Delocalisation, of charge

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

This type of chemistry is also observed with 1-methyl isoquinoline 6.28. However 3-methyl isoquinoline is much less activated because delocalisation of charge in 6.29a,b involves disruption of aromaticity of the benzenoid ring. This phenomenon is closely related to the reluctance of 3-halo isoquinolines to undergo nucleophilic substitution. 

The electrophilic character of chlorine and bromine in their attack on olefins is confirmed by recent studies of substituent effects in styrenes and cinnamic acids [ 102]. The slope of the Hammett plot for chlorination = — 4.01) is consistent with a classical carbonium ion intermediate of type (i) (p. 91), but the smaller value [q — — 2.23) for bromination indicates less delocalisation of charge into the aromatic ring, and is consistent with the bromonium ion intermediate. 

Charges on atoms in transition structure, reactant and product are independent of the distance of the polar substituent from the reaction centre as illustrated in Equation (5a), (see Chapter 4, Section 4.1). The greater sensitivity of the phenol dissociation compared with that of benzoic acid standard (Equation 5b) is interpreted on the basis of a smaller distance (d) between substituent and dissociation centre (compared with in the benzoate standard) and to the delocalisation of charge in the benzoate ion (Equation 6, where s, and 8,. are the effective charges on product and reactant respectively). 

Just as for naphthalene, the regiochemistry of attack is readily interpreted by looking at the possible intermediates those for attack at C-5/8 allow delocalisation of charge, while an intermediate for attack at C-6/7 would have a localised charge. 

Nitro substituents activate the displacement of leaving groups like halide, as in benzene chemistry, and extensive use of this has been made in thiophene work. Such nucleophilic displacements proceed at least 10 times faster than for benzenoid counterparts, and this may be accounted for by participation of the sulfur in the delocalisation of charge in the Meisenheimer intermediate. Nitrogroups also permit the operation of VNS processes (3.3.3), as illustrated below. ... 

The pA a for loss of the A -hydrogen of imidazole is 14.2 it is thus an appreciably stronger acid than pyrrole (pATa 17.5) because of the enhanced delocalisation of charge, involving both nitrogens in the imidazolyl anion. Salts of imidazoles can be alkylated or acylated on nitrogen. One convenient method is to use the dry sodium/potassium salt obtained by evaporation of an aqueous alkaline solution sodium hydride in dimethylformamide also serves very well for this purpose. 

In solution, reversible proton addition occurs at all positions, being by far the fastest at the nitrogen, and about twice as fast at C-2 as at C-3. In the gas phase, mild acids like C4H9 and NH4 protonate pyrrole only on carbon and with a larger proton affinity at C-2 than at C-3. Thermodynamically the stablest cation, the 2H-pyrrolium ion, is that formed by protonation at C-2 and observed values for pyrroles are for these 2-protonated species. The weak A -basicity of pyrroles is the consequence of the absence of mesomeric delocalisation of charge in the H-pyrrolium cation. 

Indoles, like pyrroles, are very weak bases typical p a values are indole, -3.5 3-methylindole, -4.6 2-methylindole. -0.3. This means, for example, that in 6M sulfuric acid two molecules of indole are protonated for every one unprotonated, whereas 2-methylindole is almost completely protonated under the same conditions. By NMR and UV examination, only the 3-protonated cation (3//-indolium cation) is detectable it is the thermodynamically stablest cation, retaining full benzene aromaticity (in contrast to the 2-protonated cation) with delocalisation of charge over the nitrogen and o-carbon. The spectroscopically undetectable A -protonated cation must be formed, and formed very rapidly, for acid-catalysed deuterium exchange at nitrogen is 400 times faster than at C-3, indeed the A -hydrogen... 

In fact, the low-temperature measurements (T < 50 K) of 6,5 GHz probe the delocalisation of charges along and between polymer chains within the crystalline regions. As the temperature is increased towards room temperature [60], the delocalisation between the crystalline regions occurs. 

The for loss of the N-hydrogen of imidazole is 14.2 it is thus an appreciably stronger acid than pyrrole pK 17.5) because of the enhanced delocalisation of charge, especially onto the second nitrogen, in the imidazolyl anion. 

Studies of the electrochemical reduction of tetraphenylcycloocta-tetraenes showed that the 1,3,5,7-tetraphenyl derivative was reduced more easily than cyclooctatetraene itself whereas the 1,2,4,7-tetra-phenyl isomer was more difficult to reduce [34]. This accords well with the concept that the presence of phenyl substituents should, on the one hand, make reduction easier because of increased delocalisation of charge in the resultant anion but, on the other hand, make reduction more difficult due to increased steric hindrance when the molecule is flattened [34]. Steric factors similarly result in 1,2,3,8-tetramethylcyclooctatetraene being more easily reduced than its 1,2,3,4-tetramethyl isomer [13],... 

Variable temperature H-n.m.r. studies on the mono-trons anion (XII) show that the tTcens bond migrates around the ring [63,05,67,68]. This bond migration requires a lower activation energy than isomerisation of (XII) into the all-ets isomer (XI) this difference has been ascribed to the difference in delocalisation of charge in the respective transition states [63-65]. Isomerisation of (XII) into (XI) can be brought about photochemically as well as thermally it is also catalysed by acids and by alkali metals [63,65]. 

Two interpretations have been advanced to explain the upheld shift observed for the C-6,7 carbons (91 ppm) in [27] compared to the corresponding shifts of 130.36 and 139.82 ppm (C-6 C-7) in the parent hydrocarbon [28]. If the anion is a bishomoaromatic static molecule [29], delocalisation of charge from the allyl anion moiety to C-6,7 could account for the observed upheld shift. A multiple minimum energy surface with a rapidly equilibrating mixture of allyl anion [30] and the tricyclic and tetracyclic isomers [31] and [32] would give C-6,7 in part carbanion and part cyclopropyl-carbon character and could also explain the observed upheld shift. 

As discussed in more detail in Section 11.13. the conformational flexibility and lability of the anion [N(S02CF3)2] (with significant delocalisation of charge) are associated with both frustration of crystallisation and low-melting points. The use of thermochemical and spectroscopic techniques has enabled the detection of polymorphs of salts of bis(trifluoromethanesulfonyl)amide [576]. However, relatively few have been subject to crystallographic characterisation. These include those of [CiC2pyr][NTf2l [389, 390], [Rmim]... 

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