Salt effects negative

One factor that complicates the kinetic picture is the salt effect. An increase in ionic strength of the solution usually increases the rate of an SnI reaction (p. 451). But when the reaction is of charge type II, where both Y and RX are neutral, so that X is negatively charged (and most solvolyses are of this charge type), the ionic strength increases as the reaction proceeds and this increases the rate. This effect must be taken into account in studying the kinetics. Incidentally, the fact that the addition of outside ions increases the rate of most SnI reactions makes especially impressive the decrease in rate caused by the common ion. 

The difference between A obsd and caic might be due to a specific salt effect on the rate constant for solvolysis. However, this is unlikely because perchlorate ion acts to stabilize carbocations relative to neutral substrates.At high concentrations of sodium bromide, the rate-limiting step for solvolysis of 1-Br is the capture of 1 by solvent (ks Scheme 5A). Substitution of Br for CIO4 should destabilize the carbocation-like transition state for this step relative to the starting neutral substrate, and this would lead to a negative, rather than positive deviation of obsd for equations (3A) and (3B). 

The nature of salt effects in monomolecular heterolysis has been reviewed. The experimental work of the same group on salt effects has continued with a study of the negative salt effect of lithium perchlorate on the heterolysis of 1-iodoadamantane in y-butyrolactone. It is assumed that the salt effect of lithium perchlorate is caused by the salt action on the solvent-separated ion pair of the substrate. 

The yield of the nucleophilic substitution product from the stepwise preassociation mechanism k[ = k. Scheme 2.4) is small, because of the low concentration of the preassociation complex (Xas 0.7 M for the reaction of X-2-Y). Formally, the stepwise preassociation reaction is kinetically bimolecular, because both the nucleophile and the substrate are present in the rate-determining step ( j). In fact, these reactions are borderline between S l and Sn2 because the kinetic order with respect to the nucleophile cannot be rigorously determined. A small rate increase may be due to either formation of nucleophile adduct by bimolecular nucleophilic substitution or a positive specific salt effect, whUe a formally bhnole-cular reaction may appear unimolecular due to an offsetting negative specific salt effect on the reaction rate. 

If the inequality is positive, solvent 1 (nonelectrolyte) is salted out on the other hand, if it is negative, solvent 1 is salted in. Thus the extent of the salt effect on... 

The salt effect parameter ko is plotted in Figure 2, and the data for lithium chloride and lithium bromide reported in the previous paper (3) are also plotted for the purposes of comparison. It can be seen from Figure 2 that ko depends markedly on the solvent composition. The values of ko decrease and in the extremely water-rich region k0 is negative at 50° and 25°C. In other words, 2-propanol is salted in by the addition of lithium perchlorate. The salting-in effect of 2-propanol increases with reduction in temperature. 

For the three reactions represented in Fig. 12 the maximum rate of hydrolysis in acid represents only a mpdest acceleration, compared with the rate in initially neutral solution. Bunton and Hadwick89,90 explained the maximum for methyl and phenyl trifluoroacetate in terms of negative salt effects on both acid-catalyzed and neutral reactions. Consistent with this interpretation, it was demonstrated directly that the rate of neutral hydrolysis is decreased by added salts. The effect of added salt should be to decrease the activity of water, and perhaps also to salt in the ester. 

Finally, the development of pulse radiolysis enabled a direct observation of e aq, and a direct distinction between e aq and H could easily be made. Matheson (37) (with spectroscopic data obtained by Keene) suggested that e ag has optical absorption in the visible. Hart and Boag (26) used spectrographic plates and studied this absorption. The effect of solutes, which were known as electron scavengers led to the conclusion that the absorption was due to e aq. It was confirmed later, that the absorption belonged to unit negatively charged species by means of a salt effect (20), as well as by conductivity measurements (49). Many more papers on the absorption spectrum and rate constants of the hydrated electron have since appeared (16). 

Intramolecular electron transfer in a stepwise manner from the amine substrate to die silver(III) center in a 1 2 complex, [Ag(OH)4] -iV,/V-dimcthylanilinc, has been observed.44 The kinetics of oxidation of some aliphatic, heterocyclic, and aromatic aldehydes towards bis(dihydrogentellurato)cuprate(III) and argentate(III) in alkaline medium have been studied.45 A negative salt effect was observed in the oxidation of aminoacetic acid by diperiodatocuprate(III) complex in alkaline medium.46 The oxidation of glutamic acid by thallium(III) perchlorate is catalysed by Ru(M), Os(III), and Nd(III) in a free radical mechanism and the rate is inversely dependent on [H+] concentration.47... 

A new Y solvolysis scale has been developed for benzylic species with extensive charge delocalization, based upon the solvolyses of some benzhydryl bromides and /-butyl(2-naphthyl)methyl bromides.39 Chlorides have negative salt effects on the ionization of benzhydryl bromide in 7-butyrolactone.40 The X-ray structure of the dimerization product of l,8-bis(dhnethylammonio)-4-naphthyl(phenyl)methyl carbocation has been determined it appears to be formed via a 4n + 2n-cycloaddition mechanism 41... 

A normal, a special, and a negative special salt effect have been detected in the SnI reaction between benzhydryl chloride and LiC104 in y-butyrolactone.71... 

Stage 1. The MeOH/H20/NaCl data are subjected to the correlation procedure described previously which gives values of the Wilson energy constants (Zi and Z2) and a new set of data for temperature and vapor composition that are internally consistent (see Table I). The small values of the standard deviation and the bias indicate good quality data in the salt effect field. For the analysis of serial correlation among the residuals we use the Durbin-Watson test (9). A run of positive or negative signs in the series of residuals is some indication that the model... 

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