Hydrogen Halide Elimination Reactions

Unimolecular HX elimination from haloalkanes can occur via either three- or four-centered processes. The threshold energy for the four-centered process is usually smaller than for the three-centered process and in competitive situations the four-centered pathway is favored unless substituents lower the threshold for the three-centered pathway. A five-centered elimination process was inferred from the HF emission observed in the flash photolysis of 3,3,3-trifluoropropyne however, such processes are not well characterized. Activation of most of the molecules listed in Table 2.17 was accomplished by radical recombination the chloroolefins were activated by photon absorption and excited electronic states may contribute to the elimination of HCl. The energy available is the average excitation energy of the activated molecule less the endoergicity for HX elimination. The fluoroalkane molecules have 50-60 kcal mole more energy 

Reaction Ey- kcal mole HX vibrational distribution /v(HX) /v(olefin) pr kcal mole Refer- ence 

The CF2 energy distribution for elimination of Br2 and CI2 from CF2Br2 and CF2CI2 also has been studied by laser fluorescence. In these cases, halogen elimination is the minor channel relative to Br or Cl rupture. For these halogen eliminations the distributions were Boltzmann with i(CF2) 1.5, y(CF2) 1.8-2.9, and r 1.6 kcal mole These 

Lin measured the HF vibrational distribution following three-centered elimination from CHFNO and which were formed by the 

The translational energy disposal from the four-centered HX elimination has been measured for multiphoton excitation of CH3CCI3 and CH3CF2CI4 The broad distributions were fitted to functions of the form exp (— 3 r/ r with r = 8 and 12 kcal mole . The authors estimate = 64kcalmole for CH3CCI3 which leads to /r 0.13. 

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Boehm, H.-P., Mair, G., Stohr, T., et al. (1984). Carbon as a catalyst in oxidation reactions and hydrogen halide elimination reactions. Fuel, 63, 1061-3. 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

In other systems ( 5.8.4.2.2) hydrogen halide elimination often follows oxidative addition of Ge—H, and such addition may be the initial step in other fast reactions where only the elimination is observed. 

If the decomposition is a unimolecular process then a loose, four-centred transition state has to be assumed. The log 15 value, though slightly high, is not unreasonable considering other four-centred elimination reactions such as, for instance, the hydrogen halide eliminations. However, the activation energy, determined experimentally, is higher than expected for such a reaction. 

By far the most important type of reaction displayed by halophosphines is nucleophilic substitution. This is pivotal to the preparation of many other three-coordinate compounds containing either solely P—C, P—O, P—N bonds, or mixed combinations. These reactions are often exothermic and frequently carried out at low temperatures. For the synthesis of phosphorus(III) compounds containing a P—O or P—N bond it is often necessary to add a base (triethylamine or pyridine are frequently used) to capture the hydrogen halide eliminated from these condensation reactions. In the case of P—C bond formation, a variety of routes are possible using various carbon-derived nucleophiles. 

Many cyclopropyl chlorides and bromides have been converted to alkoxycyclopropanes by treatment with a strong base, in most cases potassium rerf-butoxide, in an appropriate organic solvent (Table 13). Under such conditions, hydrogen halide elimination takes place, yielding strained cyclopropene intermediates, which are trapped by nucleophilic attack of the alkoxide. Overall, a simple substitution occurs when a bond is formed between the alkoxide group and the carbon atom to which the halide was attached. This is the case when l-chloro-5-methyl-exo-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (1) was reacted with potassium /ert-butoxide l-/er/-butoxy-5-methyl-ent/o-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2) was isolated in 94% yield.If a C-O bond is established at the other olefinic carbon atom, a C H bond is concomitantly formed at the carbon atom, to which the halide was attached. The result is a double substitution which is discussed elsewhere (see Section 5.2.1.3 ). When the substrate contains more than one halogen atom, several elimination reactions usually take place. Thus, treatment of 1 -bromo-2-chloro-2-methylcyclopropane (3) with an excess of potassium /er/-butoxide gave l-ter/-butoxy-2-methylenecyclopropane (4) in 30% yield. 

Alkyl halides containing easily abstractable p-hydrogen imdergo elimination reactions 284> ... 

Certain compounds which contain halogen, other than acyl chlorides, react with amines and form condensation-products as the result of the elimination of a hydrogen halide. The reaction of methylamine with chloroform, CHCI3, in the presence of an alkali is an example of a reaction of this type. In this case, both hydrogen atoms joined to nitrogen unite with chlorine to form hydrochloric acid, and an intermediate compound is probably formed, which is unstable and loses another molecule of the acid. The steps in the reactions may be represented by the following equations —... 

The use of sodium hydrogen carbonate under solvent-free conditions and microwave irradiation is best method for N-alkylating pyrazoles. The yields are good and the method is devoid of side reactions like quatemization, isomerization and hydrogen halide elimination. Solvent-free conditions are the only ones that allow the preparation of 1-substituted pyrazoles from secondary halides (Almena et al., 2009). 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

When allylic alcohols are used as an alkene component in the reaction with aryl halides, elimination of /3-hydrogen takes place from the oxygen-bearing carbon, and aldehydes or ketones are obtained, rather than y-arylated allylic alcohoIs[87,88]. The reaction of allyl alcohol with bromobenzene affords dihydrocinnamaldehyde. The reaction of methallyl alcohol (96) with aryl halides is a good synthetic method for dihydro-2-methylcinnamaldehyde (97). 

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... 

Phosphoms halides are subject to reactions with active hydrogen compounds and result in the elimination of hydrogen halide. They are convenient reagents in the synthesis of many esters, amides, and related compounds. However, because the involved hydrogen halide frequendy catalyzes side reactions, it is usually necessary to employ a hydrogen halide scavenger to remove the by-product. 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

The elimination of hydrogen halide from organic halides under basic conditions generates alkenes and is a fundamental reaction in organic chemistry. It is sometimes carried out with a base in aqueous media.14 In contrast, the corresponding Hofmann-type eliminations of quaternary ammonium hydroxides are frequently carried out in aqueous media,15 which will be covered in Chapter 11. 

In 1,2-eliminations involving carbon atoms (i.e. most), the atom from which Y is lost is usually designated as the l-(a-) carbon and that losing (usually) H as the 2-(/ -) carbon in the older a/J-terminology, the a- is commonly omitted, and the reactions are referred to as p-eliminations. Among the most familiar examples are base-induced elimination of hydrogen halide from alkyl halides—this almost certainly the most common elimination of all—particularly from bromides (1) ... 

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