Solvolysis chloride

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

The suitability of the model reaction chosen by Brown has been criticised. There are many side-chain reactions in which, during reaction, electron deficiencies arise at the site of reaction. The values of the substituent constants obtainable from these reactions would not agree with the values chosen for cr+. At worst, if the solvolysis of substituted benzyl chlorides in 50% aq. acetone had been chosen as the model reaction, crJ-Me would have been —0-82 instead of the adopted value of —0-28. It is difficult to see how the choice of reaction was defended, save by pointing out that the variation in the values of the substituent constants, derivable from different reactions, were not systematically related to the values of the reaction constants such a relationship would have been expected if the importance of the stabilization of the transition-state by direct resonance increased with increasing values of the reaction constant. 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

A more quantitative approach to the influence of the thiazole ring on the reactivity of a lateral functional chain was made in a recent study by Noyce and Fike (383), already discussed in Section 10.4. The first-order rates of solvolysis for three isomeric 1-thiazolylethyl chlorides were determined in 80% ethanol. The order of relative reactivity observed. 

In the second, which belongs to a systematic study of the transmission of substituent effects in heterocyclic systems, Noyce and Forsyth (384-386) showed that for thiazole, as for other simple heterocyclic systems, the rate of solvolysis of substituted hetero-arylethyl chlorides in 80% ethanol could be correlated with a constants of the substituent X only when there is mutual conjugation between X and the reaction center. In the case of thiazole this situation corresponds to l-(2-X-5-thiazolyl)ethyl chlorides (262) and l-(5-X-2-thiazolyl)ethyl chlorides (263). 

Although Noyce and Fike have recently found for the solvolysis of 2-thiazolyl-ethyl chlorides analogous modality of substituent electronic effect transmission from position 2 toward position 5 and from position 5 toward position 2(60). a more general conclusion indicates that the... 

Solvent Effects on the Rate of Substitution by the S l Mechanism Table 8 6 lists the relative rate of solvolysis of tert butyl chloride m several media m order of increasing dielectric constant (e) Dielectric constant is a measure of the ability of a material m this case the solvent to moderate the force of attraction between oppositely charged par tides compared with that of a standard The standard dielectric is a vacuum which is assigned a value e of exactly 1 The higher the dielectric constant e the better the medium is able to support separated positively and negatively charged species 8olvents... 

Relative Rate of SnI Solvolysis of te/t-Butyl Chloride as a Function of Solvent Polarity ... 

A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations For example the rate of solvolysis of a chlonde that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride... 

Alkanesulfinyl chlorides have been prepared by the action of thionyl chloride on alkanesulfinic acids and by the solvolysis of alkylsulfur trichlorides with water, alcohols, and organic acids. The present procedure, which appears to be general for the preparation of sulfinyl chlorides in either the aliphatic or the aromatic series, is based on an improvement in the solvolysis method whereby the use of inert solvent is eliminated and the reaction is carried out in a one-phase system. ... 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Studies of the solvolysis of 1-phenylethyl chloride and its p-substituted derivatives in aqueous trifluorethanol containing azide anion as a potential nucleophile provide details relative to the mechanism of nucleophilic substitution in this system. 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

We have already encountered the ir, a, and p quantities. The 8h term is inserted to account for the cavity effect. Equation (8-80) is a 12-parameter equation for which considerable generality is claimed, in that it is said to be applicable to chemical rates and equilibria, spectra, solubilities, partition coefficients, and even biological responses. Usually, of course, by judicious selection of solvents, it is possible to reduce the number of parameters by ensuring that some terms are negligible.An example requiring most of the parameters in Eq. (8-80) is the solvolysis/dehydrohalogenation of r-butyl chloride in 21 HBD and non-HBD solvents, for which this correlation was found ... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

Psetjdo-Fibst-Obdeb Rates fob the Solvolysis of Cyantjbic Chloride in Various Alcohols"... 

In hydroxyUc solvents, the reaction with aniline follows a bi-molecular course but is complicated by competing solvolysis. This is a striking result when compared with the behavior of picryl chloride, which is much more selective with regard to the same reagents (aniline and alcohol), and has been interpreted to mean that bond-breaking has made appreciable progress in the rate-determining step of the reaction of phosphonitrilic chloride. Furthermore, the same indication is obtained from the fact that in the reactions of the halides, the fluorine chlorine ratios are less than one. ... 

Reduction of the imine with sodium borohydride leads to an intermediate amino-ester that cyclizes spontaneously to the <5-lactam function. Solvolysis of the acetyl group with methoxide followed by acylation of the hydroxyl group thus liberated with trimethoxybenzoyl chloride leads to 38. Bischler-Napieralski cyclodehydration (phosphorus oxychloride) effects closure of the remaining ring. Reduction of the imine thus formed with sodium borohydride gives 39. This, it should be noted, leads to the... 

C-6 with sulfuryl chloride. 181 Solvolysis of sulfonate esters,... 

---