Acid halides, solvolysis

Oxetanes are much less susceptible to cleavage by nucleophiles than oxiranes, except in the presence of acids. Several types of acid-catalyzed nucleophilic reactions are described in the previous section, such as reaction of hydrogen halides to give 3-halogeno-1-propanols and various acid-catalyzed solvolysis reactions. Another example of this type is the reaction of thiourea with 2-alkyloxetanes in the presence of hydrochloric or perchloric acid to give excellent yields of 3-alkyl-3-hydroxybutylisothiouronium salts (equation 39) <67CR(C)(264)1309>. 

Reactions corresponding to acidic solvolysis of acid halides have also been studied in the gas phase (McMahon, 1978). The results which bear close resemblance to solution processes are illustrated in (82) and (83). 

KINETIC METHODS USED IN STUDIES OF THE SOLVOLYSIS OF ACID HALIDES... 

The S j km is responsible for the deep blue color of ihese solutions (Amax = 610 nm). This ion is also responsible for the color of sulfur dissolved in chloride melts (see below) and in the aluminosilicate known as ultramarine (see Chapter 16). Many nonmetal halides behave as acid halides in solvolysis reactions ... 

In order to compare S l substitution rates in a range of alkyl halides experimental con ditions are chosen m which competing substitution by the 8 2 route is very slow One such set of conditions is solvolysis m aqueous formic acid (HCO2H)... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

Common-ion rate depressions and the involvement of vinyl cation ion pairs similar to those observed by Miller and Kaufman (131) were observed by Rappoport and Gal (136) in the solvolysis of 150 in acetic acid. Further interesting evidence for ion-pair involvement in the solvolytic generation of some vinyl cations comes from the investigation of cis- and trans-1,2-dianisyl-2-phenylvinyl halides, 151 (1937). In 80% ethanol, solvolysis... 

Phosphonoformic acid (85) decarboxylated in acid solution, and it was proposed that the uncatalysed reaction involved a simple decarboxylation of the zwitterion. The acid-catalysed reaction showed some kinetic similarity to that of mesitoic acid and an elimination of carbon dioxide as trihydroxymethylcarbonium ion was preferred. Participation of the trans vicinal phosphonyl group in the solvolysis of the halides (86) and (87) has been deduced from rate measurements. In the norbornene derivatives, the relative rates of loss of chloride from (87a) and (87b) were 5 x 10 1. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

The addition of carboxylic acids to bicyclo[2.2.1]hepta-2,5-diene has been described by several authors the method described here is a modification of these procedures. Nortricyclanol has been prepared by the hydration of bicyclo[2.2.1]hepta-2,5-diene and the solvolysis of nortricyclyl and bicyclo[2.2.1]hept-2-en-5-yl halides, as well as by the saponification and... 

Crunden and Hudson80, by comparison of solvolysis rates in formic acid and aqueous acetone (although the same authors have stated that this may be coincidental), obtained a value of 40% SN1 participation in aqueous acetone (65%) they also estimated the contribution of the SN1 route to the solvolysis of substituted benzoyl halides (Table 6). It must, however, be emphasized that the nature of the intermediate trapped in the work of Gold et al81 could either be the acylium ion RCO+ or the hydrated acylium ion [RC0H20]+. 

Volume 10 is devoted to formation and solvolysis of esters and related reactions, with discussion of the effect of neighbouring groups and biological implications, e.g. enzyme action, where appropriate. The first chapter deals mainly with esters of the inorganic acids of phosphorus and sulphur, Chapter 2 with the formation and solvolysis of esters of organic acids and the final chapter with the solvolysis of related derivatives of carboxylic acids, e.g. halides, amides, anhydrides, cyanides, carbamic acid derivatives. 

When, however, solvolysis is carried out in the absence of an acid acceptor (and, therefore, in acid medium), orthoester formation stops, and simple replacement occurs, with net retention of configuration01 as in (11)— (lla). Under the same conditions, 1,2-dfi halides undergo solvolysis with inversion of configuration01 as in (12)— (12a) therefore, irrespective of whether a 1,2-trana or a 1,2-da halide is the starting material, the result... 

Although most laser flash photolysis studies have been performed in acetonitrile, transient vinyl cations have also been observed in acetonitrile/methanol mixtures and even in pure methanol309. In acetic acid, the solvent of choice for most thermal solvolysis studies of vinylic halides, the presence of polar additives such as sodium acetate is necessary for transient vinyl cations to be observed335. 

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