Hydrogen halides, molecular elimination

The carbonylation of some alkyl halides such as iodocyclohexane (911) can be carried out under neutral conditions in the presence of N,N,N.N-tetre,-methylurea (TMU), which is a neutral compound, but catches generated hydrogen halide. Molecular sieves (MS-4A) are used for the same pur-pose[768]. Very reactive ethyl 3-iodobutyrate (912) is carbonylated to give ethyl methylsuccinate (913) in the presence of TMU. The expected elimination of HI to form crotonate, followed by carbonylation, does not occur. 

There have been both experimental and theoretical studies to probe the degree of concertedness in gas-phase substitutions as shown in Scheme 1. Is (2) an intermediate with a finite lifetime, or are the addition and elimination steps concerted so that (2) is a transition state Experimental molecular beam studies on the femtosecond time-scale have been reported for the reaction of chloride ions with the iodobenzene cation to yield chlorobenzene and iodine. The results show an 880 fs reaction time for the elimination process, indicating a highly non-concerted process, so that here the er-complex is an intermediate rather than a transition state.12 The reactions of halobenzene cations with ammonia have been interpreted in terms of the formation of an addition complex which may eliminate either halogen, X, or hydrogen halide, HX, depending on the nature of the halogen.13... 

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. 

The deactivation of OCD2) by halocarbons has been studied in a number of laboratories and both the overall kinetics and reaction mechanisms quite thoroughly examined (Fletcher and Husain, 1976 Davidson ei al., 1978 Force and Wiesenfeld, 1981). Both quenching and reaction play significant roles here, although reaction dominates except in the cases of CF4 and CHFi. Electrophillic attack upon the chlorine atom may well dominate deactivation of O ( D2) by the chloro-carbons (Addison et al., 1979). Molecular elimination of hydrogen halides as in... 

Reactions which yield cyclic products from open-chain starting materials by the elimination of water, hydrogen halides or other low molecular mass compounds are called cyclocondensations. They are important methods for synthesizing five-, six- und larger-membered heterocycles. 

Purpose. A three-step synthesis of 2 -bromostyrene is carried out starting with benzaldehyde. You will gain experience working with semimicroscale quantities of organic materials. (See Sequence A, where benzaldehyde is also used as the starting material, in that case for the formation of two different intermediates in the seven-step synthesis of hexaphenylbenzene.)You will become familiar with the stereo-chemistry involved in the addition of molecular halogen to an aUcene (see also Experiment [A2b]). You will observe the elimination of a hydrogen halide promoted by a concerted decarboxylation reaction (see also Experiment [A3b]). As an option, H NMR spectroscopy will be used to determine the ds/trans ratio of isomers in the product (see also Experiment [5B]). 

The alternative suggestion that dehydrochlorination of poly(vinyl chloride) involves unimolecular elimination has been made by analogy with low molecular weight alkyl halides. It has been found that the homogeneous gas phase pyrolysis of many alkyl halides to hydrogen halide and olefin follows first order kinetics, which is consistent with a one-step mechanism proceeding through a cyclic transition state ... 

Some fragmentations involve cleavage of two bonds simultaneously. In this process, an elimination occurs, and the odd-electron molecular ion yields an OE and an even-electron neutral fragment N, usually a stable small molecule of some type H2O, a hydrogen halide, or an alkene. Some examples of two-bond cleavages of this type are shown in Figure 4.3. 

Alternatively, under strong reductive conditions, e.g. electrochemical reduction, arylpalladium(II) halide (XIII) is reduced to arylpalladium(0) which, upon the second oxidative addition, gives diarylpalladium(II) complex, also capable of undergoing reductive elimination of biaryl similarly to XIV [10]. Isopropanol [50-52], amines [4,17,52-54], hydrazine [55], tetrakis(dimethylamino)ethylene [18], hydroquinone [56], tetrabutylammonium ftuoride [57], formates [58-60], zinc [61-64], and molecular hydrogen [65] have been used as stoichiometric reductants. Electro-reductive homocoupling of aryl halides was also reported [10]. The most important methods for... 

---