfer stages. The first step (typically metal catalysed) yields the semiquinone radical intermediate, which is resonance stabilised. The second step involves electron transfer to molecular oxygen to generate superoxide and quinone [70]. This reaction mechanism is common to all hydroquinones, catechols, resorcinols, and so forth. 

The 1,4-dUiydropyridines, for example, nifedipine, nicardipine and amlodip-ine, are a well-established class of anti-hypertensive drugs. They are photolabile, some markedly so, for example, nifedipine. In all cases, the major light degradation product is the resonance-stabilised, fuUy aromatic, pyridine analogue [38]. 

In some cases, the effect of reactant structure may outweigh the influence of catalyst nature. This is seen by comparison with the dehydration of aliphatic secondary alcohols and substituted 2-phenylethanols on four different oxide catalysts (Table 4). With aliphatic alcohols, the slope of the Taft correlation depended on the nature of the catalyst (A1203 + NaOH 1.2, Zr02 0.3, Ti02—0.8, Si02—2.8 [55]) whereas for 2-phenyl-ethanols, the slope of the corresponding Hammett correlation had practically the same value (from —2.1 to —2.4) for all catalysts of this series [95]. The resonance stabilisation of an intermediate with a positive charge on Ca clearly predominates over other influences. 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

The amide group is resonance-stabilised to a greater extent than the ester group, as shown by the extensive evidence of X-ray analysis of the peptide bond10 and the carbon-nitrogen bond distance in amides8. 

The resonance-stabilised acylium ion is formed in this case. 

Alcohols, phenols, ethers. The molecular ion of alcohols is weak or undetectable. Characteristic ions result from alpha-cleavage giving rise to resonance-stabilised carbocations ions the loss of the largest alkyl group is the preferred pathway although ions resulting from losses of the other groups may also be observed. 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Barbituric acid (83) may be regarded as 2,4,6-trihydroxypyrimidine, but in the crystalline state it exists as the triketo-form (95). In aqueous solution the compound is remarkedly acidic as the result of ionisation of the mono-enolic form (96) with the formation of a resonance stabilised anion (97). 

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