Benzhydryl chloride, ionization

The source of some of the difficulties encountered in trying to explain the effects of structural changes on ionization rates may be due to the different parts played by the solvent, as for example, the sulfur dioxide of the trityl chloride equilibrium experiments and the aqueous acetone of the benzhydryl chloride rate data. The solvent is bound to modify the effect of a substituent, and although the solvent is usually ignored in discussing substituent effects this is because of a scarcity of usable data and not because the importance of the solvent is not realized "... solvation energy and entropy are the most characteristic determinants of reactions in solution, and... for this class of reactions no norm exists which does not take primary account of solvation. 220 Precisely how best to take account of solvation is an unanswered problem that is the subject of much current research. 

Rate constants and products have been reported for solvolysis of benzhydryl chloride and /7-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)-water and-ethanol, along with additional kinetic data for solvolysis of r-butyl and other alkyl halides in 97% TFE and 97% hexafluoropropan-2-ol. The results are discussed in terms of solvent ionizing power Y and nucleophilicity N, and contributions from other solvation effects are considered. Comparisons with other 3 nI reactions show that the solvolyses of benzhydryl chloride in TFE mixtures are unexpectedly fast an additional solvation effect influences solvolysis leading to delocalized cations. 

The benzhydryl chlorides and BC13 react with formation of ion pairs (ionization constant, Ki) which dissociate to give the free ions (dissociation constant, KD). Because paired and free diarylcarbenium ions show only slightly different UV-visible spectra, [41], spectrophotometric measurements allow the determination of the total carbocation concentration. On the other hand, only free ions are detected by conductometric analysis, and a combination of both methods allows the determination of Ki and Kd using the theory of binary ionogenic equilibria [42,43]. 

The dimethoxy-substituted benzhydryl chloride is also fully ionized in a SnCl4/EtOAc/CH2Cl2 solution [60], and the reactivity toward 2-methyl-l-pentene is identical with that in the BCI3/CH2Q2 solution (Fig. 19). AnPhCHCl is not fully ionized under these conditions, however, and... 

Ionization is exothermic and favored at lower temperatures in systems such as trityl, alkoxycarbenium, and benzhydryl derivatives which generate stabilized carbenium ions due to their electron-donating substituents. The exothermicity of ionization of benzhydryl species with BCl3 in CH2CI2 decreases from AH = -62 kJ/mol for di(p-anisyl) to -22 kJ/ mol for less stabilized p-tolyl and phenyl derivatives, and to approximately -8 kJ/mol for unsubstituted benzhydryl chloride [193]. These values, based on the solvolysis rates, extrapolate to AH = -12 kJ/mol for cumyl chloride (HCl adduct of a-methylstyrene) and AH = 0 kJ/mol for 1-phenyl-ethyl chloride (styrene adduct) and ( -butyl chloride (isobutene adduct) [240]. The reported entropies of ionization do not vary significantly and... 

The weak Lewis acid SbCl3 does not even fully ionize the p,p -dimethoxy substituted benzhydryl chloride, and only one branch of the SbCl3/CH2Cl2 graph can be seen in Figure 6. 

Neutral salts facilitate RX ionization affecting it through the ionic strength of the solution. For example, 0.1 mol/1 LiCl increases the hydrolysis rate of/er/-butyl bromide in 90% acetone (10% water) by 40% (323 K), 0.1 mol/1 LiBr accelerates the hydrolysis of benzhydryl chloride in 80% acetone (20% water) by 17% (298 K), and 0.1 mol/1 LiCl accelerates the hydrolysis of benzhydryl bromide, under the same conditions, by 27%. 

That the situation may be even more complex than it seems is suggested by some recent work of Pocker (98a) on the solvolysis of variously deuterated benzhydryl chlorides, also in 80% aqueous acetone.. Comparing the rates of solvolysis, racemization, and radio-chloride exchange, he concludes that solvolysis follows the prior formation of a rather loose ion-pair. Moreover, the isotope effect on solvolysis due to a-deuteration (k /ko = 1.15) is the same as that on the more rapid racemization, suggesting that the isotope effect in solvolysis can nearly be identified with that on an ionization pre-equilibrium. In addition, nuclear deuteration, according both to Pocker and to Klein... 

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 very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

The c coefficients measure the substrate response to nucleophilicity, dY, and electrophilicity, d2. Again, 80% aqueous ethanol was chosen as the standard solvent, and a set of c values was chosen in accord with supposed mechanistic behavior of model substrates cY = 3c2 for methyl bromide, = c2 for tert-butyl chloride, and 3cx = c2 for (C6H5)3CF. This approach was largely unsuccessful, probably in large part because tert-butyl chloride is assumed to be equally sensitive to nucleophilicity and ionizing power (29). The equation predicts, for example, that tertiary, benzhydryl, and bridgehead substrates are more sensitive to solvent nucleophilicity than typical primary and secondary SN2 substrates (31). 

In the case of triphenylsilane and trityl chloride, this reaction is first order in both reactants in benzene (233). The reaction rate for modified substrates indicates that with respect to R3CX, rate is a function of proclivity to ionize, trityl halides being more rapid than benzhydryl or allyl or tcr/-butyl halides. Indeed the latter required the presence of BBr3 as a catalyst. Within a given series. Cl > Br > I, phenylsilanes react more rapidly than alkyl silanes. A mechanism involving simultaneous nucleophilic attack of halide on silicon, and electrophilic attack on hydrogen by the carbonium ion is reasonable,... 

MeO , OH , or EtO and methyl fluoride, anisole, and 4-fluoroanisole on the gas-phase 5 2 reactions between dimethylmethylphosphonate and methylformate and HOO versus HO , MeO , or EtO in an attempt to discover the origin of the O -effect on the Sf 2 reaction of carbanions with 4-substituted benzyl chlorides in liquid ammonia " on the solvolysis reaction and the 2 reaction between phenoxide ions, and both neutral and negatively charged amines and 4-substituted benzyl chlorides in liquid ammonia on the ionization rates (the step) of the 5" 1 reactions of many substituted trityl halides and carboxylates in aqueous acetone and in aqueous and pure acetonitrile in the presence of piperidine on the ionization rates ( i) of the 5 reactions of various diarylmethyl chlorides in the presence of piperidine, pyridine, or PPh3 in several dipolar aprotic solvents on the solvolyses of X,Y-substituted benzhydryl acetates in various aqueous MeOH and EtOH solutions and on the dispersions observed in Grunwald-Winstein correlations of 5 solvolyses of substrates with substituents containing adjacent tt-electrons. ... 

The ionization (fcj) and the reverse (A i) rate constants for the 5 1 solvolyses of several 4,4 -disubstituted benzhydryl flourides were determined in several different aqueous and alcoholic solvents. The Nf and if values for fluoride ion in the log ki = s Nf + Ef) equation were found in these solvent mixtures. The Nf decreased significantly and the if values decreased slightly but remained close to 1.0 as the percentage water increased in each solvent. The results suggest that the fluoride reactions have earlier transition states than either the corresponding bromides or chlorides. The data was used to construct quantitative energy profiles for the reactions of the different fluorides in 80% MeCN/20% water. 

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