Haloalkanes conversion

The simplest C-C bond formation reaction is the nucleophilic displacement of a halide ion from a haloalkane by the cyanide ion. This was one of the first reactions for which the kinetics under phase-transfer catalysed conditions was investigated and patented [l-3] and is widely used [e.g. 4-12], The reaction has been the subject of a large number of patents and it is frequently used as a standard reaction for the assessment of the effectiveness of the catalyst. Although the majority of reactions are conducted under liquiddiquid two-phase conditions, it has also been conducted under solidrliquid two-phase conditions [13] but, as with many other reactions carried out under such conditions, a trace of water is necessary for optimum success. Triphase catalysis [14] and use of the preformed quaternary ammonium cyanide [e.g. 15] have also been applied to the conversion of haloalkanes into the corresponding nitriles. Polymer-bound chloroalkanes react with sodium cyanide and cyanoalkanes under phase-transfer catalytic conditions [16],... 

Selected examples of the conversion of haloalkanes into alkyl cyanides... 

The dehydrohalogenation of 1- or 2-haloalkanes, in particular of l-bromo-2-phenylethane, has been studied in considerable detail [1-9]. Less active haloalkanes react only in the presence of specific quaternary ammonium salts and frequently require stoichiometric amounts of the catalyst, particularly when Triton B is used [ 1, 2]. Elimination follows zero order kinetics [7] and can take place in the absence of base, for example, styrene, equivalent in concentration to that of the added catalyst, is obtained when 1-bromo-2-phenylethane is heated at 100°C with tetra-n-butyl-ammonium bromide [8], The reaction is reversible and 1-bromo-l-phenylethane is detected at 145°C [8]. From this evidence it is postulated that the elimination follows a reverse transfer mechanism (see Chapter 1) [5]. The liquidrliquid two-phase p-elimination from 1-bromo-2-phenylethanes is low yielding and extremely slow, compared with the PEG-catalysed reaction [4]. In contrast, solid potassium hydroxide and tetra-n-butylammonium bromide in f-butanol effects a 73% conversion in 24 hours or, in the absence of a solvent, over 4 hours [3] extended reaction times lead to polymerization of the resulting styrene. 

Catalysed oxidation of non-activated haloalkanes by hypochlorite provides an attractive low-cost and convenient procedure for their conversion into carbonyl compounds [6] primary haloalkanes produce carboxylic acids and secondary haloalkanes are converted into ketones (Table 10.12). Secondary amines are oxidized to ketones under analogous conditions, whereas primary amines yield nitriles (Table 10.13) [1,2], o-Nitroanilines are oxidized to benzofurazan-1-oxides [15]. 

When a hydrocarbon is substituted with other than alkyl groups a new problem arises, which can be illustrated by CH3CH2C1. This substance can be called either chloroethane or ethyl chloride, and both names are used in conversation and in print almost interchangeably. In the IUPAC system, halogens, nitro groups, and a few other monovalent groups are considered to be substituent groups on hydrocarbons and are named as haloalkanes, nitro-alkanes, and so on. 

RCO , an indifferent nucleophile in prohc solvents, enjoys a large rate enhancement, permitting rapid alkylation with haloalkanes in hexamethylphosphoric triamide [301, 302], When the Williamson ether synthesis is carried out in dimethyl sulfoxide [303], the yields are raised and the reaction time shortened. Displacements on unreactive haloarenes become possible [304] (conversion of bromobenzene to tert-butoxybenzene with tert-C UgO in dimethyl sulfoxide in 86% yield at room temperature). The fluoride ion, a notoriously poor nucleophile or base in protic solvents, reveals its hidden capabilities in dipolar non-HBD solvents and is a powerful nucleophile in substitution reactions on carbon [305],... 

Tertiary phosphine oxides are also produced as significant by-products in several of the reactions of phosphines that have been noted previously, including the Wittig olefination and the conversions of alcohols to haloalkanes with triphenylphosphine as an adjunct reagent. The tertiary phosphine oxides produced in such reactions present a problem in chemical economics, as they themselves possess little chemical utility. The phosphine may be regenerated, but several steps are required, as previously noted with preparations of phosphines by reduction (see Section 3.2). 

Unsymmetrically substituted compounds have been obtained by stepwise preparation of a monoalkylcyclopropane from cyclopropene, followed by reaction with sodium amide and a second haloalkane. Thus, conversion of cyclopropene to l-butyl-2-ethylcyclopropcne (5a) and l-ethyl-2-isopropylcyclopropene(5b) via 1-ethylcyclopropcnc occurred in yields of 75 and 18%,... 

In addition to the conversion of unactivated alkanes to alcohols, cytochrome P450 hemes transform alkenes to epoxides, arenes to phenols, and sulfides to sulfoxides to sulfones. Furthermore, they are involved in the biosynthesis and biodegradation of endogenous compounds such as steroids, fatty acids, prostaglandins and leukotrienes. Under anaerobic conditions, P450 will reductively dehalogenate haloalkanes to the corresponding alkanes. 

Eecently a study was made of the intracrystalline catalytic dehydro-halogenation patterns of two-carbon haloalkanes over zeolite catalysts in continuous flow systems (108). At 260°, REX, NaX, NiX, AgX, HY, metal-hydrogen Y, and Linde 5A zeolites all converted substantial amounts of ethyl chloride to ethylene in gas phase reactions, while amorphous silica-alumina was considerably less reactive. Catalysts such as y—AI2O3 or CaCl2 require temperatures of 360-420° for comparable conversions (109). With REX, conversion increased rapidly from about 5% at 150° to nearly 100% at 316° and higher. 

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