Pyrolysis alkyl halides

The negative reaction constant p for electron-donating substituents seemed to support Maccoll s view1 on the heterolytic nature of the transition state for alkyl halide pyrolysis in the gas phase. 

The 5-substituted 1,3-dioxolan-4-one 23 is readily deprotonated at the 5 position and can be alkylated with a variety of alkyl halides. The resulting products 24 decompose upon flash vacuum pyrolysis (FVP) at 600°C with loss of acetone... 

Unsaturated alkyl halides react first by ir-complexation (141), followed by C-X oxidative addition, probably on matrix warm-up [but see the preceding point 3, and see ref. (81), which suggests that pyrolysis and radical production can occur on the crucible insulating material to cause reaction]. 

Disulfides can be prepared by treatment of alkyl halides with disulfide ions and also indirectly by the reaction of Bunte salts (see 10-41) with acid solutions of iodide, thiocyanate ion, or thiourea, or by pyrolysis or treatment with hydrogen peroxide. Alkyl halides also give disulfides when refluxed with sulfur and NaOH, and with piperidinium tetrathiotungstate or piperidinium tetrathiomolybdate. ... 

B. By Hydrolysis Reactions.—Details have appeared of the synthesis of dibenzophosphorin oxides (15) from 5-alkyldibenzophospholes, by reaction with methyl propiolate in the presence of water, and of confirmatory syntheses from phosphinic acid chlorides, as shown below. Evidence for the suggested mechanism of the ring-expansion reaction is presented. The hydrolysis of enamine phosphine oxides is an efficient, although somewhat indirect, method for the preparation of j8-ketoalkylphosphine oxides (16) [see Section 3(iii), for the preparation of enamine oxides]. Reasonable yields (48—66%) of trialkylphosphine oxides (17) have been obtained by the alkaline hydrolysis of the products from the pyrolysis at 220 °C of red phosphorus with alkyl halides, in the presence of iodine. 

The other cause for the xmusually high values of the slopes may be the absence of a solvent as all the data on catalytic eliminations have been obtained in gas-phase experiments. With highly polar transition states, the solvent compensates for the influence of the separation of charges. It should be noted that the correlation of the data for the pyrolysis of alkyl halides similarly gave very high negative values of the slopes (65). 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

Oxathiolane 3,3-dioxide (332) metallates in its 2-position to yield an anion which reacts with various electrophiles (alkyl halides and carbonyl compounds) to give substituted oxathiolanes (333) in good to excellent yield (79TL3375). Pyrolysis of these alkylated products affords the corresponding aldehydes or 2-hydroxyaldehydes in addition to sulfur dioxide and isobutylene (Scheme 71). The oxathiolane (332) thus becomes another member of the already burgeoning class of carbonyl anion equivalents. 

The chemical reactivities of organohalide compounds vary over a wide range. The alkyl halides are generally low in reactivity, but may undergo pyrolysis in flames to liberate noxious products, such as HC1 gas. Alkenyl halides may be oxidized, which in some cases produces highly toxic phosgene, as shown by the following example ... 

Pyrolysis of the quaternary salts of pyrazoles causes loss of alkyl halide and reformation of N-substituted pyrazoles. If the substituents in the 3- and 5-positions are similar in character, as well as those on the two nitrogen atoms, then the two possible products will be formed in comparable quantities.81 83,152, 412 711... 

Prior to 1953, few kinetic works on the homogeneous, unimolecular gas-phase pyrolysis or elimination of simple alkyl halides were reported. According to these experimental data the commonly accepted mechanism consisted of a concerted four-membered cyclic transition state yielding the corresponding olefin and hydrogen halide as shown in equation 1. For molecular dehydrohalogenation, the presence of a /i-hydrogen adjacent to the C—X bond is necessary. 

The lactonization of y-bromoesters, which involves neighboring group participation and an ionic intermediate in solution, was not observed in the gas phase. Kwart and Waroblak13 reported that only HBr gas was produced from ethyl y-bromobutyrate pyrolysis at 450 °C. Consequently, they doubted the heterolytic nature via an ion-pair intermediate of alkyl halides pyrolyses. 

According to the data reported in the literature up to 1972, the gas-phase pyrolysis of alkyl halides was to be described in terms of a discrete polar transition state which yields products directly without the formation of an ion-pair intermediate. The only apparent exception to this consideration is the Wagner-Meerwein rearrangement of neopentyl chloride8,9. 

This study on the kinetic chlorine isotope effect in ethyl chloride50 was extended to secondary and tertiary alkyl halides pyrolyses51. The isotope effects on isopropyl chloride and terf-butyl chloride pyrolysis were found to be primary and exhibited a definite dependence on temperature. They increased with increasing methyl substitution on the central carbon atom. The pyrolysis results and model calculations implied that all alkyl chlorides involve the same type of activated complex. The C—Cl bond is not completely broken in the activated complex, yet the chlorine participation involves a combination of bending and stretching modes. 

Compounds of type R3ASX2 are generally low-melting, crystalline solids, soluble in alcohol but only slightly soluble or insoluble in nonpolar solvents. These can readily be reduced to tertiary arsanes. They are hydrolyzed by water or alkali. Compounds containing at least one alkyl group yield on pyrolysis an alkyl halide and a haloarsane (49) ... 

Sodium thiosulfate reacts with alkyl halides to form salts of the type RSSOjNa (Bunte salts). Alkyl disulfides may be obtained from these salts by pyrolysis or reaction with iodine or hydrogen peroxide. The yields range from 47% to 6S>%. Cyano and carboxyl groups do not interfere. Benzoylation of sodium thiosulfate produces benzoyl disulfide in 58% yield. ... 

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