Hydrocarbons formation from alkyl halides

Organometallic compounds or carbanions undergo a number of reactions in which the carbanion or carbanion-like moiety of the organometallic compound acts as a nucleophilic displacing agent. Examples are the formation of hydrocarbons from alkyl halides, alkyl halides from halogens, and ketones from acid chlorides or esters. The latter two reactions are closely related to the base-catalyzed condensations and are perhaps additions as well as displacement reactions. Related addition reactions are the carbonation of organometallic compounds and the addition to ketones or aldehydes. 

The condensation of saturated hydrocarbons with haloolefins in the presence of anhydrous aluminum chloride also results in the formation of alkyl halides, as in the preparation of l-chloro-3,4-dimethylpentane from isobutane and allyl chloride (40%). Under the same conditions, alkyl halides react with olefinic halides to give dihaloalkanes. unsym-Heptachloropropane is synthesized from tetrachloroethylene and chloroform (93%). ... 

This reaction can be used for the synthesis of hydrocarbons but it may also take place as a side-reaction during generation of a Grignard reagent from an alkyl halide and magnesium, then leading to formation of undesired side-products. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

Alkylation is a very broad reaction type and it can, depending on the nature of the alkylating agent, proceed either as a substitution or as an addition reaction. The alkylation by substitution of, for example, aromatic hydrocarbons, phenols or amines is based on the reaction with alkyl halides or alcohols. Some evidence indicates that, at least partly, the alkylation proceeds through the intermediate formation of alkenes from the alkylating agent when the reaction is conducted at atmospheric pressure and at high temperature. 

The results obtained in the gas-phase isopropylation of various aromatic hydrocarbons with isopropyl chloride over Nafion-H catalyst showed only a relatively small variation of reactivity in going from fluorobenzene to xylenes.235 Therefore, it has been assumed that the reaction rate is controlled by the formation of a reactive electrophilic intermediate (possibly, protonated alkyl halide 61, or some form of incipient alkyl cation) rather than by cr-complex formation between the electrophile and the aromatic nucleus [Eq. (5.89)]. 

Thus Eq. (1) represents the dissociation of alkyl (aryl) halide by Me3Al and the formation of alkyl (aryl) cation and counter anioa Equation (2) shows the alkylation of the cation methide anion from the counter anion and the formation of a hydrocarbon. 

The ethereal solutions of these iodides do not fume in air, and removal of the solvent gives a liquid, which on further heating evolves dense white fumes, probably of beryllium oxide. Heating changes the alkyl beryllium halides to beryllium dialkyls. All the alkyl halide compounds are decomposed by water, with formation of the corresponding hydrocarbon. When carbon dioxide is passed through ethereal beryllium methyl iodide for three hours, the solution still gives a positive test and no acetic acid is found after hydrolysis. Acetanilide is formed from beryllium methyl iodide and phenyl isocyanate. 

Alcohols react with HX to form alkyl halides, but the reaction works well only for tertiary alcohols, R,COH. Primaiy and secondary alkyl halides are normally prepared from alcohols using either SOClj or PBr ). Alkyl halides react with magnesium in ether solution to form organomagnesium halides, or Grignard ret ents (RM O- Since Grignard reagents are both nucleophilic and basic, they react with acids to yield hydrocarbons. The overall result of Grignard formation and protonation is the conversion of an alkyl halide into an alkane (RX— RM RH). 

The formation of organoberyllium halides is favored by solvents, especially aromatic hydrocarbons and ethers. The best results (yield ca. 70 %) are obtained in boiling Et20 after activation of Be with Hg or from anhydr BeCl2 in ether. However, the alkylberyl-lium halides obtained are ether complexes from which the ether cannot be removed in vacuum on heating. The rate is slow when alkyl chlorides are used, hence, the bromides and iodides are preferred. 

The hydrocarbon a-cedrene (61) has been converted to its isomer /3-cedrene (62) in 52% yield by a two-step process involving first addition of HCl and then elimination of HCl (equation 10.88). Propose conditions that would optimize the formation of 62 from the intermediate alkyl halide. 

---