Dehydrohalogenation state

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

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. 

A method that is stated to be applicable to residues of benzene hexachloride (20) is based on the fact that benzene hexachloride yields essentially 1,2,4-trichlorobenzene on dehydrohalogenation with alkali. This product possesses a characteristic absorption band in the ultraviolet, which permits its quantitative determination. 

The antimony oxide/organohalogen synergism in flame retardant additives has been the subject of considerable research and discussion over the past twenty-five years (1-17). In addition to antimony oxide, a variety of bismuth compounds and molybdenum oxide have been the subject of similar studies (18-20). Despite this intensive investigation, relatively little has been conclusively established about the solid state chemical mechanisms of the metal component volatilization, except in those cases where the organohalogen component is capable of undergoing extensive intramolecular dehydrohalogenation. 

The rate of catalytic dehydrohalogenation is influenced by the structure of the reactants, but the extent of this effect varies from one catalyst to another with change of mechanism, i.e. with the timing of the fission of the Ca—X and Cp—H bonds. This is best seen from the published data on the deuterium kinetic isotope effect in Table 8. Their significance for the elucidation of the mechanism will be dealt with in Sect. 2.4.4 and here we can simply state that the value of the isotope effect depends on the nature of the catalyst. However, with a different reactant and within a series of related catalysts, kH/kD values independent of the catalyst were obtained (Table 9) [183],... 

Dehydrohalogenation of alkyl halides is stereospecific, requiring an anti arrangement between the hydrogen being lost and the leaving group in the transition state. (Z)-l,2-Diphenylpropene must therefore be formed from the diastereomer shown. 

If an alkyl halide contains more than two carbons in its chain, and the carbon atoms adjacent to the carbon atom bonded to the halogen each have hydrogen atoms bonded to them, two products will form. The major product is predicted by Zaitsev s Rule, which states that the more highly branched alkene will be the major product. For example, in the dehydrohalogenation reaction between 2-chlorobutane and sodium methoxide, the major product is 2-butene. 

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. 

As with other aromatic substitutions, the substitution step itself can be considered to involve an approximately sps hybridization at the carbon atom under attack (10). In the idealized substitution process shown in Eq. (16), 10 may constitute either an intermediate or a transition state. If proton loss ensues directly, the process is properly called a substitution. In other situations the intermediate 10 may become allied with a radical or an anion, leading thereby to a covalent adduct 11. The final substituted product 12 may then be formed either by the elimination of H—Z (first H, then Z) or by the reversal to 10, followed by proton loss. The first case is a classical example of an addition-elimination halogenation, where the adduct is an essential species in the process. In the second case, structure 10 is a common intermediate for both the substitution and the addition reactions. Being merely a diversion of 10, the addition product is not essential to the substitution. In consequence of this, the isolation of adduct 11 may not mean that addition-elimination is the principal pathway of substitution reversal to 10 may be faster than the elimination of H—Z ( 2, k3>ki). On the other hand, the mere failure to detect adduct 11 does not rule out an addition-elimination process, for dehydrohalogenation of adduct 11 may be much faster than its formation (ki>klt k2). 

Why is a stronger base needed in this dehydrohalogenation The transition state for the second elimination reaction includes partial cleavage of a C - H bond. In this case, however, the carbon atom is sp hybridized, and sp hybridized C-H bonds are stronger than sp hybridized C-H bonds. As a result, a stronger base is needed to cleave this bond. 

DehydrohalogenatUm. A preliminary report states that the reagent is useful for the dehydrohalogenation of a-halo ketones under mild conditions. On treatment with the salt in acetone, 4 3-bromocoprostanone afforded cholestenone in good yield. 

The dehydrohalogenation products of poly(ethylene-a/z-chlorotrifluor-oethylene) were also studied using solid-state NMR spectroscopy and the cross-polarization magic angle spinning (CP MAS) technique [18]. The elimination reaction induced by potassium ZerZ-butoxide (z-BuOK) in tetrahydrofuran (THF) was found to proceed slowly only 50% of hydrogen... 

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