Hydrohalogenation mechanism

No data on the biodegradation of nitrogen mustards were located. Nitrogen mustards can theoretically be biodegraded via reductive dehalogenation and de-hydrohalogenation mechanisms, but these processes would be very slow. HNl and HN2 can be degraded via oxidation dealkylation (N-dealkylation for HNl and C-dealkylation for HN2) the metabolites would possess vesicant properties (Morrill et al. 1985). 

In each mechanism above, the first step involves protonation of the alkene to form a carbocation. Then, in both cases, a nucleophile (either X or H2O) attacks the car-bocation to give a product. The difference between these two reactions is in the nature of the product. The first reaction above (hydrohalogenation) gives a product that is neutral (no charge). However, the second reaction above (hydration) produced a charged species. Therefore, one more step is necessary at the end of the hydration reaction— we must get rid of the positive charge. To do this, we simply deprotonate ... 

The mechanism for the acid-catalysed hydration reaction is very similar to that for the hydrohalogenation of alkenes and also proceeds via a carbocatlon intermediate. It is outlined below using water and propene. 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

Only a few speculations concerning the mechanism of the hydrohalogenation reactions can be found in the literature. They are mostly based on the better knowledge of the reverse reaction, the decomposition of haloalkanes. It seems evident that catalytic hydrohalogenation must also involve transfer of paired valence electrons and that a free radical-like process is highly improbable. The existence of 7r-complex intermediates has been suggested [321,336,337] but the hypothesis lacks experimental evidence. 

A study of the solid-state hydrohalogenation (HC1, HBr) of 2-methyl-2-butene led to formulation of a mechanism involving the 2HX.C5H10 complex76. 

The mechanism of hydrohalogenation illustrates why two enantiomers are formed. Initial addition of the electrophile (from HCl) occurs from either side of the planar double bond to form a carboca-... 

Because hydrohalogenation begins with a planar double bond and forms a planar carbocation, addition of H and Cl occurs in two different ways. The elements of H and Cl can both be added from the same side of the double bond— that is, syn addition—or they can be added from opposite sides—that is, anti addition. Both modes of addition occur in this two-step reaction mechanism. 

Scheme 85. (a) Overall reaction and (b) mechanism of Pt-catalyzed, hydrohalogenation-promoted... 

Besides the addition of halogens and hydrohalogens across the double bond just covered, there are many other reagents that will react similarly with unsaturated polymers by free radical, ionic, or radical-ion mechanisms. Of prime importance is the addition of ethylene derivatives to polydienes. One of the earliest reactions of natural rubber to be studied in detail was the combination with maleic anhydride (Cunneen and Porter, 1965). Depending on the reaction conditions and the presence or absence of free radical initiators, one or more of four basic reactions may take place, with the products shown (the arrows indicate where the addition has taken place and the new bonds formed). 

CHEMICAL MODIFICATION The following chemical modifications of cis-1,4-polyisoprene are employed as a convenient way of altering physical and mechanical properties hydrohalogenation, halogenation, oxidation, ozonolysis, hydrogenation, carbene addition, cyclization. ... 

ABSTRACT The gas-solid halogenation and hydrohalogenation using micro-crystalline cyclodextrin complexes are found to be efficient for production of the optical active halides of ethyl trans-cinnamate in moderate optical yields On exposure to HBr at 2QOC for 15-20 hr, the cinnamate in solid a- and S-cyclodextrin complexes yields ethyl R-(+)-3-bromo-3-phenylpropanoate in 46% e.e., and S-(-)-enantiomer in 31% e.e., respectively. No addition nor substitution products are obtained with HCl vapor at 0-50°C for 15-65 hr. Bromination of the B-cyclodextrin complex results in the formation of optical active ethyl erz/t/zrc>-2,3-dibromo-3-phenylpropanoate, while chlorination gives the optical active mixture of trans and cis addition products, ethyl erythro- and threo-2,3-di-chloro-3-phenylpropanoates in 60-80% yields. Mechanism of chiral induction in the present gas-solid reaction has been proposed on the basis of the crystal structure of the complex. 

In this reaction, a hydrogen atom and a halogen are added across an alkene. The halogen (Cl) is ultimately positioned at the more substituted carbon, which verifies that this reaction takes place via an ionic mechanism (Markovnikov addition). The ionic mechanism for hydrohalogenation has two steps (1) protonation of the alkene to form the more stable carbocation and (2) nucleophilic attack. Each step must be drawn precisely. 

DRAWING A MECHANISM FOR A HYDROHALOGENATION PROCESS WITH A CARBOCATION V REARRANGEMENT... 

The proposed mechanism is consistent with the experimental observations discussed earlier in this section. The reaction proceeds via a Markovnikov addition, just as we saw for hydrohalogenation, because there is a strong preference for the reaction to proceed via the more stable carbocation intermediate. Similarly, reaction rates for substituted alkenes can be justified by comparing the carbocation intermediates in each case. Reactions that proceed via tertiary carbocations will generally occur more rapidly than reactions that proceed via secondary carbocations. 

One possible mechanism for the hydrohalogenation of alkynes involves a vinylic carbocation, while another possible mechanism is termolecular. 

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