Reactions dehydrohalogenation

Similarly, the reaction with bromine yields the corresponding 5-bromo derivatives, which are suitable substrates for nucleophilic displacement or dehydrohalogenation reactions. Typical nucleophiles (Nu) are H2O, NH, RNH2, and sodium barbiturates ... 

A combined halogenation-oxidation-dehydrohalogenation reaction has been described. ... 

Cyclohexyl bromide, for exfflnple, is converted to cyclohexene by sodium ethoxide in ethanol over 60 times faster than cyclohexyl chloride. Iodide is the best leaving group in a dehydrohalogenation reaction, fluoride the poorest. Fluoride is such a poor leaving group that alkyl fluorides are rarely used as starting materials in the preparation of alkenes. 

Dibromo-7-oxabicyclo[4.1.0]heptanes can be obtained by bromination of the respective 7-oxabicyclo[4.1.0]hept-3-ene, the monoepoxide of cyclohcxa-1,4-diene, and when submitted directly to the dehydrohalogenation reaction give products 3 and 4.2 149 150... 

The dehydrohalogenation reaction has been extended to benzannulated oxepins. Elimination of hydrogen bromide from 3-bromo-4-phenyl-2,3-dihydro-l-benzoxepin with 1,5-diazabicyclo-[4.3.0]non-5-ene gives 4-phenyl-1-benzoxepins 15a15 and 15b16 in low yield. 

LaClair (44) and Soloway et al. (SO) have modified the dehydrohalogenation reaction to permit the determination of p,p -DDT in dusts and oil solutions containing technical DDT. The reaction between the base and the halide is carried out at 20° to 30° C., as Cristol (16) has found that under proper conditions at this temperature the p,p -DDT reacts completely, whereas the o,p -DDT and most of the impurities react only slightly. The Association of Official Agricultural Chemists (3) tentative method for the determination of the purity of p,p -DDT, which employs the dehydrohalogenation procedure, has been modified by Fleck (30) so that the reaction is carried out at 25° C. instead of under reflux conditions. 

The dehydrohalogenation reaction has been applied by Goldenson and Sass (32) to the determination of benzene hexachloride in impregnated cloth. Howard (39) proposed the reaction for the determination of benzene hexachloride residues in food, and Barlow (6) used it for benzene hexachloride in the blood of cattle. 

LaClair (43) has employed the dehydrohalogenation reaction to determine the gamma isomer of benzene hexachloride in the technical product and in dust mixtures. Two identical samples are dissolved in 95% ethyl alcohol and treated with 1 N ethanolic potassium hydroxide at 0° C. for 15 and 50 minutes, respectively. The 15-minute period is sufficient to dehydrochlorinate most of the alpha and the delta isomers without appreciably affecting the gamma. In 50 minutes the gamma isomer is also dehydrochlorinated. The beta isomer does not react under these conditions, and usually the epsilon isomer is present in quantities too small to interfere seriously. 

The thermolysis of a linear (fluorostannyl)silylphosphine has been reported to give the corresponding stannaphosphene.8 This reaction can be considered to be a defluorosilylation and, as such, is analogous to the dehydrohalogenation reactions discussed below. 

For some dehydrohalogenation reactions, a single elimination product is possible . 

For those organohalogen compounds which can undergo intramolecular dehydrohalogenation, reaction sequence [1] has been proposed as the principal route to the generation of volatile antimony containing species (3, 5, 7, 18, 20). ... 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

Several dehydrohalogenation reactions have been carried out with the help of DMSO. Thus isopropyl bromide has been successfully converted to propene by heating with potassium tertiary butoxide-DMSO mixture at 550°. 

As an alternative to the extraction and interfacial mechanisms, hydroxide promoted dehydrohalogenation reactions may also occur, not by proton removal from the... 

Dehydrohalogenation reactions appeared to be a convenient route to double bond germanium nitrogen species and were commonly carried out with an organolithium compound as a base.3,4 5b 6 7 96,97 123 Accordingly, the synthesis of two moderately hindered stable germanimines Mes2Ge = NR 136 and 137 has been reported.59 Stabilization in these cases is achieved... 

Isotope effects and element effects associated with hydron-transfer steps during methoxide promoted dehydrohalogenation reactions of jo-CF3C6H4C HClCH2X (X=Br, Cl, or F) have also been discussed, with regard to distinction between E2 and multi-step pathways. The Arrhenius behaviour of hydrogen isotope effects was used to calculate the amounts of internal hydrogen return associated with the two-step mechanism. 

If there is possibility of formation of more than one alkene due to the availability of more than one 3-hydrogen atoms, usually one alkene is formed as the major produet. These form part of a pattern first observed by Russian ehemlst, Alexander Zaitsev (also pronounced as Saytzeff) who In 1875 formulated a rule which can be summarised as in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. Thus, 2-bromopentane gives pent-2-ene as the major product. 

Lithium bromide is used in absorption, refrigeration and air-conditioning systems. A highly concentrated solution of the salt is an efficient absorbent of water vapor. The vapor pressure of such solution is very low. Other applications include the use of the salt as a swelling agent for wool, hair and other organic fibers as a catalyst in dehydrohalogenation reactions and as a sedative and hypnotic in medicine. 

A small amount of acetaldehyde formed during the dehydrohalogenation reaction is removed by resinification under the influence of KOH. 

Scheme 16.19. 2-Chloro-l,3-didehydronaphthalene was postulated as an intermediate of a complex dehydrohalogenation reaction by Billups et al. (Tetrahydrofuran = THE)...

Dehydrohalogenation reactions have been involved in several routes for the total synthesis, involving pyran intermediates, of... 

In a /3-elimination (dehydrohalogenation) reaction a halogen and a hydrogen atom are removed from adjacent carbon atoms to form a double bond between the two C s. The reagent commonly used to remove HX is the strong base KOH in ethanol (cf. Section 6.2). 

The catalytic activity of polyethylene glycol (PEG) phosphonium salts has been evaluated, in phase-transfer dehydrohalogenation reactions, as slightly better than that of the corresponding PEG ammonium compounds886 (reaction 271). By comparison... 

Table 13.7 Kinetic Data on Nucleophilic Substitution and Nonreductive Elimination (Dehydrohalogenation) Reactions of Some Polyhalogenated Hydrocarbons in Aqueous Solution at 25 °C ... 

Roberts, A. L., P. M. Jeffers, N. L. Wolfe, and P. M. Gschwend, Structure-reactivity relationships in dehydrohalogenation reactions of polychlorinated and polybrominated alkanes , Crit. Rev. Environ. Sci. Technol., 23, 1-39 (1993). 

The illustrative preparative example is the conversion of cyclohex-1 -en-l-yl-acetic acid into the y-iodolactone (12) (cis-l-iodo-7-oxabicyclo[4.3.0]nona-8-one,188 Expt 5.172). In general the simpler iodolactones tend to be unstable the preparative sequence therefore recommends conversion into the unsaturated lactone by a dehydrohalogenation reaction. 

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