Positional Reactivities

Calculations for electrophilic substitution in the quinolinium ion can be compared with experiment, and for a range of values of h the predicted order of positional reactivities, s 8>6>3>7, agrees moderately well in a qualitative sense with the observed order of s 8>6>7>3 (table 10.3). Further evaluation of the method must await the results of more extensive calculations for a range of aromatic systems. 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

Comparison of localization energies has frequently been applied to prediction of the relative positional reactivity in polycyclic aromatic hydrocarbons. Simple HMO calculations have only marginal success. CNDO/2 and SCF calculations give results which show good correlation with experimental data on the rate of proton exchange. ... 

The rate constants reported in Table 4 are double the specific rate constant for a given site when two identical sites are present, so that the actual position reactivity is half of the level indicated. Since the rates are reported as seeond order, the... 

Another factor in determining eomparative positional reactivity is the localization energy required to produce 50 or some form approaching 50 as the substrate reaehes the transition state under the influence of the nueleophile. Experimental results on azines and theoretieal considerations warrant the general postulate that the localization energy will be lower when a nitrogen atom is at the... 

The effects of the nucleophile on aromatic substitution which are pertinent to our main theme of relative reactivity of azine rings and of ring-positions are brought together here. The influence of a nucleophile on relative positional reactivity can arise from its characteristics alone or from its interaction with the ring or with ring-substituents. The effect of different nucleophiles on the rates of reaction of a single substrate has been discussed in terms of polarizability, basicity, alpha effect (lone-pair on the atom adjacent to the nucleophilic atom), and solvation in several reviews and papers. ... 

I. Interrelation of Reactivity of Rings and Ring-Positions. Reactivity Rules... 

As for regioselectivity in the electrophilic substitution reactions, we have assumed that introducing a methoxy group to the 1 position of indole nucleus might alter its positional reactivity. 

Partial rate factors calculated for indazole bromination indicate that the benzo derivative is less reactive than pyrazole a positional reactivity order of 5 > 3 > 7 (in the ratio 10.7 6.9 I) was obtained [78JCS(P2)865]. 

Antibodies that recognize oxidized LDL but not native LDL show positive reactivity in human or animal atherosclerotic lesions, but not the normal arterial wall (Haberland etal., 1988 Palinski et al., 1989, 1990 Rosenfeld etal., 1990). 

Base-catalyzed deprotonation of pjn idine 1-oxides should occur much more readily than in the pjrridines themselves and this prediction has been borne out by several workers. These compounds exhibit the same relative positional reactivity as do the pyridinium ions. For example, for 3-bromopyridine 1-oxide in 0.1 N NaOD-DjO the order is 2>6>4>>5, as would be expected on the basis of the net (it -t- a) electron densities at the various nuclear positions. For pyridine 1-oxide the relative rates of exchange in methanolic methoxide solutions at 50° are... 

Base-catalyzed hydrogen exchange in pyridazine (74) occurs in NaOD-DgO and MeONa-MeOD, ° the positional reactivity being 4(5) > 3(6) in both cases. Once more the decreased reactivity of a center ortho to a nitrogen atom relative to a more removed center is evident like pyridine, pyridazine does not have the regular geometry of benzene. 

In the absence of any accurate isomer distribution studies the question of theoretical positional reactivities in dibenzothiophene becomes somewhat meaningless. However, when viewed within the context of the behavior of the related heterocycles dibenzofuran, carbazole, and fluorene, specific studies on dibenzothiophene are more valid. Such studies have been made by Eaborn and co-workers. The rates of cleavage of the four isomeric trimethylsilyldibenzothiophenes (Section VI, H, 2) were studied at 50° using a mixture of methanol and aqueous perchloric acid and compared with the rates of protodesilylation of 2- and 4-trimethylsilyldiphenyl sulfide. The reactivities shown in Fig. 3... 

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