Solvolytic solvents

The present results are well understood by the above mechanism. Itmann which is fusible at relatively high temperature was not liquefied below 420°C with a non-solvolytic solvent such as pyrene, however it was significantly liquefied at 480°C of its maximum fluidity temperature in decacyclene of a stable aromatic compound. 

No. Solvolytic Solvent Acid tion of Time, Extract... 

This type of reaction is accelerated by small amounts of peroxide and may occur both in inert and solvolytic solvents, such as methanol and 2-propanol. 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

Trifluoromethyl-l-phenylethyl tosylate has been used to differentiate as shown in Table 1, the solvolytic power of three fluorinated solvents and to compare these with formic and acetic acids The three fluorinated solvents are trifluoroacetic acid, trifluoroethanol, and 1,1,1,3,3,3-hexafluoroisopropyl alcohol [55]... 

Most of the kinetic measures of solvent effects have been developed for the study of nucleophilic substitution (Sn) at saturated carbon, solvolytic reactions in particular. It may, therefore, be helpful to give a brief review of aliphatic nucleophilic substitution. Two mechanistic routes have been clearly identified. One of these is shown by... 

In 1955 Swain, et al. proposed a four-parameter equation, Eq. (8-74), to describe the solvent dependence of solvolytic reactions. 

Liquid solvents are used to extract either desirable or undesirable compounds from a liquid mixture. Solvent extraction processes use a liquid solvent that has a high solvolytic power for certain compounds in the feed mixture. For example, ethylene glycol has a greater affinity for aromatic hydrocarbons and extracts them preferentially from a reformate mixture (a liquid paraffinic and aromatic product from catalytic reforming). The raffinate, which is mainly paraffins, is freed from traces of ethylene glycol by distillation. Other solvents that could be used for this purpose are liquid sulfur dioxide and sulfolane (tetramethylene sulfone). 

The limitations of the reaction have not been systematically investigated, but the inherent lability of the aziridines can be expected to become troublesome in the case of epoxyketones which are slow to form hydrazones. The use of acid catalysis is curtailed by the instability of the aziridines, particularly the diphcnylaziridine, in acidic media. Because of their solvolytic lability, the hydrazones are best formed in inert solvents. A procedure proven helpful in some cases is to mix the aziridine and the epoxyketone in anhydrous benzene, and then to remove the benzene on a rotary evaporator at room temperature. Water formed in the reaction is thus removed as the azeotrope. This process is repeated, if necessary, until no carbonyl band remains in the infrared spectrum of the residue. 

In solvolytic reactions like those we have just been considering, where the solvent itself is the nucleophile, such mixed kinetics may not be detectable, irrespective of what is actually happening, as both SN1 and Sn2 pathways are likely to follow a rate equation of the form ... 

The present authors studied the solvolytic liquefaction process ( ,7) from chemical viewpoints on the solvents and the coals in previous paper ( 5). The basic idea of this process is that coals can be liquefied under atmospheric pressure when a suitable solvent of high boiling point assures the ability of coal extraction or solvolytic reactivity. The solvent may be hopefully derived from the petroleum asphaltene because of its effective utilization. Fig. 1 of a previous paper (8) may indicate an essential nature of this process. The liquefaction activity of a solvent was revealed to depend not only on its dissolving ability but also on its reactivity for the liquefying reaction according to the nature of the coal. Fusible coals were liquefied at high yield by the aid of aromatic solvents. However, coals which are non-fusible at liquefaction temperature are scarcely... 

In contrast, the non-fusible coal requires the solvation (extraction) or solvolytic reaction to be liquefied. The solvation of non-polar organic compounds including the pitch may be rather limitted, so that the solvolytic reaction is necessary for the high liquefaction yield between the coal and the solvent. 

The second series of data on protic solvent effects in bromination that are related to transition states comprises the m-values of solvent-reactivity correlations. First, it is important to underline that 7-parameters, the solvent ionizing powers, established from solvolytic displacements, work fairly well in this electrophilic addition. This is expected since bromination, like SN1 reactions, leads to a cation-anion pair by heterolytic dissociation of the bromine-olefin CTC, a process similar to the ionization of halogenated or ether derivatives (Scheme 14). 

The rigorous analogy between electrophilic halogenation and solvolytic displacements was recognized only recently (Ruasse et al., 1991). Consequently, for most of the previously published solvent effects, the above procedure has not been applied systematically. Winstein Grunwald, YW G, parameters have... 

Acid-induced wagner-meerwein rearrangements in chiral alcohols. In view of the considerable interest on ion-molecule complexes involved in gas-phase analogues of solvolytic reactions," ° " a sustained research effort has been directed to the study of Wagner-Meerwein rearrangements in cationized )8-arylaIkyl systems, under conditions excluding nucleophilic assistance by the solvent which in these systems normally interferes with anchimeric assistance of groups adjacent to the... 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

Before we turn to other investigations of the stabilized ion it is interesting briefly to compare the rates of hydride shifts found in the strong acid systems with rates obtained under solvolytic conditions. Collins and Lietzke (1967) and Berson et al. (1967) have deduced the rates of 3,2- and 6,2-hydrogen migration relative to the rate of solvent capture for norbornyl ions in acetic acid, formic acid and other common solvents from elaborate C scrambling studies due to Roberts and Lee (1951) and Roberts et al., (1954) and tritium labelling studies of Lee and Lam (1966). From the ratio of these... 

Jencks and Richard, and others, had pioneered the use of the azide elock to quantitatively assess the lifetime of carbenium ions generated under solvolytic conditions. The method relies on the use of product yield data collected at varying [N3 ] to determine the N /solvent selectivity, expressed as the ratio of the second-order rate constant for trapping of the ion by N3 and the pseudo-lirst-order rate constant for trapping of the ion by solvent k Jk. The assumption is made that k z is diffusion limited at ca. 5 x 10 M"" s This assumption allows k to be estimated, and l/kg provides the lifetime of the ion in the solvent in the absence of added nucleophiles. McClelland and Steenken showed by direct measurement of k. , for a series of diarylmethyl and triarylmethyl carbocations that k is approximately constant at... 

Shortly after Anderson and Falvey reported the first observation of a shortlived nitrenium ion in CH3CN by UV spectroscopy, Novak and McClelland and co-workers demonstrated that the nitrenium ions 75h and 75o could be observed in aqueous solution after LFP of the pivalic acid ester 76h, the sulfuric acid ester 76o, and its N-chloro analogue N-chloro-4-phenylacetanilide. The transients with A ax of ca. 450 nm were identified as singlet nitrenium ions, based on the kinetics of their decomposition in the presence of NJ, the equivalence of kaz/ks determined by the azide clock method and by direct observation, the lack of sensitivity of the transients to O2, product studies that showed similar products from solvolytic and photolytic decomposition of N-chloro-4-phenylacetanilide, and identical transient UV spectra for 75o derived either from 76o or its N-chloro analogue. A comparison of azide/solvent selectivity data obtained by azide clock and direct observation of 7Sh and 75o is presented in Table 1. 

In this paper we will discuss the substitution reactions of complexes of the type, [Co en2 A X]+n in nonaqueous solvents and will show how the general conditions that apply here can be extended to aquation and other solvolytic reactions, and to the base hydrolysis reaction. 

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